Acclimation by diverse phytoplankton species determines oceanic carbon to nitrogen ratios

The carbon to nitrogen (CN) ratio of phytoplankton connects the carbon and nitrogen cycles in the ocean. Any variation in this ratio under climate change will alter the amount of carbon fixed by photosynthesis, and ultimately the amount sequestered in the ocean. However, a consistent mechanistic explanation remains lacking for observed species-specific variations in phytoplankton CN ratios. We show that acclimation to ambient environmental conditions explains the observed variation of phytoplankton CN ratios by incorporating phytoplankton acclimation theory based on a resource allocation trade-off between carbon vs. nitrogen acquisition capacity into a three-dimensional marine ecosystem model. Inter-specific differences in CN ratio and its sensitivity are caused by inter-specific differences in Droop's minimum nitrogen cell quota. Our model, constrained by observed phytoplankton parameters, shows that the global mean phytoplankton CN ratio is greater than the canonical Redfield ratio, as suggested previously based on regional in situ observations.