Cyanobacterial diazotrophs are major contributors of marine nitrogen fixation. Iron (Fe) and phosphorus (P) availability constrain the growth and N₂ fixation of diazotrophic cyanobacteria in the oligotrophic oceans. CO₂ enrichment on natural cyanobacterial diazotrophs assemblies in the western North Pacific generally decreased N₂ fixation rates, with the negative effects exacerbated by Fe/P-limitation. We further cultured Fe/P-limited filamentous Trichodesmium erythraeum IMS101 (Trichodesmium) and unicellular Crocosphaera watsonii WH8501 (C. watsonii ) under ambient and acidified conditions (400 µatm vs 750 µatm). Results showed that Fe in Fe-limited Trichodesmium and C. watsonii was reallocated to the synthesis of nitrogenase to compensate for the decrease of nitrogenase efficiency caused by lower pH levels, consequently inhibiting the main energy production process and exacerbating the adverse effects of ocean acidification. Moreover, under P limitation, acidification enhanced phosphorus demands and decreased phosphorus-specific nitrogen fixation rates in Trichodesmium. However, acidification had little or no effect on P-limited C. watsonii. These findings highlight the distinct responses of different cyanobacterial diaozotrphs to acidification under different nutrient conditions, improving a predictive understanding of global N₂ fixation in future acidified oceans.

N2 fixation, nutrient limitation, ocean acidification, marine diazotrophic cyanobacteria