Iron and phosphorus limitations modulate the effects of carbon dioxide enrichment on a unicellular nitrogen‐fixing cyanobacterium

Abstract Iron (Fe) and phosphorus (P) availability constrain the growth and N 2 fixation of diazotrophic cyanobacteria in the global ocean. However, how Fe and P limitation may modulate the effects of ocean acidification on the unicellular diazotrophic cyanobacterium Crocosphaera remains largely unknown. Here, we examined the physiological responses of Crocosphaera watsonii WH8501 to CO 2 enrichment under both nutrient‐replete and steadily Fe‐ or P‐limited conditions. Increased CO 2 (750 μ atm vs. 400 μ atm) reduced the growth and N 2 fixation rates of Crocosphaera , with Fe limitation intensifying the negative effect, whereas CO 2 enrichment had a minimal impact under P limitation. Mechanistically, the high CO 2 treatment may have led to a reallocation of limited Fe to nitrogenase synthesis to compensate for the reduction in nitrogenase efficiency caused by low pH; consequently, other Fe‐requiring metabolic pathways, such as respiration and photosynthesis, were impaired, which in turn amplified the negative effects of acidification. Conversely, under P limitation, CO 2 enrichment had little or no effect on cellular P allocation among major P‐containing molecules (polyphosphate, phospholipids, DNA, and RNA). Cell volumes were significantly reduced in P‐limited and high CO 2 cultures, which increased the surface : volume ratio and could facilitate nutrient uptake, thereby alleviating some of the negative effect of acidification on N 2 fixation. These findings highlight the distinct responses of Crocosphaera to high CO 2 under different nutrient conditions, improving a predictive understanding of global N 2 fixation in future acidified oceans.