Defining the ecological interactions that drove the evolution of biological nitrogen fixation
Hamilton, Trinity Lynn
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All life requires fixed forms of nitrogen (N). On early Earth, fixed N was supplied through abiotic mechanisms, which became limiting to an expanding biome, precipitating the emergence of biological nitrogen fixation. Today, most biological nitrogen fixation is catalyzed by molybdenum (Mo)-dependent nitrogenase (Nif). Alternative forms of the enzyme contain either vanadium (V) or only iron (Fe) instead of Mo, but are only found in taxa that encode Nif. Geochemical evidence suggests Mo bioavailability was limited on the early Earth, leading to the hypothesis that alternative forms of nitrogenase are ancestral. Evidence presented here suggests that in fact Nif emerged first in a methanogenic archaeon. Previous studies revealed a widespread distribution of nif along geochemical gradients but little is known about the environmental conditions that drove its evolution. An analytical approach allowed examination of the role environment played in shaping the evolution of Nif across geochemical gradients in Yellowstone National Park. The distribution of nifH was widespread and not constrained by temperature or pH alone, but exhibited evidence of niche conservatism imposed by salinity, and seemed dispersal limited. Activity measurements in sediments collected from high-temperature acidic springs confirmed the potential for N ₂ fixation in these environments. These data expand our understanding of the habitat range and environmental drivers of N ₂ fixing organisms. In organisms that encode alternative nitrogenases, Nif is preferred for nitrogen fixation. In addition, the alternative forms of the enzyme do not encode the full suite enzymes necessary for assembling the active site metal cofactors. Presumably, the selective pressure driving the evolution of alternative nitrogenase would have been provided by Mo limitation. Transcriptome studies of a model organism which encodes all three forms of nitrogenase reveals the genes associated with expression of each nitrogenase and the interplay between systems that enables nitrogen fixation in the absence of Mo and fixed N. These analyses suggest the alternative nitrogenases would not function in the absence of Nif biosynthetic machinery and expression of nitrogenase is regulated by fixed N limitation and metal availability. The results presented here help elucidate the environmental conditions that have driven nitrogenase evolution, resulting in the extant enzyme.