Rising CO2 Is Rewiring the Phosphorus Metabolism of Ocean Plankton

The ocean’s smallest workers are quietly rewriting the chemistry of the seas. Phytoplankton, the drifting microscopic plants responsible for roughly half of the photosynthesis on Earth, are being pushed into a new metabolic regime as carbon dioxide dissolves into seawater and makes the ocean more acidic. A study published in Nature Geoscience now shows that this acidification, when it collides with the chronic scarcity of phosphate that already constrains much of the marine biosphere, fundamentally reshapes how these organisms handle phosphorus — the nutrient that limits life across vast stretches of the global ocean.

The research, led by Bingkun Wang and Yapeng Wang of the Yellow Sea Fisheries Research Institute and Harbin Institute of Technology, together with colleagues in China and the United States, took an unusually long view of the problem. Rather than measuring how phytoplankton react to elevated carbon dioxide over days or weeks, the team followed experimental populations of the model diatom Thalassiosira pseudonana through more than 1,000 generations of evolution under high-CO2 conditions. This timescale matters, because it allowed the researchers to separate two very different biological phenomena: genuine evolutionary adaptation, in which genetic changes become heritable across lineages, and acclimation, the flexible physiological adjustments individual cells make to their immediate environment.

The results reveal a striking asymmetry. Long-term selection under elevated CO2 produced heritable increases in growth and carbon fixation that persisted regardless of how much phosphate was available to the cells. In other words, the diatoms’ carbon-side response to acidification appears to be locked in by evolution. But the phosphorus side of the story was entirely different. When the researchers measured particulate organic phosphorus — the phosphorus bound into living biomass — they found that ocean acidification drove it down under phosphate-limited conditions. Crucially, this decline was reversed when lineages were transferred back and forth between selection treatments, showing that the phosphorus response was not a fixed evolutionary outcome but a plastic, environment-dependent shift in how cells allocate their scarce phosphorus reserves.

To understand what was happening inside the cells, the team deployed a multi-omics arsenal, combining transcriptomics, proteomics and metabolite profiling across the long-term experiment. The picture that emerged was one of biochemical triage. Under the combined stress of acidification and phosphate scarcity, the diatoms depleted their cellular phosphorus reserves, including polyphosphate stores that normally serve as intracellular batteries of the nutrient. At the same time, they enriched their membranes with non-phosphorus lipids, swapping out the phospholipids that ordinarily form cellular boundaries for phosphorus-free substitutes. This membrane lipid remodeling is a well-documented survival strategy in phosphorus-starved phytoplankton, first described in landmark work on natural plankton communities, but the new study shows that acidification amplifies and reshapes the process, effectively squeezing phosphorus out of the structural fabric of the cells.

The laboratory findings did not stay confined to culture flasks. In coastal mesocosm experiments — large enclosures that contain natural plankton communities under controlled conditions — the researchers observed the same signature: reduced particulate organic phosphorus in natural phytoplankton assemblages at low-phosphate stations exposed to ocean acidification. This convergence between a decade-scale laboratory evolution experiment and real-world community responses is one of the study’s most persuasive elements, because it suggests the mechanism operates not just in a single cultivated diatom but across the messy diversity of wild plankton.

The team then scaled up to the planetary level. Mining global meta-transcriptomic datasets collected across the world ocean, they found widespread upregulation of genes involved in non-phosphorus membrane lipid synthesis in regions that combine high partial pressure of CO2 with low phosphate. These are precisely the conditions expected to expand as the ocean absorbs more anthropogenic carbon and as nutrient ratios shift. Independent observations have already documented declining phosphate relative to nitrate in upper ocean waters, and atmospheric nitrogen deposition continues to push many regions toward phosphorus limitation, making the overlap between acidification and phosphate stress a growing feature of the modern ocean rather than a rare corner case.

When the researchers fed these mechanisms into Earth system models, the projections were sobering. Under the intermediate Shared Socioeconomic Pathway SSP2-4.5, the models predict a 211 percent decline in particulate organic phosphorus by the year 2100, and under the high-emissions SSP5-8.5 scenario the projected decline reaches a staggering 2,185 percent. These are not marginal adjustments to the ocean’s elemental bookkeeping. Particulate organic phosphorus is the currency through which phosphorus enters marine food webs and is exported to the deep sea, so its collapse in phosphate-limited regions would ripple through ecosystem productivity, food web structure and the biological carbon pump that carries carbon away from the atmosphere.

The stoichiometric implications are profound. Since the era of Alfred Redfield, oceanographers have recognized that marine plankton maintain remarkably consistent ratios of carbon, nitrogen and phosphorus, and these ratios underpin much of how we model ocean biogeochemistry. Recent work has documented global-scale shifts in plankton elemental ratios over the past half-century, and the new study adds a mechanistic explanation for how acidification could accelerate that drift. If phytoplankton under future conditions carry systematically less phosphorus per unit of carbon, then the same amount of primary production would transfer less phosphorus to grazers, microbes and the deep ocean, intensifying nutrient imbalance throughout marine ecosystems. Models that assume static elemental ratios could therefore misjudge future carbon sequestration and nutrient cycling in ways that matter for climate projections.

There is also a subtler lesson in the study’s experimental design. By running reciprocal transfers between CO2 treatments, the researchers demonstrated that not all responses to ocean acidification are created equal. Carbon fixation gains can be genuinely heritable, meaning lineages evolved under high CO2 retain their enhanced performance even when returned to ambient conditions. Phosphorus allocation, by contrast, is reversible and context-dependent, which means it will track the environment the cells actually experience rather than the environment their ancestors evolved in. For forecasters, this distinction is critical: plastic responses can buffer populations in the short term but can also produce rapid, nonlinear shifts in biogeochemistry as conditions cross thresholds, without any evolutionary change being required.

As atmospheric CO2 continues to climb, the ocean will grow warmer, more acidic and, in many regions, more phosphorus-starved. This study makes clear that these stressors do not act independently; they converge inside the cells of phytoplankton, where a single nutrient must be rationed among DNA, RNA, energy metabolism and membrane construction. The diatoms that anchor marine food webs from coastal seas to the open ocean are already reorganizing their biochemistry in response, and the models suggest the reorganization will deepen dramatically by century’s end. What happens to phosphorus at the base of the ocean’s food web, the researchers conclude, may ultimately reshape the global phosphorus cycle itself — a reminder that climate change’s reach extends into the molecular plumbing of life in the sea.

Subject of Research: Effects of ocean acidification and phosphate limitation on phytoplankton phosphorus metabolism

Article Title: Ocean acidification reshapes phytoplankton phosphorus metabolism under phosphate-limited conditions

Article References: Wang, B., Wang, Y., Sun, Y., Huang, X., Yang, F., Wang, Z., Chi, H., Tong, S., Fan, X., Zhang, X., Sun, K., Fu, F., Wang, Y., Hutchins, D. A., Chen, G., Xu, D., & Ye, N. (2026). Ocean acidification reshapes phytoplankton phosphorus metabolism under phosphate-limited conditions. Nature Geoscience. https://doi.org/10.1038/s41561-026-02113-y

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02113-y

Keywords: ocean acidification, phytoplankton, phosphorus cycle, phosphate limitation, diatoms, marine biogeochemistry, particulate organic phosphorus, membrane lipid remodeling, multi-omics, Earth system models, climate change, Thalassiosira pseudonana