The sensitivity of marine N2 fixation to dissolved inorganic nitrogen

The dominant process adding nitrogen (N) to the ocean, di-nitrogen (N(2)) fixation, is mediated by prokaryotes (diazotrophs) sensitive to a variety of environmental factors. In particular, it is often assumed that consequential rates of marine N(2) fixation do not occur where concentrations of nitrate (NO(-) (3)) and/or ammonium (NH(+) (4)) exceed 1μM because of the additional energetic cost associated with assimilating N(2) gas relative to NO(-) (3) or NH(+) (4). However, an examination of culturing studies and in situ N(2) fixation rate measurements from marine euphotic, mesopelagic, and benthic environments indicates that while elevated concentrations of NO(-) (3) and/or NH(+) (4) can depress N(2) fixation rates, the process can continue at substantial rates in the presence of as much as 30μM NO(-) (3) and/or 200μM NH(+) (4). These findings challenge expectations of the degree to which inorganic N inhibits this process. The high rates of N(2) fixation measured in some benthic environments suggest that certain benthic diazotrophs may be less sensitive to prolonged exposure to NO(-) (3) and/or NH(+) (4) than cyanobacterial diazotrophs. Additionally, recent work indicates that cyanobacterial diazotrophs may have mechanisms for mitigating NO(-) (3) inhibition of N(2) fixation. In particular, it has been recently shown that increasing phosphorus (P) availability increases diazotroph abundance, thus compensating for lower per-cell rates of N(2) fixation that result from NO(-) (3) inhibition. Consequently, low ambient surface ocean N:P ratios such as those generated by the increasing rates of N loss thought to occur during the last glacial to interglacial transition may create conditions favorable for N(2) fixation and thus help to stabilize the marine N inventory on relevant time scales. These findings suggest that restricting measurements of marine N(2) fixation to oligotrophic surface waters may underestimate global rates of this process and contribute to uncertainties in the marine N budget.