Isotope discrimination provides new insight into biological nitrogen fixation

摘要

Biological N2 fixation, the assimilation of atmospheric N2 into NH3, is the province of highly specialized microorganisms, and a key entry point for atmospheric nitrogen (N) into terrestrial ecosystems (Vitousek etal, 2002). It is probably the most important biologically mediated process after photosynthesis, and is universally carried out by the enzyme nitrogenase. Collectively, N2 fixing organisms are termed diazotrophs, some of which can fix N2 in a 'free-living' state, while others fix N2 in loose association with plants, and a select few in highly evolved, complex symbioses on plant roots or stems. Despite its importance, physiological control of biological N2 fixation is only partially understood and quantification of N2 fixation at the fieldlevel is difficult (Unkovich et al, 2008). Further progress in quantifying N cycle fluxes in ecosystems will rely heavily on stable isotope (l3N) investigations. These powerful techniques can be used at scales ranging from cell to globe (Vandover et al,1992; Robinson, 2001; Werner & Schmidt, 2002), but require an understanding of the isotope discrimination associated with N transformations. Generally, compounds containing the lighter of two isotopes react more quickly, resulting in reaction products being isotopically lighter than the substrate, unless all of the substrate is converted to the product (Dawson & Brooks, 2001). In the case of biological N2 fixation this equates to differences in the relative abundance of the stable isotopes 15N and l4N between atmospheric N2 and the fixed NH3 produced by the nitrogenase enzyme in the diazotroph. Natural N isotope abundances (delta 15N) are expressed as a parts per thousand (%o) deviation from the 15N composition of atmospheric N2 (0%o) (Mariotti, 1983)and thus one might anticipate fixed NH3 to have a negative delta l3N given that the N2 fixation substrate substrate (N2) would be in unlimited supply and one would anticipate preferential reduction of 'N 'N (mass 28) over 'N 'N (mass 29) or 15N15N (mass30). The aim of the present paper is to highlight uncertainties surrounding the extent of isotope fractionation associated with N2 fixation, and to provide a possible working framework for interpretation of the available data.