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Abstract
SummaryThe production of hydrogen (H2) is an inherent component of biological dinitrogen (N2) fixation, and there have been several studies quantifying H2 production relative to N2 fixation in cultures of diazotrophs. However, conducting the relevant measurements for a field population is more complex as shown by this study of N2 fixation, H2 consumption and dissolved H2 concentrations in the oligotrophic North Pacific Ocean. Measurements of H2 oxidation revealed microbial consumption of H2 was equivalent to 1–7% of ethylene produced during the acetylene reduction assay and 11–63% of 15 N2 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 N2 fixation and H2 oxidation. However, no corresponding changes were observed in the dissolved H2 concentrations which remained consistently supersaturated (147–560%) relative to atmospheric equilibrium. The results from this field study allow the efficiency of H2 cycling by natural populations of diazotrophs to be compared to cultured representatives. The findings indicate that dissolved H2 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 H2‐metabolizing microorganisms.
Highlights
In the surface waters of the tropical and subtropical open ocean, dissolved H2 concentrations typically range from [1,2,3] nmol l-1, equivalent to 300–900%supersaturation relative to atmospheric equilibrium (Herr et al, 1984; Conrad and Seiler, 1988; Moore et al, 2009)
While N2 fixation is more commonly measured than H2 production, it is unwise to use the theoretical stoichiometry predicted in Equation 1: (Eq 1) to provide an estimate of H2 production associated with nitrogenase activity
This is due to several inherent issues associated with H2 cycling linked to N2 fixation, as listed below: (i) Measurements of H2 production alongside measurements of N2 fixation are always less than the equimolar stoichiometry predicted in Equation 1 (Schubert and Evans, 1976; Wilson et al, 2010)
Summary
In the surface waters of the tropical and subtropical open ocean, dissolved H2 concentrations typically range from [1,2,3] nmol l-1, equivalent to 300–900%supersaturation relative to atmospheric equilibrium (Herr et al, 1984; Conrad and Seiler, 1988; Moore et al, 2009). While N2 fixation is more commonly measured than H2 production, it is unwise to use the theoretical stoichiometry predicted in Eq 1 to provide an estimate of H2 production associated with nitrogenase activity. (i) Measurements of H2 production alongside measurements of N2 fixation are always less than the equimolar stoichiometry predicted in Equation 1 (Schubert and Evans, 1976; Wilson et al, 2010). This is because all diazotrophs contain uptake hydrogenases that reassimilate a variable portion of H2 released during N2 fixation to conserve energy (Burns and Hardy, 1975, Tamgnini et al, 2007)
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