Nitrogen fixation is driven by cellular nitrogen stores in a diazotrophic cyanobacterium: Evidence from culture experiments and model validation

Diazotrophic cyanobacteria, including those responsible for harmful algal blooms, fix atmospheric dinitrogen (N 2 ) to sustain growth when they are limited by dissolved inorganic nitrogen (DIN) supply. Conventional models fail to accurately predict the timing and magnitude of N 2 fixation, partly because of oversimplified assumptions about diazotrophic species responses to N. We hypothesized that cellular carbon (C) and N would co‐vary with DIN availability, and cellular N quota reduced below a critical threshold would trigger heterocyst differentiation and N 2 fixation. Conversely, we hypothesized that heterocysts would be discarded, and N 2 fixation would cease at another higher cellular N quota threshold. To test this hypothesis, we undertook experiments to provide data to calibrate a model of cellular C and N of N 2 fixation in the cyanobacterium Raphidiopsis raciborskii . The model incorporated internal N quotas, external DIN supply and a time lag for N 2 fixation as key regulators of heterocyst production and N 2 fixation, explaining > 70% of the variation in measured cellular N and C : N molar ratios. Heterocysts were predicted to form at a cellular N quota of 0.085 mg N mg −1 C and were discarded from filaments at 0.275 mg N mg −1 C. There was a 5‐d time lag between DIN deprivation and initiation of N 2 fixation. Continuous DIN supply was predicted to enhance N 2 fixation more than an equivalent one‐off pulse. Our study highlighted the need to consider cellular N and N supply regimes in driving N 2 fixation to provide more accurate predictions of the response of cyanobacteria to DIN availability.