Dissolved hydrogen and nitrogen fixation in the oligotrophic North Pacific Subtropical Gyre

The production of hydrogen (H(2)) is an inherent component of biological dinitrogen (N(2)) fixation, and there have been several studies quantifying H(2) production relative to N(2) fixation in cultures of diazotrophs. However, conducting the relevant measurements for a field population is more complex as shown by this study of N(2) fixation, H(2) consumption and dissolved H(2) concentrations in the oligotrophic North Pacific Ocean. Measurements of H(2) oxidation revealed microbial consumption of H(2) was equivalent to 1–7% of ethylene produced during the acetylene reduction assay and 11–63% of (15)N(2) assimilation on a molar scale. Varying abundances of Crocosphaera and Trichodesmium as revealed by nifH gene abundances broadly corresponded with diel changes observed in both N(2) fixation and H(2) oxidation. However, no corresponding changes were observed in the dissolved H(2) concentrations which remained consistently supersaturated (147–560%) relative to atmospheric equilibrium. The results from this field study allow the efficiency of H(2) cycling by natural populations of diazotrophs to be compared to cultured representatives. The findings indicate that dissolved H(2) concentrations may depend not only on the community composition of diazotrophs but also upon relevant environmental parameters such as light intensity or the presence of other H(2)-metabolizing microorganisms.