Cellular-level control on global ocean deoxygenation driven by phytoplankton ecophysiology

Phytoplankton elemental composition shapes the distribution of dissolved nutrient concentrations and thus plays a key role in ocean biogeochemistry. While the carbon-to-nitrogen-to-phosphorus ratio (C:N:P) in marine phytoplankton has been extensively studied, the hydrogen (H) and oxygen (O) content have received less attention despite their critical role in determining dissolved oxygen (O ₂ ) consumption rates in the ocean. Here, we estimated the elemental composition of marine phytoplankton, including the H and O content, from first principles, using a cellular allocation model embedded in a global ocean model. We estimated that an average phytoplankton cell has a chemical formula of C ₁₀₇ H ₁₉₀ N ₁₆ O ₅₃ P, with an O ₂ demand of 149 mol O ₂ /mol P and respiration quotients of 1.40 mol O ₂ /mol C, suggesting a lower H and O content, and higher O ₂ demand than commonly assumed. We found global variations in the O ₂ demand of organic matter respiration driven by population structure and cellular reorganisation under different environmental conditions. By testing how shifts in the macromolecular composition of phytoplankton cells affect the ocean’s O ₂ budget, we found that O ₂ consumption increases significantly when shifting cell composition from carbohydrate-rich to protein- or lipid-rich cells. As a result, low-O ₂ (hypoxic) zones in the ocean expanded by 75%. These findings demonstrate that cellular-level processes in marine phytoplankton shape the global O ₂ cycle and large-scale patterns of ocean biogeochemistry.