C:H:N:O:P stoichiometry and oxygen demand of marine organic matter are driven by phytoplankton ecophysiology

Although the hydrogen and oxygen content of marine phytoplankton plays a critical role in determining oceanic O₂ consumption rates, shaping habitat for anaerobic metabolism, they are often assumed to be constant in biogeochemical studies, resulting in biased estimates of respiration rates. Here, by simulating phytoplankton allocation in a general circulation model, we predict latitudinal variations in phytoplankton C:H:N:O:P composition and corresponding O₂ demand. These patterns are driven by physiological adaptation to environmental conditions and are consistent with observations. Subtropical phytoplankton have more C─O bonds and lower O₂ demand, as they contain more carbohydrates than higher-latitude populations. We test the sensitivity of O₂ inventory and regenerated C via the biological pump (C^soft) to plausible bounds of cellular allocation and find that the global O₂ inventory and C^soft decrease by ∼11 and 14%, respectively, when a cell shifts from carbohydrate-rich to protein-rich biomass. Our findings demonstrate that biochemical remodeling of phytoplankton composition mediates large-scale patterns and oceanic budgets of O₂ and C.