Regulation of nitrogen metabolism in the marine diazotroph Trichodesmium IMS101 under varying temperatures and atmospheric CO₂ concentrations

We examined the influence of forecasted changes in global temperatures and pCO₂ on N₂ fixation and assimilation in the ecologically important cyanobacterium Trichodesmium spp. Changes of mRNA transcripts (nifH, glnA, hetR, psbA, psaB), protein (nitrogenase, glutamine synthetase) pools and enzymatic activity (nitrogenase) were measured under varying pCO₂ and temperatures. High pCO₂ shifted transcript patterns of all genes, resulting in a more synchronized diel expression. Under the same conditions, we did not observe any significant changes in the protein pools or in total cellular allocations of carbon and nitrogen (i.e. C : N ratio remained stable). Independently of temperature, high pCO₂ (900 μatm) elevated N₂ fixation rates. Levels of the key enzymes, nitrogenase and glutamine synthetase that mediate nitrogen assimilation did not increase, implying that the high pCO₂ allowed higher reaction turnover rates through these key enzymes. Moreover, increased temperatures and high pCO₂ resulted in higher C : P ratios. The plasticity in phosphorous stoichiometry combined with higher enzymatic efficiencies lead to higher growth rates. In cyanobacteria photosynthesis, carbon uptake, respiration, N₂ fixation and nitrogen assimilation share cellular components. We propose that shifted cellular resource and energy allocation among those components will enable Trichodesmium grown at elevated temperatures and pCO₂ to extend its niche in the future ocean, through both tolerance of a broader temperature range and higher P plasticity.