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The effects of Ni availability on H2 production and N2 fixation in a model unicellular diazotroph: The expression of hydrogenase and nitrogenase

We investigated the effects of Ni concentration on hydrogen (H2) net production and nitrogen (N2) fixation rates in a model unicellular diazotrophic cyanobacterium, Cyanothece. We quantified NiFe uptake hydrogenase of the diazotroph to examine how Ni deficiency affects the expression of hydrogenase and nitrogenase and N2 fixation rates. We found that Ni availability controls its H2 net production and the expression of HupL, the major subunit of the uptake hydrogenase. Low Ni availability induced low HupL expression levels while yielding high H2 net production, and vice versa. We also identified the threshold range of Ni concentrations that may differentiate low and high H2 net production patterns in the diazotroph. However, the substantial variations in Ni availability and H2 net production did not affect nitrogenase expression and N2 fixation rates in Cyanothece. From two independent studies in the Atlantic tropical and subtropical oceans, we found that dissolved Ni concentrations and H2 oversaturation percentage exhibited a statistically significant negative correlation, which is consistent with the findings of this culture study. Overall, these results indicate that Ni availability may affect H2 net production and concentrations in the euphotic zone of the oceanic regions. H2 concentrations may be used as a promising proxy to reflect N2 fixation activities in tropical and subtropical oceans only under Ni‐deficient conditions.