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

Abstract

We investigated the effects of Ni concentration on hydrogen (H 2 ) net production and nitrogen (N 2 ) 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 N 2 fixation rates. We found that Ni availability controls its H 2 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 H 2 net production, and vice versa. We also identified the threshold range of Ni concentrations that may differentiate low and high H 2 net production patterns in the diazotroph. However, the substantial variations in Ni availability and H 2 net production did not affect nitrogenase expression and N 2 fixation rates in Cyanothece. From two independent studies in the Atlantic tropical and subtropical oceans, we found that dissolved Ni concentrations and H 2 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 H 2 net production and concentrations in the euphotic zone of the oceanic regions. H 2 concentrations may be used as a promising proxy to reflect N 2 fixation activities in tropical and subtropical oceans only under Ni-deficient conditions.