Reduced nitrogenase efficiency dominates response of the globally important nitrogen fixer Trichodesmium to ocean acidification

The response of the prominent marine dinitrogen (N 2 )-fixing cyanobacteria Trichodesmium to ocean acidification (OA) is critical to understanding future oceanic biogeochemical cycles. Recent studies have reported conflicting findings on the effect of OA on growth and N 2 fixation of Trichodesmium . Here, we quantitatively analyzed experimental data on how Trichodesmium reallocated intracellular iron and energy among key cellular processes in response to OA, and integrated the findings to construct an optimality-based cellular model. The model results indicate that Trichodesmium growth rate decreases under OA primarily due to reduced nitrogenase efficiency. The downregulation of the carbon dioxide (CO 2 )-concentrating mechanism under OA has little impact on Trichodesmium , and the energy demand of anti-stress responses to OA has a moderate negative effect. We predict that if anthropogenic CO 2 emissions continue to rise, OA could reduce global N 2 fixation potential of Trichodesmium by 27% in this century, with the largest decrease in iron-limiting regions.