A first-principles look at phytoplankton chemistry reveals an overlooked driver of the ocean’s oxygen budget.
Reporting by Helen Hill for CBIOMES
For nearly a century, the Redfield ratio has served as a cornerstone of biological oceanography, describing the characteristic elemental composition of marine life and linking the cycles of carbon, nitrogen, and phosphorus throughout the ocean. But a new study led by Shlomit Sharoni suggests that the chemistry of marine life is more dynamic than this classic picture implies. By connecting phytoplankton physiology to global ocean circulation, the study reveals how differences in cellular composition can influence oxygen consumption, carbon storage, and the distribution of low-oxygen waters across the planet.
The biochemical makeup of phytoplankton matters because it determines how much oxygen is consumed when organic matter is respired. Yet most large-scale biogeochemical models simplify this chemistry by treating marine organic matter as having a largely fixed composition. Real phytoplankton cells are far more complex, containing different mixtures of proteins, lipids, carbohydrates, pigments, and nutrient-storage compounds depending on the environmental conditions they experience.
To explore the implications of this complexity, the researchers embedded a mechanistic model of phytoplankton physiology within the Darwin ecosystem model and the MIT General Circulation Model (MITgcm) framework, allowing cellular biochemistry to interact with simulated environmental variables. Rather than assigning a fixed composition to marine organisms, the model allows cells to adjust their internal chemistry in response to changing light, temperature, and nutrient conditions.
What emerged was a picture of an ocean whose biology is far less uniform than classic stoichiometric models assume. In nutrient-poor subtropical gyres, phytoplankton tend to accumulate carbon-rich storage compounds such as carbohydrates and lipids. In higher-latitude regions, where nutrients are more abundant but light is scarcer, cells invest more heavily in proteins needed for growth and photosynthesis. Those physiological differences translate into systematic differences in the elemental composition of marine organic matter and in the amount of oxygen required to break it down. “The chemistry of a phytoplankton cell isn’t fixed,” explains Sharoni, “as cells adapt to different environments, they change their biochemical makeup, and those small cellular changes can scale up to influence the chemistry of the global ocean.”
The consequences scale all the way up to the global ocean. The model predicts substantial geographic variation in oxygen demand, with patterns that closely match independent estimates derived from observations. The study also arrives at an updated global-average chemical formula for marine phytoplankton that departs from the classical Redfield description, reflecting the important contributions of proteins and lipids to marine biomass.
Perhaps most strikingly, the authors show that relatively modest changes in the biochemical composition of phytoplankton can have large effects on the ocean’s oxygen inventory. In long-term simulations, shifts in the total respiration quotient of just 0.1 can expand the volume of oceanic hypoxic zones where dissolved oxygen is lower than 62 mmol O2 m⁻³ by ~35% for the same amount of organic carbon degradation. The results suggest that changes in plankton physiology could influence how marine ecosystems respond to ongoing ocean deoxygenation.
The findings also have implications for how scientists estimate the strength of the biological carbon pump. Many common techniques infer carbon cycling from measurements of oxygen utilization. If the relationship between carbon and oxygen varies across ocean regions, those calculations may need to account for more physiological complexity than previously recognized.
“Taken together, our study highlights how the chemistry of individual cells can shape the chemistry of the planet,” says Sharoni. “By linking phytoplankton physiology to global patterns of oxygen consumption and carbon storage, the work offers a new perspective on how marine ecosystems help regulate Earth’s climate and biogeochemical cycles.”
Publication:
Shlomit Sharoni, Keisuke Inomura, Stephanie Dutkiewicz, Oliver Jahn, Gregory L. Britten, Michael J. Follows (2026), C:H:N:O:P stoichiometry and oxygen demand of marine organic matter are driven by phytoplankton ecophysiology, Science Advances, doi: 10.1126/sciadv.aeg6228