Dissolved hydrogen and nitrogen fixation in the oligotrophic North Pacific Subtropical Gyre (vol 5, pg 697, 2013)

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 N-15(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.