Dinitrogen fixation in the world's oceans

The surface water of themarine environment has traditionally beenviewed as a nitrogen (N) limited habitat, andthis has guided the development of conceptualbiogeochemical models focusing largely on thereservoir of nitrate as the critical source ofN to sustain primary productivity. However,selected groups of Bacteria, includingcyanobacteria, and Archaea canutilize dinitrogen (N2) as an alternativeN source. In the marine environment, thesemicroorganisms can have profound effects on netcommunity production processes and can impactthe coupling of C-N-P cycles as well as the netoceanic sequestration of atmospheric carbondioxide. As one component of an integrated ‘Nitrogen Transport and Transformations’ project, we have begun to re-assess ourunderstanding of (1) the biotic sources andrates of N2 fixation in the world'soceans, (2) the major controls on rates ofoceanic N2 fixation, (3) the significanceof this N2 fixation for the global carboncycle and (4) the role of human activities inthe alteration of oceanic N2 fixation. Preliminary results indicate that rates ofN2 fixation, especially in subtropical andtropical open ocean habitats, have a major rolein the global marine N budget. Iron (Fe)bioavailability appears to be an importantcontrol and is, therefore, critical inextrapolation to global rates of N2fixation. Anthropogenic perturbations mayalter N2 fixation in coastal environmentsthrough habitat destruction and eutrophication,and open ocean N2 fixation may be enhancedby warming and increased stratification of theupper water column. Global anthropogenic andclimatic changes may also affect N2fixation rates, for example by altering dustinputs (i.e. Fe) or by expansion ofsubtropical boundaries. Some recent estimatesof global ocean N2 fixation are in therange of 100–200 Tg N (1–2 × 1014 g N)yr−1, but have large uncertainties. Theseestimates are nearly an order of magnitudegreater than historical, pre-1980 estimates,but approach modern estimates of oceanicdenitrification.