Physiology of Mo-, V- and Fe-only based biological nitrogen fixation in the anaerobic photoheterotroph Rhodopseudomonas palustris

Abstract

Nitrogen is an essential nutrient for life. Specialized microorganisms called diazotrophs convert inert atmospheric nitrogen gas into ammonia, a more reactive form of nitrogen. This keystone process, called nitrogen fixation, is a biologically costly reaction catalyzed by the metalloenzyme nitrogenase. Nitrogenase exists in three forms, a primary isoform that contains molybdenum (Mo) and two complementary isoforms that contain vanadium (V) or iron (Fe)-only at the active site. Because of methodological limitations, V- and Fe-only nitrogenase contributions to global nitrogen inputs were largely unquantified and assumed to be low until the past decade. Consequentially, although nitrogenase is one of the most important enzymes in the biosphere, few studies have characterized V- and Fe-only based diazotrophy. This limits our ability to predict how natural nitrogen inputs are likely to respond to changes in metal deposition and climate. To address this gap, my doctoral research tests how metabolic and environmental context influence nitrogen fixation and growth based on each of the three nitrogenase isoforms in the metabolically versatile model diazotroph Rhodopseudomonas palustris. The key findings reported in this dissertation are that (1) the complementary V-nitrogenase can enable growth as fast as the Mo-nitrogenase depending on the carbon substrate and temperature, (2) the Mo- and V-nitrogenase produce similar amounts of the important metabolite and biofuel hydrogen under many growth conditions, (3) the use of Fe-only nitrogenase leads to the slowest growth and greatest hydrogen production under all conditions tested here, and (4) the V- and Fe-only nitrogenases produce methane with highly depleted hydrogen isotope composition distinguishable from other natural sources. This research demonstrates that the metabolic cost of using different nitrogenases varies based on environmental context and that the efficiency of nitrogenase is not always the most important aspect of growth under nitrogen limited conditions. The result that Mo- and V-nitrogenase activity can result in similar growth and nitrogen fixation rates is unexpected based on previous in vitro work and suggests that shifts from Mo- to V-nitrogenase activity based on Mo limitation are not inherently associated with a decrease in nitrogen fixation, especially at cooler temperatures ≤ 19°C.