The sensitivity of marine N2 fixation to dissolved inorganic nitrogen

• Published in: Frontiers in microbiology 2012, Vol.3
• Main Author: Knapp, Angela N.
• Format: Article
• Language: English
• Subjects: ammonium; diazotroph; inhibition; Microbiology; N2 fixation; nitrate; Sensitivity
• ISSN: 1664-302X; 1664-302X
• DOI: 10.3389/fmicb.2012.00374

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.