Answers sought to the enigma of marine nitrogen fixation

Nitrogen fixation is a biologically catalyzed reaction that converts dinitrogen gas (N 2 ) to ammonium (NH 4 + ), which can then be utilized to support autotrophic growth. Early research suggested that N 2 ‐fixation in the open ocean was not a significant source of nitrogen for phytoplankton growthsemi; virtually all of the nitrogen needed for growth was thought to come from the deep ocean through mixing and upwelling. A growing body of evidence now suggests that N 2 ‐fixers may be far more ubiquitous in the open ocean than previously thought and that nitrogen fixation is quantitatively significant in the global nitrogen cycle. This finding is important because primary production supported by N 2 ‐fixation can result in a net export of carbon from the surface waters to the deep ocean and a net draw‐down of atmospheric carbon dioxide and may therefore play a significant role in the global carbon cycle as well. In fact, N 2 ‐fixation may be the only biologically mediated process in the open ocean that drives a significant net export of atmospheric carbon dioxide over greater than annual timescales. In addition, the global balance between N 2 ‐fixation and denitrification determines the degree to which the oceans are nitrogen limited. Thus global warming‐induced changes in N 2 ‐fixation could have significant long‐term effects on oceanic productivity and the global carbon cycle. If this is the case, then we must understand the global controls on N 2 ‐fixation and find ways to represent it in models that are currently being developed to predict future climate.