Alternative nitrogenase activity in the environment and nitrogen cycle implications

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 N-2 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 C-13 fractionation of acetylene reduction to ethylene ((13)epsilon(Mo) = 13.1-14.7 aEuro degrees, (13)epsilon(V) = 7.5-8.8 aEuro degrees, (13)epsilon(Fe) = 5.8-6.5 aEuro degrees). ISARA analyses indicate significant contributions of alternative nitrogen fixation in boreal cyanolichens and salt marshes (similar to 10-40 % acetylene reduction, similar to 20-55 % N-2 fixed). These results affect the quantitative interpretation of natural abundance N-15 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.