Alternative nitrogenase activity in the environment and nitrogen cycle implications

TLDR: ISARA analyses indicate significant contributions of alternative nitrogen fixation in boreal cyanolichens and salt marshes and suggest significant interactions between the cycles of nitrogen and trace metals, which affect the quantitative interpretation of natural abundance 15N data or traditional acetylene reduction assays.

Abstract: Biological nitrogen fixation, the main natural input of fixed nitrogen into the biosphere, is catalyzed by Mo-, V-, or Fe-only nitrogenase metalloenzymes. Although “alternative” V- and Fe-only nitrogenase genes are found in many environments, the contribution of these isoenzymes to N2 fixation is unknown. Here we present a new method (ISARA, isotopic acetylene reduction assay) that distinguishes canonical Mo and alternative nitrogenase activities based on in vivo 13C fractionation of acetylene reduction to ethylene (13eMo = 13.1–14.7 ‰, 13eV = 7.5–8.8 ‰, 13eFe = 5.8–6.5 ‰). ISARA analyses indicate significant contributions of alternative nitrogen fixation in boreal cyanolichens and salt marshes (~10–40 % acetylene reduction, ~20–55 % N2 fixed). These results affect the quantitative interpretation of natural abundance 15N data or traditional acetylene reduction assays. They also invite a reexamination of the conditions under which the different nitrogenase isozymes are active and suggest significant interactions between the cycles of nitrogen and trace metals.