Isotopic Biomarkers of Nitrogenase Metalloenzymes: Forging Links Between the Cycles of Nitrogen and Trace Metals

Abstract:

Biological N₂ fixation, catalyzed by the metalloenzyme nitrogenase, is a critical process that makes life possible on Earth. Environmental N₂ fixation has been automatically attributed to canonical Mo-based nitrogenases despite over two decades of knowledge that two other metalloenzyme forms of nitrogenase exist: those containing catalytic V or Fe-only. A key area of missing information is the contribution of the “alternative” V and Fe-only nitrogenases, as the interpretation of field data to construct budgets and assess N availability depends on the type of nitrogenase metalloenzyme used to fix N₂. Additionally, substantial changes in metal speciation over geological time may have favored the use of different metalloenzymes, with implications for evolution of the biosphere. Despite the potential importance of alternative nitrogenases in modern and ancient N cycling, few methods can determine their contributions to environmental N₂ fixation. Here, we present new isotopic methods to distinguish between the activities of Mo, V, and Fe-only nitrogenases. We show evidence for alternative N₂ fixation in diverse environments (cyanolichens, microbial mats, sediments, leaf litter), thereby linking a key process in the nitrogen cycle to specific metalloenzyme forms of nitrogenase. The results invite a reexamination of the conditions under which the different nitrogenase metalloenzymes are active and may lead to new insights into the coupling of the cycles of nitrogen and trace metals.