Physiological Implications of Alternative Nitrogen Fixation

Biological nitrogen fixation by the prokaryotic metalloenzyme nitrogenase (Nase) converts inert N₂ gas into fertilizing ammonia, producing H₂ as an obligatory reaction byproduct. All known nitrogen fixers (diazotrophs) encode a molybdenum (Mo-) based Nase metalloenzyme. Some also encode "alternative" vanadium (V-) and iron-only (Fe-) Nases, enabling diazotrophy in the absence of Mo. Recent results from our group have demonstrated that alternative Nases can be present and active in Mo-replete environments, suggesting that additional factors may control alternative Nase usage. To elucidate these factors, we are investigating how alternative Nases affect diazotroph physiology. Using wild type and Nase mutant strains of the metabolically versatile photoheterotroph Rhodopseudomonas palustris, we tested the influence of alternative nitrogen fixation on cellular redox homeostasis, biomass composition, and carbon acquisition. Despite assumptions that alternative nitrogen fixation is less efficient than canonical nitrogen fixation, we find that, depending on the carbon source, use of the alternative V-Nase does not necessarily slow photoheterotrophic growth or result in substantially greater H₂ production. The data indicate that the metabolic costs of alternative nitrogen fixation could be less significant than previously assumed, possibly explaining why alternative Nase genes persist in diverse diazotroph lineages and are broadly distributed in the environment.