The submission took an unusual and deliberately democratic form. Rather than speaking with a single voice, AMI combined a survey reflecting the majority view of participating members with a detailed supporting document that sets out the scientific case for greater recognition of marine microorganisms in European ocean policy. The society’s central message is unambiguous: marine microbes must be explicitly recognised as fundamental components of ocean ecosystems, and their needs and contributions must be considered across biodiversity conservation, ecosystem management, ocean-climate policy and the assessment of overall marine health. In other words, an ocean act that only protects what can be seen with the naked eye is, in the view of these scientists, an act that protects only a fraction of what matters.

The scientific rationale for this position is compelling. Microorganisms, including phytoplankton, bacteria, archaea and viruses, drive many of the biological and chemical processes that regulate the ocean. They are responsible for recycling around half of global primary production, a staggering figure that underscores just how much of the planet’s carbon and nutrient throughput depends on organisms too small to see. Through the biological carbon pump, phytoplankton fix carbon dioxide at the surface and export it to the deep sea, while the microbial carbon pump converts dissolved organic matter into long-lived reservoirs that can keep carbon out of the atmosphere for centuries. Microbes also orchestrate the cycling of nitrogen, phosphorus and iron, elements that determine how productive marine food webs can be and how much carbon the ocean can ultimately sequester.

The urgency of this agenda is growing as the ocean changes. As marine waters warm, acidify and lose oxygen, microbial communities are being pushed into conditions they have not experienced on human timescales. Because microbes reproduce quickly and respond rapidly to shifting temperatures, pH levels and nutrient supplies, changes in their community composition and metabolic function often precede visible changes in larger organisms and habitats. Understanding how these communities respond to ocean warming, acidification and deoxygenation will therefore be increasingly important for predicting future shifts in ecosystem function, for anticipating how climate regulation services may weaken, and for designing interventions that support microbial processes capable of buffering environmental stress rather than amplifying it.

Beyond free-living microbes, AMI’s response draws attention to the intimate partnerships between microorganisms and larger marine life. Corals, sponges, seagrasses and fish all exist in close association with complex microbial communities that can influence nutrition, disease resistance and resilience to environmental stress. The coral microbiome, for example, can determine whether a reef survives a bleaching event, while the microbial partners of seagrasses are essential for nutrient uptake in the sediments that anchor these meadows. AMI argues that recognising these host-associated microbiomes could materially strengthen conservation and restoration efforts, by ensuring that the microscopic processes underpinning ecosystem health are not overlooked when protected areas are designated, when species are translocated, or when damaged habitats are actively restored.

There is also a powerful economic argument embedded in the society’s submission. Marine microbial communities represent a largely unexplored reservoir of genetic and biochemical diversity, a living library of molecules shaped by billions of years of evolution in some of the most extreme environments on Earth. Potential applications range from novel medicines and industrial enzymes to new biomaterials and biotechnological processes, with microbial enzymes from cold-adapted and pressure-tolerant species already finding uses in industry and research. AMI stresses that protecting marine microbial biodiversity is therefore not only an environmental priority but also an investment in future innovation, feeding directly into the ambitions of the European blue bioeconomy and the continent’s broader strategy for sustainable growth grounded in living resources.

Translating this science into law, AMI offers a series of concrete recommendations. First, the society calls for strengthening the definition and assessment of Good Environmental Status, the benchmark against which European seas are measured, so that it explicitly considers microbial diversity, community composition, functional diversity and the ecosystem services that microbes provide. Second, the submission urges that host-associated microbiomes be formally recognised wherever they are important to the health and resilience of protected species, habitats and ecosystems. These changes would mean that monitoring programmes, conservation targets and restoration criteria under the Ocean Act would no longer treat microbes as an afterthought but as measurable, reportable components of environmental quality in their own right.

The response also argues that an ecosystem-based approach to ocean management must move beyond a focus on visible organisms and iconic habitats. Microorganisms drive the processes that make marine ecosystems possible, including nutrient cycling, carbon sequestration, primary productivity and decomposition. Their rapid responses to environmental change could provide early-warning indicators of ecosystem condition and recovery, offering policymakers near-real-time signals of stress long before fish stocks decline or habitats collapse. Incorporating microbial indicators into routine monitoring would give regulators a finer-grained, more responsive picture of ocean health, and could reveal whether management measures are actually working at the functional level where the ocean’s essential chemistry is performed.

By bringing together the expertise and perspectives of its global membership, AMI is advocating for an Ocean Act that recognises the full biological complexity of marine ecosystems and puts microbiology firmly within Europe’s vision for a healthy, resilient and sustainable ocean. The society, the oldest microbiology society in the United Kingdom with more than half of its members based outside the country, publishes The Microbiologist magazine and three internationally recognised journals in partnership with Oxford University Press, and it frames this intervention as part of its wider mission to amplify the collective influence of applied microbiologists in evidence-based decision making. The consultation response makes clear that the microbes drifting in every litre of seawater are not a footnote to ocean policy; they are its scientific foundation, and the legislation that will govern European seas for decades should say so explicitly.

Subject of Research: The role of marine microorganisms in European ocean policy and the proposed EU Ocean Act

Article Title: AMI calls for marine microbes to be at the heart of Europe’s future ocean policy

Article References: AMI calls for marine microbes to be at the heart of Europe’s future ocean policy. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: marine microbes, EU Ocean Act, Applied Microbiology International, ocean policy, microbial carbon pump, Good Environmental Status, blue bioeconomy, host-associated microbiomes, ocean acidification, biodiversity conservation, ecosystem monitoring, phytoplankton

A new landmark editorial launching Ocean Microbiology, an open-access journal dedicated to marine and aquatic microorganisms, argues that the field stands at a unique crossroads. The publication, authored by Ramiro Logares of the Institute of Marine Sciences in Barcelona, describes a golden age of marine microbial research, powered by an extraordinary convergence of technologies that have transformed what scientists can see and measure. Over the past two decades, high-throughput DNA sequencing, high-resolution imaging, microfluidics and single-cell technologies, advanced bioinformatics, autonomous monitoring systems, predictive modeling and new theoretical frameworks have together revealed the diversity and complexity of ocean microbes at unprecedented scales, opening windows onto a world that was almost entirely hidden a generation ago.

The technical achievements of this era have been stunning in their scope. Large-scale sequencing campaigns have uncovered millions of novel microbial genes, dramatically expanding understanding of the metabolic potential harbored in ocean waters—genes that hint at undiscovered biochemistries operating in every liter of seawater. Surveys of plankton across ocean basins have exposed the existence of a vast ‘rare biosphere,’ an enormous collection of low-abundance microbial taxa that nonetheless perform ecologically relevant roles in nutrient cycling and energy flow. Completely new lineages of microorganisms have been described, some of which are forcing biologists to redraw portions of the tree of life itself. And beneath the boundaries of species definitions, researchers are detecting a large but still overlooked diversity within microbial populations, revealing evolutionary dynamics occurring at fine genetic scales that classical methods could never resolve.

Equally transformative has been the growing appreciation of microbial ecology as a web of interactions rather than a roster of species. Scientists are now characterizing the myriad ecological relationships that sustain marine food webs and underpin ocean ecosystem functioning, including newly mapped interaction networks, syntrophic associations in which different microbes cooperate metabolically to complete chemical transformations neither could achieve alone, and symbioses that bind microorganisms to hosts ranging from corals to fish larvae. These interactions play key roles in ecosystem functioning and nutrient cycling, and their disruption—by warming, acidification or pollution—could cascade through marine ecosystems in ways scientists are only beginning to anticipate. The editorial emphasizes that unveiling these interaction networks is one of the central tasks facing the next generation of marine microbiologists.

But the same editorial does not celebrate unreservedly. It arrives at a moment when accelerating climate change, including ocean warming and acidification, together with other anthropogenic pressures such as pollution, is impacting marine ecosystems at an unprecedented pace. Because microbes mediate the biogeochemical reactions that determine how much carbon the ocean absorbs, how efficiently nutrients are recycled, and how productive fisheries will be, understanding how microbial communities respond and adapt to these pressures has become a matter of global urgency. The convergence of unprecedented technological capability with unprecedented environmental stress, the editorial argues, creates both opportunities and responsibilities for the marine microbiology community worldwide.

The technical toolkit for meeting that responsibility is maturing rapidly. Long-term ocean observatory programs now monitor microbial communities across years and decades, producing time series that can distinguish genuine trends in microbial abundance and composition from natural seasonal and interannual variability. When coupled with new predictive models—drawing on machine learning, ecosystem modeling and increasingly sophisticated representations of microbial metabolism—these observatories are expected to yield essential insights into how the ocean microbiome may be changing over time, and into its resistance, resilience, or potential fragility in the face of multiple simultaneous stressors. Such forecasting capability is exactly what policymakers and ocean managers will need as climate impacts intensify through the middle of the century.

Timing matters in another sense as well. The editorial notes that as the United Nations Decade of Ocean Science for Sustainable Development, running from 2021 to 2030, reaches its midpoint, advancing understanding of microbial responses to global change has become essential for forecasting the future health of the ocean. International frameworks like the Ocean Decade have elevated the microbiome from a specialist concern to a central pillar of ocean sustainability science, reflecting the recognition that no meaningful model of the future ocean can be built without representing the microbial processes that regulate it. The launch of a dedicated, open-access venue for the field is framed as part of this broader institutional shift toward treating ocean microbes as infrastructure of the biosphere.

The new journal is explicitly designed around the interdisciplinary character of modern marine microbiology. The editorial stresses that the field requires integration across scales: from molecular mechanisms inside single cells to ecosystem processes spanning whole basins, from tightly controlled laboratory experiments to global ocean observations gathered by ships, floats and satellites, and from micro- to macroecological perspectives. Accordingly, the journal welcomes research spanning microbial diversity, genomics and evolution; biogeochemical cycles and microbial metabolism; host–microbe interactions; microbial responses to global change; marine biotechnology and applied microbiology, including so-called blue biotechnology and nature-based solutions to societal challenges; methodological and theoretical advances; and microbial oceanography, including studies of large-scale ecological patterns across the ocean.

The applied dimension of the field deserves particular attention, because the same microbial capabilities that run the planet’s cycles also represent a resource for human society. Marine microbes are a reservoir of enzymes, natural products and metabolic pathways with potential uses in medicine, industry and environmental remediation. Nature-based solutions inspired by microbial processes—harnessing communities that degrade pollutants, cycle nutrients in aquaculture systems or buffer coastal ecosystems against climate impacts—form a growing frontier where fundamental ocean microbiology translates directly into tools for sustainability. The editorial signals that such translational research will sit alongside discovery-driven science in the new journal’s scope, reflecting the field’s expanding societal relevance.

The invitation to the community is broad. The editorial calls on marine microbiologists across all career stages, in every region of the world, to submit their research, contribute as reviewers and help shape a publication intended not merely to document progress but to influence the future direction of marine and aquatic microbial science. As Logares writes in closing, sitting by the ocean one wonders what new secrets it will unveil in the coming years. Given the pace of discovery, the accelerating pressures of global change, and the sheer scale of the microbial world that remains unexplored, that question may well define ocean science for the rest of the decade.

Subject of Research: The ocean microbiome and its role in Earth system functioning under global change

Article Title: The ocean microbiome on a changing planet

Article References: The ocean microbiome on a changing planet. (n.d.). https://doi.org/10.1186/s44375-025-00004-y

Image Credits: AI Generated

DOI: 10.1186/s44375-025-00004-y

Keywords: ocean microbiome, marine microbiology, climate change, biogeochemical cycles, rare biosphere, microbial genomics, ocean warming, ocean acidification, symbiosis, blue biotechnology, microbial ecology, Ocean Decade

Colistin, also called polymyxin E, occupies a special and precarious position in modern medicine. It is a cationic polypeptide that attacks the outer membrane of Gram-negative bacteria, and after decades of limited use because of kidney toxicity, it was reintroduced in recent years as a final defense against multidrug-resistant infections. With few new antibiotics in the development pipeline, clinicians increasingly depend on colistin when carbapenem-resistant pathogens strike hospital patients. The emergence of bacteria that resist this drug is therefore not a routine scientific concern but a genuine emergency in global public health, and the new study demonstrates that the ocean may be serving as an unexpected reservoir and highway for the genes responsible.

The mechanics of colistin resistance matter for understanding why the new results are so significant. Resistance can arise in two ways. Intrinsic resistance develops through chromosomal mutations that alter lipid A, the lipopolysaccharide component of the bacterial outer membrane that colistin targets, reducing the drug’s ability to bind. Acquired resistance, by contrast, depends on the horizontal transfer of mcr genes, usually carried on plasmids, which are mobile DNA elements that can shuttle between bacterial cells. Since the discovery of the mcr-1 gene in Escherichia coli plasmids a decade ago, researchers have identified ten mcr variants, from mcr-1 through mcr-10, and watched them disperse across continents, animal populations, and clinical settings. What remained poorly understood was how extensively these genes had colonized marine ecosystems.

To answer that question, the Brazilian team designed a natural experiment spanning roughly 330 kilometers of the Rio de Janeiro coastline plus the Fernando de Noronha Archipelago, a volcanic island group about 360 kilometers offshore in the Western Atlantic. Their sampling sites deliberately covered a gradient of human influence. At one extreme sat Bom Jesus Cove in Guanabara Bay, a tropical urban estuary receiving raw sewage, oils, and industrial contaminants from Rio de Janeiro city. At the other extreme lay the submarine caves of Fernando de Noronha, protected within a national marine park and accessible only by SCUBA diving. Between those poles, the researchers sampled the Cagarras Archipelago, a no-take marine protected area unfortunately positioned near the Ipanema submarine sewage outfall; the biodiverse waters of Arraial do Cabo, including the Gruta Azul submarine cave; and Ilha Grande Bay, one of the world’s largest tropical bays, which faces growing pressure from coastal settlements and maritime traffic.

The laboratory work was methodical and technically demanding. From 1,550 total bacterial isolates, the team focused on 308 Gram-negative bacilli identified as potential hosts of acquired colistin resistance determinants. Bacteria were cultured on Luria Bertani and MacConkey agar from water samples, washed and swabbed from pieces of floating plastic, and extracted from sponge tissue through serial dilution across four growth media. Identification proceeded by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry on a Bruker Microflex LT platform, and DNA was extracted using a thermal lysis protocol with Chelex 100 resin. Polymerase chain reaction screening then probed each strain for eight mcr variants, and any positive strains were tested further for beta-lactamase genes, quinolone resistance genes, aminoglycoside resistance genes, sulfonamide resistance genes, and class 1, 2, and 3 integron-integrase genes.

The results were striking in both breadth and detail. Of the 308 potential host strains, 101 carried at least one mcr variant, generating 128 total gene occurrences. Water samples contributed 59.4 percent of the positive strains, plastic litter 35.6 percent, and marine sponges 5.0 percent. The mcr-9 variant dominated with 33 occurrences, followed by mcr-3 with 23, mcr-2 with 18, and mcr-1 with 17. That mcr-9 topped the list is noteworthy because global metagenomic surveys have often ranked mcr-1 as the most dispersed variant. The mcr-9 gene is known to circulate on IncHI2 plasmids that interconnect human, animal, and environmental isolates, suggesting an efficient dissemination network that the ocean may now be extending.

The bacterial hosts carrying these genes were equally concerning. Among ten identified genera, Enterobacter, Acinetobacter, Vibrio, and Klebsiella predominated. Enterobacter species alone accounted for 31 strains and harbored the greatest mcr diversity, with every variant except mcr-5 and mcr-7 detected in the genus. More alarming still, mcr-positive Enterobacter was not confined to polluted Bom Jesus Cove but turned up in marine sponges and cave waters in Fernando de Noronha, an area with restricted human access. One Enterobacter hormaechei strain, isolated from a sponge in Sapata Cave, simultaneously carried mcr-2, mcr-9, and the sulfonamide resistance gene sul1. Four strains carried three mcr variants at once, including Klebsiella pneumoniae isolates from pristine sponges and from floating plastic, and an Enterobacter cloacae from Noronha water. Both K. pneumoniae and E. cloacae belong to the notorious ESKAPE group of pathogens, and their carriage of multiple resistance genes in aquatic matrices highlights the risk of waterborne transmission to humans.

The co-occurrence of resistance genes painted an even darker picture. Nearly half of the mcr-positive strains, 46.5 percent, also carried other antimicrobial resistance genes or integron-integrases, with sulfonamide resistance genes, particularly sul1, the most frequent companions, followed by the beta-lactamase genes blaTEM and blaSHV. In Bom Jesus Cove, the team detected the carbapenemase gene blaKPC alongside mcr-3 in Enterobacter bugandensis and Raoultella ornithinolytica strains, a combination that effectively eliminates two of the last therapeutic options for infections caused by these organisms. Integron-integrases, genetic platforms that capture and shuffle gene cassettes, co-occurred with mcr exclusively in the polluted cove, suggesting bacterial adaptation to intense anthropogenic selective pressure. Because changes in membrane permeability in mcr-positive bacteria can reduce sensitivity to multiple drugs, these combinations can compound into true multidrug resistance, complicating treatment of hospital-associated infections.

The gradient of pollution left a measurable fingerprint on the data. In Bom Jesus Cove, 27 of 104 water-isolated strains tested positive for mcr, and 36 of 82 strains from floating plastic litter carried the genes, a prevalence of roughly 44 percent that represents the highest in the study. The finding positions plastic debris as both a reservoir and a vector for antimicrobial resistance, offering microbes a stable, drifting substrate on which biofilms can form, exchange genes, and travel with currents. At the Cagarras Archipelago, 40 percent of water-isolated strains were positive despite formal protection, a legacy of the nearby sewage outfall. Arraial do Cabo showed 36.5 percent prevalence among water isolates and Ilha Grande Bay 36.3 percent. In Fernando de Noronha, prevalence dropped to about 5 percent of candidate host strains, with two of 17 water isolates and five of 15 sponge isolates positive, proving that resistance persists even where human pressure is minimal.

What emerges from the study is a warning that antimicrobial surveillance cannot remain confined to clinics and farms. Marine sponges, submarine caves, and drifting plastic are now documented hotspots of colistin resistance, and the genes involved sit on mobile elements capable of jumping into human pathogens. The Brazilian coastline, with its juxtaposition of dense urban pollution and remote protected archipelagos, offered an ideal natural laboratory, but the pattern it revealed is almost certainly global. The researchers argue for integrated monitoring programs that treat the ocean as a critical node in the antimicrobial resistance network, alongside conservation strategies that reduce sewage discharge and plastic pollution. As colistin remains a last resort for patients out of options, every mcr gene circulating in seawater, attached to plastic, or sheltering inside a sponge represents a card stacked against future medicine, and the ocean, it turns out, is dealing them freely.

Subject of Research: Distribution of mobile colistin resistance (mcr) genes in bacteria from Brazilian marine environments

Article Title: Widespread occurrence of mobile colistin resistance genes in Brazilian marine environments

Article References: Brunelli, R. C., de Jesus Carvalho Baptista, T. V., de Oliveira Nithack Marques, M., da Silva Oliveira Alves, G., Abdon, B. B., Mello, M. P., Paranhos, R., Gallo, M. N., Vinzon, S. B., Lage, A., Sandes, J., Muricy, G., Klautau, M., Lopes, M. V., Dias, G. R., Canellas, A. L. B., & Laport, M. S. (2025). Widespread occurrence of mobile colistin resistance genes in Brazilian marine environments. Ocean Microbiology, 1(1), Article 3. https://doi.org/10.1186/s44375-025-00003-z

Image Credits: AI Generated

DOI: 10.1186/s44375-025-00003-z

Keywords: antimicrobial resistance, colistin, mcr genes, marine microbiology, plastic pollution, Guanabara Bay, Fernando de Noronha, Enterobacter, Klebsiella pneumoniae, integrons, marine sponges, One Health

The study, conducted by Borna Branimir Vuković and Jelena Godrijan of the Ruđer Bošković Institute in Zagreb, Croatia, focuses on the Syracosphaeraceae family, the most species-rich family of modern coccolithophores. While most research attention has gone to Gephyrocapsa huxleyi, formerly known as Emiliania huxleyi, the bloom-forming workhorse of coccolithophore science, the Syracosphaeraceae account for roughly a quarter of all living coccolithophore species. Fossil evidence traces the order to which they belong back to the Cretaceous period, and the family survived the catastrophic mass extinction that wiped out the dinosaurs, diversifying steadily ever since. That deep evolutionary pedigree, the authors argue, makes them an ideal natural experiment in biological resilience.

The family comprises four genera: Syracosphaera, Michaelsarsia, Ophiaster and Calciopappus. Three of these are distinguished by extraordinary arm-like appendages built from highly modified coccoliths that extend from the cell surface. These structures look like limbs, but they do not help the cells swim or capture food. In Ophiaster, elongated string-like appendages coil around the spherical cell or radiate outward, and may uncoil in response to stress, enlarging the cell’s effective size and deterring grazing predators. Michaelsarsia and Calciopappus carry whorls and spine-like structures at the flagellar pole that can be swept back to create a streamlined profile, reducing drag as the cells move through the water. Such passive but dynamic architecture may help explain the remarkable ecological success of these appendage-bearing algae.

Syracosphaera, the largest genus with 36 described species, takes a different approach. Its cells build a double-layered coccosphere, an architecture almost exclusive to this genus and its closest relatives. An inner endotheca provides structural support and shields the cell membrane from mechanical damage and ultraviolet radiation, while an outer exotheca made of more elaborate coccoliths serves as the primary defense against physical damage and predation. The outer layer may also help regulate buoyancy, keeping cells suspended at optimal depths for light capture in stratified, nutrient-poor waters where sinking out of the sunlit zone would be fatal. By enlarging the coccosphere, the exotheca may even push the cell beyond the gape size of small predators such as microzooplankton. Although Syracosphaera coccospheres lack the interlocking coccoliths that give G. huxleyi its mechanical strength and appear fragile in laboratory conditions, the double-layered design still appears to buffer the cells against turbulence and environmental fluctuations.

Perhaps the most striking feature of Syracosphaera is its life cycle. Like many coccolithophores, the genus is haplo-diplontic, alternating between a haploid phase covered in holococcoliths and a diploid phase covered in heterococcoliths, two morphologically radically different forms that were once mistaken for entirely separate species. Each phase occupies a distinct ecological niche. The haploid holococcolithophore phase produces small, uniform, less heavily calcified plates, reducing metabolic cost and helping the cell stay buoyant in well-lit, nutrient-starved surface waters. These cells may even supplement photosynthesis with mixotrophy, absorbing dissolved organic nutrients directly. The diploid heterococcolithophore phase, with its larger and more complex mineral armor, is better suited to deeper, cooler, more nutrient-rich waters and tolerates low light and environmental stress. By shuttling between these two lifestyles, a single species can effectively inhabit two different oceans.

To map how this dual strategy plays out across the globe, the researchers mined the CASCADE dataset, a comprehensive compilation of 33,119 gridded coccolithophore observations covering 139 taxonomic units from 1964 to 2019, spanning all ocean basins to depths of 275 meters. They combined this with an unpublished dataset from the 2018 Atlantic CoccoMix cruise aboard the R/V Endeavor, in which seawater samples from eight depths at each station were filtered, gold-coated and examined under a scanning electron microscope, with researchers counting between 73 and 971 microscopic fields per sample to tally at least 100 cells. Because these datasets lacked environmental measurements, the team supplemented them with a systematic literature review linking species abundances to temperature, nutrients, light and water stratification.

The results reveal a family of specialists and generalists. Syracosphaera is widespread across tropical and subtropical gyres, with holococcolithophore phases typically concentrated in surface waters and heterococcolithophore phases in deeper layers. Michaelsarsia clusters in subtropical and temperate oligotrophic regions, possibly relying on mixotrophy or efficient nitrogen and phosphorus uptake. Calciopappus shows a more restricted distribution with notable abundance at high latitudes, suggesting adaptation to colder, seasonally productive waters. Ophiaster, by contrast, thrives everywhere from nutrient-rich upwelling zones to barren subtropical gyres and reaches abundances of up to 1.5 million cells per liter, up to two orders of magnitude higher than the other genera, hinting at regionally specific adaptations within its populations.

At the species level, the patterns become even more nuanced. The holococcolithophore phases of species such as S. histrica, S. arethusae and S. anthos consistently prefer oligotrophic conditions, correlating negatively with nitrate, phosphate and chlorophyll. Some diploid phases, like those of S. halldalii and S. ossa, flourish in nutrient-rich waters during bloom events, while S. pulchra and S. mediterranea display genuine flexibility across nutrient regimes. Temperature preferences range from the polar-to-tropical tolerance of S. corolla to the warm-water affinity of S. pulchra and the cooler, deeper-water tendencies of S. rotula. Seasonality matters too: diploid phases tend to dominate winter and early spring mixing, while haploid phases peak in stratified summer conditions. Intriguingly, some species flip their preferences between ocean basins, with S. nodosa correlating with nutrients in the Aegean Sea but showing no clear associations in the Adriatic, underscoring the power of local environmental context.

The ecological stakes are considerable. In oligotrophic tropical and subtropical waters, which cover vast swaths of the open ocean and are projected to expand and warm further as climate change progresses, Syracosphaera species are key contributors to primary production and calcification. The haploid phase drives organic carbon fixation in sunlit surface layers, while the diploid phase contributes to the carbonate pump in deeper, nutrient-rich waters, sequestering carbon into sinking mineral particles. This dual contribution to both organic and inorganic carbon cycling makes the genus a potentially important, and poorly quantified, term in the global carbon budget. Species such as S. molischii and S. pulchra, with their demonstrated seasonal and environmental versatility, exemplify how life cycle flexibility translates into biogeographical breadth.

The review also exposes sobering gaps. Only a single Syracosphaera species, S. pulchra, is currently available from public culture collections, severely limiting controlled experiments on calcification, nutrient uptake and life cycle transitions. Distribution records for many species, particularly their haploid phases, remain sparse, and environmental correlations in databases like CASCADE are incomplete. The authors call for expanded cultivation efforts, genomic studies to identify the genes underpinning calcification and phase switching, targeted field surveys of under-documented species, and biogeochemical models that incorporate species-specific distribution and seasonal data. As ocean acidification, warming and intensifying stratification reshape marine ecosystems, understanding whether these Cretaceous survivors can continue their ancient balancing act may prove essential for predicting the future of the ocean carbon cycle itself.

Subject of Research: Ecological diversity, distribution and life cycle adaptations of the coccolithophore family Syracosphaeraceae in the global ocean

Article Title: Diversity of coccolithophores in the ocean: insights from Syracosphaeraceae family

Article References: Diversity of coccolithophores in the ocean: insights from Syracosphaeraceae family. (n.d.). https://doi.org/10.1186/s44375-025-00001-1

Image Credits: AI Generated

DOI: 10.1186/s44375-025-00001-1

Keywords: coccolithophores, Syracosphaera, Syracosphaeraceae, marine microbiology, ocean carbon cycle, calcification, haplo-diplontic life cycle, oligotrophic ecosystems, phytoplankton, biogeochemistry, climate change, ocean acidification

The research, led by Cristian Villena-Alemany of the Institute of Microbiology of the Czech Academy of Sciences together with colleagues in Croatia and the Czech Republic, was published in the journal Ocean Microbiology. The team sampled seawater from Kaštela Bay in February, May and July of 2023, separating the bacterial community into a free-living fraction and a total fraction that included bacteria attached to particles. For each sample they measured AAP abundance under the epifluorescence microscope, quantified bacteriochlorophyll a by high-performance liquid chromatography, and built both DNA and RNA amplicon libraries of the pufM gene, which encodes the M subunit of the anoxygenic reaction centre and serves as the standard taxonomic marker for this group. Sequencing hundreds of thousands of reads per sample on an Illumina MiSeq platform yielded more than a thousand distinct pufM sequence variants for analysis.

The seasonal signal was unmistakable. AAP abundance climbed from roughly 1.27 × 10⁴ cells per millilitre in winter to 8.30 × 10⁴ cells per millilitre in summer, peaking at 13.8 percent of the total bacterial community, a figure well above the 0.1 to 11 percent typically reported for the open ocean, the Baltic Sea, the Arctic and the Mediterranean. Bacteriochlorophyll a concentrations followed the same trajectory, reaching maxima of 4.23 and 3.76 nanograms per litre in summer and spring respectively. Critically, cell counts and pigment concentrations correlated strongly, allowing the authors to calculate that each AAP cell carries between roughly 1,600 and 11,200 reaction centres, corresponding to 2.14 × 10¹⁰ to 8.51 × 10¹¹ photosynthetic units per litre of seawater. This near-constant complement of reaction centres per cell across seasons and fractions suggests that changes in community phototrophic capacity are driven primarily by shifts in cell numbers rather than by cells tinkering with their pigment investment.

The more surprising findings emerged when the team compared the total community, read from DNA, against the phototrophically active community, read from RNA transcripts of pufM. The two libraries told strikingly different stories. In winter, DNA amplicons suggested a community dominated by the genus Luminiphilus, but the RNA library revealed a much more diverse active assemblage in which several genera of Burkholderiales, along with Rhizobiales, Limnohabitans and Rhodoferax, were punching far above their genetic weight. In spring, Luminiphilus dominated both libraries, yet the transcript data showed that genera such as UBA9115, Limnohabitans and Rhodoferax were expressing their phototrophy genes at levels their DNA abundance never predicted. In summer, the season of peak photoheterotrophy, the gap widened further: Arenicellales UBA868 drastically overexpressed its phototrophy genes in the free-living fraction relative to its DNA signal, while the coastal lineage Rhodobacterales HIMB11 was underrepresented in DNA libraries from the particle-attached fraction despite its activity.

These discrepancies mean that the conventional practice of estimating phototrophic potential from DNA-based pufM amplicons can seriously distort the picture of which bacteria are actually harvesting light. Some abundant lineages, notably Luminiphilus, appear to coast along with their phototrophy genes largely switched down, inflating their apparent importance in gene surveys. Meanwhile, rarer lineages that barely register in DNA libraries are working overtime at the transcript level, quietly contributing to the community’s light-driven metabolism. The pattern echoes earlier observations from freshwater lakes, where gene presence likewise failed to guarantee gene expression, but this is among the clearest demonstrations that the same principle holds in the sea. For modellers of marine carbon fluxes, the implication is sobering: knowing which phototrophs are present is not the same as knowing which ones are plugged into the sun.

Lifestyle added a second, equally consequential layer of structure. Bacteriochlorophyll a concentrations were consistently higher in the particle-attached fraction than in the free-living fraction during winter and spring, and the active community compositions of the two fractions diverged markedly. Luminiphilus tended to express its phototrophy genes more when living freely, whereas Rhizobiales, Limnohabitans, Rhodoferax and other Burkholderiales preferred to switch on their light-harvesting machinery while clinging to particles. In summer, free-living Arenicellales UBA868 was highly active, while particle-attached communities saw a dramatic surge in pufM expression from Rhodobacterales HIMB11, which at its peak accounted for more than 66 percent of the active AAP assemblage when both AAP abundance and phototrophy were at their annual maximum. Because the total fraction also contains free-living cells, the authors note that these fraction differences are likely conservative estimates, and the true contrast between attached and free lifestyles may be even sharper.

The study also captured a fingerprint of the bay’s transitional character. Kaštela Bay receives freshwater from the nearby Jadro River and from rain runoff, and in winter and spring the team detected phototrophic lineages classically associated with freshwaters, including Rhodoferax, Limnohabitans and certain Rhizobiales. Remarkably, these were not merely passive immigrants swept in by currents; they were transcriptionally active, particularly in the particle-attached fraction, indicating that allochthonous phototrophs can participate meaningfully in coastal photoheterotrophy. Together with the estuary-derived Arenicellales UBA868 and the coastal HIMB11 lineage, the picture is one of a dynamic mixing zone where freshwater, estuarine and fully marine phototrophic strategies overlap and trade dominance across the seasons.

Why should a ship’s worth of seawater genetics matter to anyone beyond microbial ecologists? AAP bacteria are believed to play a significant role in the microbial loop, the pathway by which dissolved organic carbon is recycled into the food web rather than exported to the deep ocean. Culture experiments have shown that light-exposed AAP cells respire less and accumulate more biomass, and field studies have demonstrated that infrared illumination, which selectively feeds AAP phototrophy, reduces community respiration and boosts uptake of labelled organic substrates. If particular lineages perform disproportionate amounts of photoheterotrophy, and if that performance depends on whether cells are attached to particles or drifting free, then carbon models built on averages will systematically misjudge how much solar energy flows through these bacteria and where in the water column that flow occurs. The pronounced partitioning of AAP diversity and activity between fractions documented here suggests that particles may function as hotspots of light-driven organic matter consumption, especially in winter and spring.

The authors are careful about the limits of their snapshot. All samples were collected in the morning, and previous work has shown that phototrophy gene expression can vary across the day-night cycle, so lineages appearing transcriptionally silent at dawn may simply operate on a different schedule, as has been observed in the phylum Gemmatimonadota. The chemical composition of the particles themselves, which likely shapes the attached lifestyle’s appeal, was not characterised. High-resolution diel transcriptomics, the authors suggest, would clarify how daily light cycles regulate marine AAP gene expression. Even so, the central conclusion stands firmly: the marine AAP community is a heterogeneous federation of lineages, each with its own niche preferences for where to live and when to harvest light, and only transcript-based approaches can reveal which members are truly earning their living from the sun at any given moment.

For a group of bacteria first recognised in the late 1970s as curiosities of aerobic heterotrophy, AAP organisms have come a long way toward centre stage in marine biogeochemistry. This Adriatic study adds a crucial nuance: their ecological role is written not in the inventory of genes floating in the water, but in the shifting transcriptomes of cells negotiating season, substrate and lifestyle. As sequencing technologies mature and RNA-based monitoring becomes routine, coastal carbon budgets may need recalibration to account for the hidden phototrophs that DNA surveys have been overcounting and undercounting all at once. In the sunlit waters of the world’s coastal seas, the business of harvesting light, it turns out, is conducted by a rotating cast of specialists whose identities change with the calendar, and whose work only becomes visible when scientists listen for the transcripts rather than tally the genes.

Subject of Research: Lineage-specific phototrophy and lifestyle strategies of aerobic anoxygenic phototrophic bacteria in coastal marine waters

Article Title: Lineage-specific phototrophy and lifestyle of coastal marine aerobic anoxygenic phototrophs

Article References: Villena-Alemany, C., Tomaš, A. V., Mujakić, I., Kopejtka, K., Šantić, D., & Koblížek, M. (2025). Lineage-specific phototrophy and lifestyle of coastal marine aerobic anoxygenic phototrophs. Ocean Microbiology, 1(1), Article 5. https://doi.org/10.1186/s44375-025-00005-x

Image Credits: AI Generated

DOI: 10.1186/s44375-025-00005-x

Keywords: aerobic anoxygenic phototrophs, photoheterotrophy, pufM gene expression, Adriatic Sea, marine microbiology, particle-attached bacteria, free-living bacteria, bacteriochlorophyll a, carbon cycling, RNA transcripts, Luminiphilus, coastal waters

The team, led by P. C. Shamily Catherine of Cochin University of Science and Technology together with Graham Oliver of the National Museum of Wales, had set out on a broader bivalve diversity survey along the Kerala coast. Geloina expansa, first described by Mousson in 1849, is a large, semi-infaunal cyrenoidid clam that lives buried in the soft sediments of mangrove forests across the Indo-Pacific. These animals are remarkable survivors, equipped with thick shells bearing a pale green to black periostracum and a physiology that tolerates low oxygen, elevated organic loads, and wildly fluctuating salinity. Individuals have been reported to survive out of water for nearly two weeks, resuming filtration immediately upon re-immersion. The specimens examined in this study averaged roughly 65 millimeters in shell height, 70 millimeters in length, and 66 grams in total weight.

The environment from which the clams were collected is itself instructive. Water in the tidal channel at the sampling site, located at 11.957631 degrees North and 75.306013 degrees East, registered a temperature of 30 plus or minus 2.5 degrees Celsius, a salinity of 35 plus or minus 0.5 parts per thousand, dissolved oxygen of 3.7 plus or minus 0.5 milligrams per liter, and a slightly alkaline pH of 7.32. The redox potential of minus 39.6 millivolts betrayed the strongly reducing conditions characteristic of mangrove sediments. Sediment analysis classified the substrate as loamy sand, dominated by 77.5 percent sand with 21 percent silt and 1.5 percent clay, and revealed an organically enriched matrix containing 14.26 grams of total organic carbon per kilogram. Such conditions, warm, saline, and nutrient-rich, are exactly the kind that favor both bivalve filtration activity and the proliferation of photosynthetic dinoflagellates.

The pivotal moment came during routine morphological and anatomical examinations. Darkly pigmented regions were noticed in the mantle tissue of the clams, prompting closer inspection. Microscopic sections of the mantle, siphon, and gill tissues revealed distinct golden-brown, globular microalgal cells embedded within the epithelial layer. These structures were abundant in the mantle and occurred at lower densities within the siphons, and, strikingly, they were present in every single one of the eighteen specimens examined. The observations were made on unstained fresh tissue sections using a Leica DM6 epifluorescence microscope with LAS X software, a choice that preserved the natural pigmentation and morphology of the cells.

Under 40-times magnification, the cells appeared spherical to slightly oval, typically measuring 6 to 10 micrometers in diameter. They displayed the golden-brown coloration characteristic of photosynthetic dinoflagellates, with distinct cell walls and internal granules suggestive of chloroplast structures. Their distribution was far from random. The cells were unevenly spread, often forming small clusters along the outer mantle folds, but were conspicuously absent from deeper tissue layers. This consistent localization within the epithelium of the mantle and gills across multiple individuals strongly supports a genuine intracellular endosymbiotic association rather than surface contamination or accidental ingestion of algae during filter feeding.

The researchers attempted to confirm the taxonomic identity of the cells using molecular methods, but repeated attempts were thwarted by extremely low DNA yields from the intracellular algae. As a result, the identification rests on morphological characteristics alone: the spherical to ovoid shape, golden-brown pigmentation, and intracellular occurrence, all of which are consistent with previous descriptions of Symbiodiniaceae. The authors are appropriately candid about this limitation, noting that molecular confirmation, ideally through metabarcoding approaches, will be essential for pinning down the exact genus and species of the symbiont and for understanding its evolutionary relationship to the symbionts of corals and giant clams.

Why does this discovery matter? In well-studied systems, Symbiodiniaceae are ecological powerhouses. In reef-building corals and giant clams of the family Tridacnidae, these algae translocate enormous quantities of photosynthetically fixed carbon to their hosts, often supplying more than 50 percent of the host’s metabolic energy needs. Photosynthesis-irradiance studies on the giant clam Tridacna maxima have shown that over a 24-hour cycle, the symbionts produce more oxygen than the host consumes. The clams, in turn, exercise exquisite control over their algal partners. Daily fluctuations in hemolymph pH, rising during the day as photosynthesis peaks and falling at night, regulate the diffusion of ammonia into the host’s tissues, with more ammonium becoming available to the algae when pH drops. Nitrogen, particularly ammonium, is the key nutrient promoting algal growth, while phosphorus remains strictly regulated by the host. Recent research has even suggested that nitrogen competition is a fundamental mechanism underlying stable host-Symbiodiniaceae partnerships across cnidarians.

Symbiont-bearing bivalves also deploy sophisticated cellular machinery to support their algae. Carbon-concentrating mechanisms involving H+-ATPase and carbonic anhydrase facilitate inorganic carbon uptake for symbiont photosynthesis, and light-enhanced ion transport processes have been linked to shell formation in giant clams. Known examples of Symbiodiniaceae associations extend beyond corals and tridacnids to radiolarians, jellyfish such as the mangrove-associated Cassiopea xamachana, and even flatworms that partner with a different microalga, Tetraselmis convolutae. In giant clams, the algae reside within specialized tubular structures called Z-tubes, and individuals can host multiple genetically distinct symbiont types simultaneously, suggesting flexibility to adjust symbiont communities in response to changing environmental conditions. Yet comparable relationships in bivalves inhabiting mangrove habitats have rarely been investigated, which is precisely what makes the Kerala discovery so significant.

The authors caution that their findings do not provide direct evidence that the Symbiodiniaceae association helps Geloina expansa tolerate desiccation or anoxia, despite the clam’s legendary ability to survive prolonged aerial exposure. Rather, they suggest the association is more likely related to nutritional or physiological enhancement under the warm, saline, nutrient-rich estuarine conditions where these clams live. If the partnership mirrors what is seen in giant clams, the photosynthetic contribution could meaningfully supplement the clam’s filter-feeding diet, contributing to what symbiosis researchers call the CZAR value, the contribution of zooxanthellae to animal respiration. Given the central role mangrove ecosystems play in nutrient cycling, organic matter turnover, and carbon sequestration, an abundant photosynthetic symbiont inside a common mangrove clam could have ripple effects for ecosystem functioning that extend well beyond the individual animal.

The Kerala mangroves, dominated by Avicennia officinalis and Rhizophora mucronata with scattered patches of Ceriops tagal and Sonneratia alba, stabilize shorelines, filter pollutants, nursery juvenile fish and invertebrates, and act as major carbon sinks. Positive species-specific associations between mangrove trees and bivalves, such as Aegiceras floridum with Geloina and Avicennia alba with Saccostrea, hint at tightly woven ecological networks. The discovery of a photosynthetic symbiont inside one of these clams adds an entirely new thread to that web. The researchers emphasize that quantitative estimation of symbiont density, molecular confirmation of symbiont identity, and physiological experiments measuring carbon translocation are the logical next steps. If future work confirms that Geloina expansa derives substantial nutrition from its algal tenants, mangrove conservation efforts may need to account not only for the trees and the sediments but also for the microscopic partnerships humming quietly beneath the mud, a reminder that even in the murkiest corners of tropical coastlines, symbiosis continues to surprise science.

From a methodological standpoint, the Kerala study also illustrates both the power and the constraints of morphology-based symbiosis research in understudied tropical environments. Unstained fresh preparations allowed the investigators to document the natural golden-brown autofluorescence and pigmentation of the algal cells, but low DNA yields from cells buried inside host tissue are a well-recognized obstacle in this field. Intracellular symbionts are often present in small numbers per sample, and host tissue can inhibit polymerase chain reactions, which is why future studies of mangrove bivalves may need to rely on fluorescence in situ hybridization, symbiont culturing, or careful cell isolation before genetic sequencing becomes feasible.

The finding also raises evolutionary questions about how such partnerships arise in estuarine settings. Mangrove tidal creeks experience strong diel and seasonal swings in light, temperature, and salinity, conditions that would seem hostile to photosynthetic dinoflagellates that are famous for their sensitivity to environmental stress in coral reefs. Yet the reducing, organically loaded sediments where Geloina lives did not preclude a stable association across every specimen sampled during the 2023 survey window. Whether the symbionts persist in the clams year-round, are reacquired repeatedly from the water column, or are transmitted vertically during reproduction remains unknown, and each scenario would carry different implications for how resilient the partnership might be to coastal development, pollution, and climate-driven changes in monsoon patterns that shape Kerala’s mangrove waterways.

Subject of Research: Symbiotic association between Symbiodiniaceae dinoflagellates and the mangrove mud clam Geloina expansa in Kerala, India

Article Title: Discovery of Symbiodiniaceae symbiosis in the mangrove mud clam Geloina expansa (Mousson, 1849) from Kerala, India

Article References: Catherine, P. C. S., Oliver, G., Nazar, S. M., P, K. P., & Nandan, S. B. (2026). Discovery of Symbiodiniaceae symbiosis in the mangrove mud clam Geloina expansa (Mousson, 1849) from Kerala, India. Discover Oceans, 3(1), Article 55. https://doi.org/10.1007/s44289-026-00168-8

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00168-8

Keywords: Symbiodiniaceae, zooxanthellae, mangrove ecosystems, Geloina expansa, bivalve symbiosis, dinoflagellates, Kerala, giant clams, endosymbiosis, marine biology, coastal ecology, microscopy

To understand why this matters, it helps to begin with the chemistry. The ocean is Earth’s largest reservoir of iodine, and most of that iodine exists as iodate, a negatively charged ion written chemically as IO₃⁻. Certain bacteria can breathe iodate the way we breathe oxygen, converting it to iodide, I⁻, and harvesting energy in the process. Iodide is far more mobile than iodate, and where it reaches the sea surface it reacts with atmospheric ozone to generate volatile iodine compounds that escape into the air. Once aloft, these species participate in ozone destruction, influence the cycling of mercury, seed the formation of aerosol particles that reflect sunlight, and eventually rain back down onto land, supplying terrestrial ecosystems with iodine. In short, where DIRMs live determines, in a very real sense, how the ocean talks to the atmosphere about iodine.

The classical prediction rested on thermodynamics. On paper, iodate reduction yields more energy than nitrate reduction, so microbial ecologists reasoned that DIRMs should gorge on iodate first, then turn to nitrate. That logic placed them in a narrow ecological window just above the ocean’s oxygen minimum zones—regions where dissolved oxygen has vanished but where denitrifying microbes have not yet exhausted the nitrate. Below that window, nitrate would be scarce; above it, oxygen would suppress anaerobic respiration altogether. Textbook-style reasoning thus confined DIRMs to a thin slice of the ocean.

But field observations from an unexpected quarter—high-iodine groundwater in China—told a stubbornly different story. Professor Junxia Li of China University of Geosciences, the first author of the new paper, had previously isolated a DIRM strain called Azonexus hydrophilus NCP973 from iodine-rich aquifers. Curiously, this organism kept turning up in waters where nitrate had already been depleted, and across high-iodine groundwaters nationwide, iodide concentrations showed a consistent negative correlation with nitrate. “Thermodynamic prediction and field observation were clearly at odds,” Li explains. “We were thus curious to re-examine the ecological niche of these microorganisms.”

The team resolved the conflict in the laboratory. Working with two representative DIRM strains—NCP973 from groundwater and Denitromonas iodatirespirans IR-12, originally isolated from seawater—they supplied cultures with both iodate and nitrate simultaneously. In every experiment, the bacteria reduced nitrate first, and only began consuming iodate once the nitrate had been exhausted. The molecular mechanism emerged from transcriptomic analysis: genes encoding nitrate reductase (narGHI) switched on first, while the iodate reductase genes (idrABP1P2) remained silent until nitrate was gone. Nitrate, it turns out, actively suppresses expression of the iodate reductase machinery, a regulatory strategy analogous to its suppression of perchlorate reductase in other bacteria. Iodate adds a further twist: on its own it imposes oxidative stress that forces the cells into a prolonged lag phase, giving nitrate reduction an unassailable head start.

The implication is striking. If iodate reduction always follows nitrate reduction, then DIRMs should not live above the oxygen minimum zones but inside them, in waters where denitrification is actively consuming nitrate. To test this, the researchers turned to environmental genomics, mining metagenomic and metatranscriptomic datasets from the world’s three major oxygen minimum zones—the Eastern Tropical North Pacific, the Eastern Tropical South Pacific, and the Arabian Sea—along with metagenome-assembled genomes (MAGs) from global OMZs and the Tara Oceans expeditions. The pattern was unambiguous. The idrA gene, the genetic signature of iodate reduction, appeared and was expressed almost exclusively within OMZ depth profiles. Among 962 MAGs recovered from global oxygen minimum zones, 32 carried the full idrABP1P2 gene cluster; among 2,631 Tara Oceans MAGs, only 9 carried it, and every single one came from OMZ samples.

The genomic survey did more than confirm the habitat—it dramatically expanded the known cast of iodate-breathing microbes. The OMZ-dwelling DIRMs belonged predominantly to the candidate phylum SAR324 and to the class Alphaproteobacteria, lineages previously not recognized as iodate reducers. Intriguingly, the SAR324 genomes carrying idrABP1P2 also harbor sulfur oxidation genes, hinting that these organisms may couple the oxidation of sulfide to the reduction of iodate—a chemoautotrophic lifestyle that would let them flourish in the chemically stratified heart of oxygen-depleted waters. To verify that the newly discovered genes actually do what they appear to do, the team expressed two representative idrABP1P2 sequences heterologously in laboratory hosts and confirmed iodate-reducing activity.

Rewriting the map of DIRM habitats is more than an exercise in microbial cartography; it changes projections of the marine iodine cycle in a warming world. Global warming lowers the solubility of oxygen in seawater and intensifies ocean stratification, both of which encourage the expansion of oxygen minimum zones. Over the past six decades, the global area covered by OMZs has grown from roughly 5 percent of the ocean to 14 percent, and the trend is expected to continue. Every square kilometer of new oxygen-depleted water is, under the revised model, potential habitat for DIRMs—more microbes converting iodate to iodide, and more iodide available for transport to the surface ocean by circulation. Model simulations cited in the study indicate that a 1 percent increase in global sea-surface iodide concentration produces approximately a 0.7 percent rise in oceanic iodine emissions to the atmosphere.

Those emissions matter on several fronts. Volatile iodine compounds destroy tropospheric ozone, alter the oxidation chemistry of the marine boundary layer, contribute to the formation of new aerosol particles that influence cloud cover and climate, and govern the atmospheric fate of mercury, a potent neurotoxin. They also deliver iodine back to land, where adequate dietary intake prevents goiter and other thyroid disorders that still afflict populations in iodine-poor regions. A microbe’s preferred address in the water column thus ripples outward to human nutrition and planetary climate chemistry alike.

The study also offers a methodological lesson that extends well beyond iodine. Thermodynamic calculations, however elegant, describe what is energetically possible, not what regulatory networks actually permit. The discovery that nitrate represses idrABP1P2 expression—inverting the presumed order of substrate use—illustrates how gene regulation can override energy-yield logic in shaping microbial niches. For biogeochemists accustomed to predicting microbial distributions from redox potentials alone, the message is clear: transcriptomics and genome-resolved surveys of the actual environment must carry at least equal weight. Ecological niches, this work reminds us, are written in regulatory circuits as much as in reaction energetics.

Li and colleagues argue that their findings should now be integrated into marine iodine biogeochemical models. Doing so would sharpen predictions of how oceanic iodine emissions will respond as oxygen minimum zones continue to expand through the coming century. Given that iodine touches everything from stratospheric ozone recovery to cloud droplet formation to human endocrine health, the stakes of getting this cycle right are considerable. The humble iodate-breathing bacteria of the deep, once relegated to a theoretical sliver of the ocean, now appear to command a far larger territory—and their expanding domain may help determine the atmospheric chemistry of a warming planet. “Given the importance of iodine for human and environmental health,” Li notes, “integrating this pathway into marine iodine biogeochemical models will improve our capability of understanding and predicting the future changes in oceanic iodine emissions.”

In August 2023, the German research vessel METEOR set sail on Expedition M192 to the Greek island of Milos with Dr. Solveig Bühring as the chief scientist. The mission was to locate and investigate so far unknown hydrothermal systems. Now, three years later, a new study highlights the surprising discoveries resulting from this expedition. At the newly discovered hydrothermal system in intermediate water depths of 100 to 250 meters, researchers were able to demonstrate how different intensities of hydrothermal fluid flow shape microbial communities and control mineral formation on the ocean floor.

Around Milos, two fundamentally different types of hydrothermal venting occur relatively close to each other: slowly diffusing fluids and vigorously venting (‘advective’) hot fluids. “These two hydrothermal regimes create completely different habitats for microorganisms,” says Dr. Joely Maak, the study’s lead author and researcher at MARUM. In areas dominated by diffuse fluid flow, seawater penetrates multiple centimeters into the sediments. As seawater infiltrates the sediment, it supplies dissolved sulfate, which is utilized by sulfate-reducing microorganisms. Their metabolism promotes the formation of pyrite within the sediment. In contrast, where hot, acidic fluids are discharged through vigorous venting, sulfate-rich seawater is absent. Instead, sulfur-oxidizing bacteria colonize the interface between reduced hydrothermal fluids and oxygenated seawater. At the interface, elemental sulfur precipitates.

Mineral precipitation is not only limited to geological processes

For a long time, mineral formation in active hydrothermal systems was considered a consequence of primarily abiotic geological processes. “The new study now demonstrates that microorganisms actively contribute to processes shaping the ocean floor. This study is the first to investigate the newly discovered hydrothermal systems in detail following their initial description at the end of 2025 in Scientific Reports and provides the foundation for future investigations of these unique environments,” explains Dr. Marcus Elvert, the study’s project leader.

Biological and geological processes at the ocean floor are closely linked

To identify the different microbial metabolisms, the research team combined a wide range of analytical approaches, including compound-specific isotope analyses of fatty acids to identify various metabolic pathways, mineralogical analyses, sulfur isotope measurements, and porewater geochemistry. Only by integrating these complementary methods was it possible to reveal how closely biological and geological processes are interconnected.

This work was made possible through the close collaboration of a highly interdisciplinary team. The team included Clemens Röttgen, Birte Winkelhues, Eirini Anagnostou, Solveig I. Bühring, Andrea Koschinsky, Jianlin Liao, Harald Strauss, Christoph Vogt, Wolfgang Bach, Enno Schefuß, and Marcus Elvert. Bringing together these diverse areas of expertise, including geomicrobiology, mineralogy, and geochemistry, made it possible to comprehensively unravel the interactions between hydrothermal fluid flow, microorganisms, and mineral formation.

The study is an integral part of research in the Cluster of Excellence “The Ocean Floor – Earth’s Uncharted Interface.” The cluster aims to better understand ocean floor ecosystems under changing environmental conditions, as well as central material cycles, such as the carbon cycle.

The findings are based on samples and data collected during Expedition M192 aboard the German research vessel METEOR III. Although METEOR III has now completed its final voyage after nearly four decades of scientific service, the samples and data collected during its expeditions continue to provide new insights into previously hidden processes occurring on the ocean floor.

Contact:

Dr. Joely Maak
MARUM – Center for Marine Environmental Sciences, University of Bremen
Phone: 0421 218 65585
Email: jmaak@marum.de

Further Information

• Documentation (in German): ‘Meteor im Mittelmeer – Leben auf dem Forschungsschiff
• To the special issue of JGR Biogeosciences, “Microbial Geochemistry, Chemical Geobiology, and Geobiochemistry: Integrated Approaches to Understanding the World Around Us—a tribute to Jan Amend”:

• First description of the newly found hydrothermal systems:
• The Cluster of Excellence “The Ocean Floor – Earth’s Uncharted Interface.”:

Participating Organizations:

• MARUM – Center for Marine Environmental Sciences, University of Bremen (Germany)
• Faculty of Geosciences, University of Bremen (Germany)
• School of Science, Constructor Universität, Bremen (Germany)
• Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens (Greece)
• European Synchrotron Radiation Facility (France)

MARUM produces fundamental scientific knowledge about the role of the ocean and the seafloor in the total Earth system. The dynamics of the oceans and the seabed significantly impact the entire Earth system through the interaction of geological, physical, biological, and chemical processes. These influence both the climate and the global carbon cycle, resulting in the creation of unique biological systems. MARUM is committed to fundamental and unbiased research in the interests of society, the marine environment, and in accordance with the sustainability goals of the United Nations. It publishes its quality-assured scientific data to make it publicly available. MARUM informs the public about new discoveries in the marine environment and provides practical knowledge through its dialogue with society. MARUM cooperation with companies and industrial partners is carried out in accordance with its goal of protecting the marine environment.

Journal

Journal of Geophysical Research Biogeosciences

DOI

10.1029/2026JG009869