Biochemical remodelling of phytoplankton cell composition under climate change

Although the macromolecular composition of phytoplankton shapes the nutrition available to marine ecosystems and regulates global biogeochemistry, there are no mechanistic, predictive models for its global distribution. Here, using a cellular allocation model, we simulate phytoplankton allocation to proteins, carbohydrates and lipids in the present day and a warming scenario. Our simulations predict spatial variations consistent with available observations: in nutrient-sufficient, low-light, high-latitude regions, phytoplankton allocate more to nitrogen-rich proteins, while in nutrient-depleted subtropical regions, allocation favours carbohydrates and lipids. Under warming, subtropical phytoplankton increase protein allocation by ~20%, as subsurface populations rich in light-harvesting proteins thrive, whereas high-latitude protein allocation declines by ~15–30% due to warming and relief from light limitation. In situ macromolecular measurements in polar regions show recent trends consistent with our predictions. These results suggest that macromolecular composition responds measurably to changing environmental conditions, reshaping the nutritional landscape at the base of the marine food web.