How Life Survives at the Limits: New Insights from Marine Extremes

From shallow waters bathed in intense ultraviolet radiation to the crushing darkness of hadal trenches, the oceans contain environments that push life to its physiological limits. A new editorial published in Frontiers in Marine Science on 20 October 2026 brings together a collection of research articles examining how marine organisms persist, and even flourish, under some of the most challenging conditions on the planet. The editorial, authored by Daniela Giordano, Laura Núñez-Pons, Jérémy Carlot and Cristina Purcarea, introduces a Research Topic titled Adaptation and evolution in marine extreme environments, and argues that these hostile habitats serve as natural laboratories for probing the boundaries of life itself.

The environments covered in the collection span a remarkable range. Polar ecosystems impose low temperatures and strong seasonal constraints. The deep sea combines high pressure with perpetual darkness. Yet, as the editorial emphasizes, none of these settings is devoid of life. On the contrary, they support diverse and often highly specialized communities whose members have evolved an impressive array of physiological, biochemical, morphological and ecological strategies for survival.

What makes the new collection distinctive is its insistence that adaptation operates at multiple levels of biological organization simultaneously. Protective metabolites can be produced and redistributed within a single organism. Cellular processes can be modified. Entire communities of hosts, microbial symbionts and viruses can reorganize in response to stress. Evolutionary responses can also emerge through changes in dispersal, population connectivity, developmental trajectories and morphological traits, sometimes revealing cryptic genetic diversity that was previously hidden. Understanding these layered mechanisms, the editors argue, is essential not only for explaining how organisms survive near their physiological limits but also for understanding how biodiversity is generated and maintained across environmental gradients.

A key theme running through the editorial is the power of modern molecular and genomic tools to illuminate these processes. The combination of field observations with high-throughput sequencing, metabolomics, phylogenetics and controlled environmental experiments is making it increasingly possible to connect specific environmental conditions with the mechanisms that underlie adaptation and evolutionary change. These approaches also reveal that adaptation is not necessarily a property of individual organisms alone. Complex associations between hosts and their microbial partners can contribute to nutrient acquisition, detoxification, protection from environmental stress and the maintenance of physiological stability. Environmental extremes, however, can destabilize these partnerships, potentially reshaping the composition and functioning of entire holobionts, the integrated units formed by a host and its associated microbes.

One of the highlighted studies, by Hammerle and colleagues, examined how Baltic Sea flatfish cope with ultraviolet radiation, a persistent stressor in shallow waters. Using a combination of targeted and untargeted metabolomics, the researchers investigated the distribution of mycosporine-like amino acids, natural UV-protective compounds, in three species of flatfish. They identified several of these compounds and found that they accumulated preferentially in the eyes, where concentrations were substantially higher than in other tissues. The detection of the compounds in internal organs also provided evidence for their transfer within the organism and supported the hypothesis that they can be acquired from dietary sources and selectively transported to tissues exposed to ultraviolet radiation. The study further revealed a diversity of related metabolites, including some that may not have been previously characterized, illustrating how metabolomic approaches can uncover biochemical mechanisms associated with environmental tolerance.

Whereas Hammerle and colleagues focused on biochemical protection within individual organisms, a study by Pascual-Benito and colleagues shifted the focus to the level of the coral holobiont. The team investigated how thermal stress affects the bacterial microbiomes of soft corals originating from polar, temperate and tropical environments. By combining field sampling, controlled heat-stress experiments and 16S rRNA sequencing, they showed that each coral species harbored a distinct bacterial community that differed substantially from the surrounding seawater. Experimental warming nevertheless disrupted these host-associated communities, reducing characteristic symbionts and increasing opportunistic or seawater-associated bacteria. Notably, the magnitude of these changes differed among host species: Antarctic and tropical corals showed substantial microbiome restructuring under moderate warming, whereas the Mediterranean species maintained greater stability until exposed to more extreme temperatures. These results suggest that the environmental history of host populations can influence the stability of their microbial associations, and that thermal stress can rapidly modify interactions within marine holobionts.

Microbial partnerships can also provide more direct routes to exploiting chemically demanding habitats. Li and colleagues investigated the microbiome and virome of an undescribed glass sponge collected from a hydrothermal vent field in the Okinawa Trough at a depth of 1,284 meters. Metagenomic analyses revealed a diverse microbial community that included sulfur-oxidizing bacteria, ammonia-oxidizing archaea, methane-oxidizing bacteria and other microorganisms involved in carbon and nitrogen cycling. The reconstructed microbial genomes indicated complementary metabolic capacities, including sulfur and methane oxidation, carbon fixation, nitrogen transformations and the degradation of potentially harmful compounds. This functional diversity suggests that the sponge holobiont can integrate multiple microbial processes in an environment where reduced chemical compounds serve as important energy sources. The study also identified a methane-oxidizing symbiont whose genomic characteristics are consistent with an early stage of symbiotic genome reduction, offering a glimpse into the evolutionary transition from free-living microorganisms to specialized host-associated lifestyles. A diverse viral community, meanwhile, may help regulate microbial populations and recycle nutrients within the holobiont.

Adaptation to extreme environments is not confined to physiology and biochemistry; it also unfolds through population-level processes. Prófumo and colleagues examined population connectivity in the Antarctic isopod Spinoserolis beddardi, a species that lacks a pelagic larval stage and might therefore be expected to show limited dispersal. Using nearly 14,000 genome-wide SNPs from individuals collected at three coastal sites on Livingston Island, the researchers found remarkably low genetic differentiation among populations separated by just 2 to 13 kilometers. The results point to substantial genetic connectivity despite the species’ life-history characteristics, suggesting that occasional dispersal through oceanographic transport, together with shared evolutionary history, may help maintain gene flow among populations. The finding underscores that dispersal and connectivity can persist even in species with limited opportunities for active long-distance movement, and that population genomic approaches can reveal evolutionary processes not readily apparent from life-history traits alone.

The deep sea offers yet another perspective on how evolution generates diversity under conditions that remain poorly explored. González-Casarrubios and colleagues described two new species of deep-sea kinorhynchs, Echinoderes prior and Echinoderes shimadai, collected from abyssal and hadal environments in the Pacific Ocean. Both species possess a middorsal spine on trunk segment 9, a feature that extends beyond the segment range traditionally considered characteristic of this morphological group. By integrating detailed morphological observations with mitochondrial COI and nuclear 18S and 28S markers, the authors confirmed the distinctiveness of the two species and identified additional genetic diversity within E. prior. Their phylogenetic analyses suggest that the segment 9 spine was secondarily reacquired within the lineage, while observations of postembryonic development showed that posterior spines can be added or lost during successive molts. These findings indicate that morphological characters used in the taxonomy of deep-sea kinorhynchs may be more evolutionarily dynamic than previously assumed, and the discovery of new species and unrecognized variation in abyssal and hadal environments highlights how much evolutionary diversity remains to be documented in the deep ocean.

Taken together, the contributions gathered in the Research Topic demonstrate that adaptation and evolution in extreme marine environments operate across multiple scales, from the redistribution of protective molecules within tissues to the restructuring of microbial communities, metabolic cooperation among hosts, bacteria, archaea and viruses, population connectivity across fragmented habitats, and morphological change over evolutionary time. The editors stress that integrating complementary approaches, from metabolomics and controlled experiments to metagenomics, population genomics, phylogenetics and detailed morphology, is particularly important in extreme environments, where ecological processes and evolutionary histories can be difficult to disentangle from observations at a single scale. As technological and methodological advances continue to expand access to these habitats, they are expected to open new opportunities to investigate how organisms and their associated communities respond to environmental challenges, and how those responses contribute to the generation and maintenance of marine biodiversity in a changing ocean.

Subject of Research: Adaptation and evolution of marine organisms in extreme environments including polar waters, hydrothermal vents, and the deep sea.

Article Title: Editorial: Adaptation and evolution in marine extreme environments

Article References: Giordano, D., Núñez-Pons, L., Carlot, J., & Purcarea, C. (2026). Editorial: Adaptation and evolution in marine extreme environments. Frontiers in Marine Science, 13, Article 2021381. https://doi.org/10.3389/fmars.2026.2021381

Image Credits: AI Generated

DOI: 10.3389/fmars.2026.2021381

Keywords: marine biology, extreme environments, adaptation, evolution, hydrothermal vents, microbiome, metabolomics, population genomics, deep sea, Antarctica, holobiont, UV protection