Dissolved hydrogen and nitrogen fixation in the oligotrophic N orth P acific S ubtropical G yre

Summary 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 N orth P acific O cean. 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 C rocosphaera and T richodesmium as revealed by nif H 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.