Tiny Ocean Algae Face a Carbon Crisis as Seawater Acidifies

Beneath the waves, some of the planet’s most consequential organisms are so small that a single drop of seawater can hold thousands of them. Diatoms, the single-celled microalgae that drift through oceans, lakes, rivers and even damp soil, are far more than microscopic specks. They generate a substantial share of the oxygen in Earth’s atmosphere, and in the oceans alone they account for an estimated 40 to 50 percent of primary production, the foundational process by which sunlight, nutrients and dissolved carbon dioxide are converted into living organic matter. When something disturbs the health of diatoms, therefore, the consequences ripple far beyond their glass-like cell walls, reaching into marine food webs, fisheries productivity and one of the planet’s most important natural mechanisms for drawing carbon out of the atmosphere and locking it away in the deep sea.

New research from Flinders University in South Australia adds a sobering detail to the growing body of evidence on how a changing ocean chemistry could weaken this biological foundation. A study published in the journal Marine Ecology set out to test, with unusually high sensitivity, how common species of marine diatoms respond to one of the defining stressors of the modern ocean: acidification caused by the absorption of excess carbon dioxide from the atmosphere. The findings suggest that shifts in ocean pH can alter the growth, abundance and elemental composition of these algae, with potential downstream effects that include disruption of marine food webs, reduced export of carbon and silicon to the deep ocean, and heightened microbial and nutrient activity in surface waters.

The senior author of the study, Professor Sophie Leterme, director of the ARC Industry Transformation Training Centre for Biofilm Research and Innovation at Flinders University’s College of Science and Engineering, emphasizes that the stakes extend well beyond the algae themselves. According to Leterme, changes in seawater pH can affect how diatoms grow and what elements they accumulate, and understanding how these shifts interact across various trace elements is essential for anticipating broader ecological impacts. Her team’s work points to a chain of consequences: altered trace metal uptake in diatoms could cascade through the organisms that graze on them, through the fisheries that depend on those grazers, and through the biological carbon pump that quietly transports carbon from the sunlit surface ocean into the abyss.

The chemistry at the heart of the problem is straightforward but relentless. As humanity emits carbon dioxide, a large fraction of it dissolves into seawater, forming carbonic acid and lowering the ocean’s pH. Since the end of the Industrial Revolution, this process has already driven a global decline in surface ocean pH of approximately 0.1 units. That figure may sound modest, but because the pH scale is logarithmic, it represents an increase in acidity of roughly 30 percent. Projections suggest the decline will not stop there: by the end of this century, ocean pH is expected to fall by a further 0.3 to 0.6 units, a rate of chemical change that marine organisms have never encountered in such a short span of evolutionary time.

For diatoms, the changing acidity matters largely because of trace metals. Elements such as iron, zinc and cadmium are absorbed from seawater and play essential roles in the algae’s metabolism, including the acquisition of inorganic carbon for photosynthesis. When pH shifts, the chemical speciation of these metals in seawater changes too, altering how readily they bind to cell surfaces and how effectively diatoms can take them up. Because trace metal availability constrains the growth of phytoplankton across vast stretches of the ocean, any systematic change in metal uptake driven by acidification could reshape which algae thrive and which falter, and by extension how much carbon dioxide the ocean’s microscopic forests continue to draw down.

To test these effects directly, the Flinders researchers turned to a powerful analytical technique. Using seawater samples collected from Gulf St Vincent in South Australia and from the CSIRO algae collection, the team ran experiments on two well-studied diatom species, Thalassiosira pseudonana and Nitzschia navis-varingica. Their method of choice, neutron activation analysis, offered by far higher sensitivity than conventional approaches, allowing the researchers to quantify how much of a range of trace elements the algae absorbed under different chemical conditions. The work was supported by expertise from ANSTO, Australia’s nuclear science organization, whose facilities underpin this kind of high-precision elemental measurement.

The experimental results demonstrated that trace metal uptake by marine diatoms responds measurably to the conditions the researchers created, and the team suggests that this approach could be extended to probe how other marine organisms absorb a wide range of elements in their environments. That versatility matters, because the same ocean chemistry that changes diatom physiology also affects bacteria, zooplankton, larvae and every other layer of the marine ecosystem. A method that can precisely track elemental transfer at the base of the food web gives scientists a sharper tool for tracing how chemical stress propagates upward through the ecosystem and downward into the carbon cycle.

Diatoms also serve a second, more practical role in this research: they are excellent bio-indicators. Because their shells and their physiological responses are sensitive to water chemistry, diatoms have long been used to assess water quality in rivers, lakes and coastal seas. The new work extends that utility into the era of ocean acidification and rising water temperatures, offering a way to monitor how ongoing environmental change degrades the physiology and functioning of organisms that sit at the base of nearly every marine food chain. In effect, the same organisms that anchor ocean food webs and carbon storage also double as living gauges of ocean health, and the readings they now display are increasingly urgent.

One of the study’s most thought-provoking implications concerns a subtle tension at the heart of the ocean carbon story. In principle, higher diatom abundance and faster growth could help reduce atmospheric carbon dioxide levels by fixing more carbon near the surface. Yet the impact of lower concentrations of major and trace elements in a more acidic ocean is not well understood, and it could constrain the very growth that would make such a carbon drawdown possible. In other words, a warmer, more acidic ocean might simultaneously encourage and undermine the biological machinery of carbon export, and the balance between those forces will determine how much carbon the deep ocean ultimately stores. Resolving that uncertainty, the researchers argue, requires a better understanding of the complex processes at work in seawater.

The Flinders team also sees practical dividends beyond climate science. Insights into how marine organisms interact with surfaces and trace elements in seawater are already informing the development of novel biofilms designed to reduce shipping pollution in harbours, linking fundamental ocean chemistry research to tangible environmental applications. Together, the studies underscore a broader message: understanding the invisible chemistry of the ocean is not an academic luxury but a prerequisite for protecting both marine life and one of humanity’s most valuable natural carbon stores. As carbon emissions continue to acidify the seas, the fate of organisms barely visible to the naked eye may help decide how much of that carbon stays buried in the deep and how much returns to the air above.

The choice of study species reflects the breadth of the question. Thalassiosira pseudonana is a coastal centric diatom whose genome has been fully sequenced, making it a standard model for probing how these algae manage silica, carbon and nutrient uptake at the molecular level. Nitzschia navis-varingica, by contrast, is notable for its tolerance of variable salinity and its capacity to accumulate unusual elements, giving the researchers a useful contrast in how different diatom lineages handle metal sorption. Comparing responses across species with different ecological strategies helps distinguish effects that are general to diatoms as a group from those tied to particular life histories.

The reliance on neutron activation analysis is also significant for the field. Traditional measurements of trace metal uptake in phytoplankton often struggle with contamination, because the concentrations involved are vanishingly small and seawater itself carries background levels of many elements. Neutron activation, which detects elements by bombarding samples with neutrons and reading the characteristic radiation emitted, sidesteps many of these limitations and can quantify dozens of elements simultaneously from a single sample. That breadth matters because trace metals rarely act in isolation; iron, zinc, cadmium and other elements compete for binding sites on cell surfaces, and acidification can shift those competitive balances in ways that single-element studies would miss.

The study also connects to a longer scientific lineage. Diatoms build their intricate shells from dissolved silicon, and the coupling of silicon and carbon export is a cornerstone of how the biological pump has operated over geological timescales. If acidification weakens that coupling, the composition and sinking speed of organic material reaching the seafloor could change, altering not only carbon storage but also the food supply for deep-sea communities adapted to a steady rain of particles from above. The researchers suggest that follow-up work examining how pH-driven changes in elemental composition propagate through grazers and decomposers will be needed to close that loop, and that the experimental framework developed here can be adapted to test other organisms and other elements under future ocean conditions.

Subject of Research: How ocean acidification affects trace metal uptake by marine diatoms and its implications for ocean health and carbon sequestration.

Article Title: Sinking feeling: Testing for ocean health and carbon storage

Article References: Sinking feeling: Testing for ocean health and carbon storage. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: ocean acidification, diatoms, trace metals, carbon sequestration, marine food webs, phytoplankton, seawater pH, neutron activation analysis, Flinders University, carbon cycling, microalgae, ocean health