Biochemical remodeling of phytoplankton cell composition under climate change

The macromolecular composition of phytoplankton shapes the nutrition available to marine ecosystems and regulates the interwoven global ocean cycles of carbon and nutrients. Despite these fundamental roles, there is currently no global-scale prediction on the distribution of phytoplankton macromolecular composition and how it varies in response to environmental changes. Here, we simulate the cellular allocation of proteins, carbohydrates, and lipids in marine phytoplankton in a global ocean model at present and over the 21st century under a climate change scenario. Our results, consistent with sparse observations, indicate systematic variations in phytoplankton macromolecular composition: In nutrient-sufficient, low-light high latitudes, phytoplankton allocate more to nitrogen-rich proteins, while in the nutrient-depleted subtropical regions, allocation favours carbohydrates and lipids. Our simulations project geographic variations in response to a warming climate: At high latitudes, cellular protein levels decline by ~10-30%, and carbohydrate and lipid levels increase due to enhanced nutrient limitation and reduction in sea ice. Conversely, despite a weakened supply of nutrients to the surface, phytoplankton in subtropical latitudes are projected to have ~20% more proteins as subsurface populations thrive. Such geographic variations will reshape the nutritional landscape at the base of the marine food web and increase the C:P and C:N in sinking organic matter, thus partially offset the reduction in oceanic carbon storage projected under warming.