NO EVIDENCE FOR ALTERNATIVE NITROGENASE EXPRESSION IN A MESOPROTEROZOIC MOLYBDENUM-LIMITED ENVIRONMENT: CONSTRAINTS FROM NITROGEN ISOTOPES AT 1.1 GA

Nitrogen (N) is a critically important element for life on Earth, and its cycling through the ocean-atmosphere system is a biologically mediated process that requires a suite of transition metals as enzyme cofactors. For example, nitrogenase—the enzyme that mediates N-fixation by prokaryotes—requires molybdenum (Mo) as a structural component. It has been proposed, however, that alternative nitrogenase enzymes may have been important under conditions of expanded euxinia and Mo-drawdown that may have characterized parts of the Proterozoic oceans. To test this hypothesis, we examined nitrogen isotopes in black and grey shale of the 1.1 Ga Atar and El Mreiti groups, Mauritania. These strata were previously investigated for iron speciation and trace metal content, and they record a period of substantial Mo-drawdown in a variably sulfidic epeiric sea. Nitrogen isotopes have the potential to distinguish between the Mo and alternative nitrogenase pathways because culture experiments have shown that alternative nitrogenase expression can produce fixed N with distinctly lower δ¹⁵N values (< −2 ‰) compared to Mo-nitrogenase. In the Atar and El Mreiti groups, both shallow epicratonic and deeper pericratonic environments record strongly positive δ¹⁵N values largely between +4 and +7 ‰. Even in the most Mo-limited environments (euxinic shale with Mo enrichment factors < 1), no negative δ¹⁵N values are recorded, suggesting that N-fixation proceeded via Mo-nitrogenase. These results are consistent with data from other Mesoproterozoic basins, and collectively suggest that alternative nitrogenase expression was not common in the Mesoproterozoic oceans, despite the potential for Mo-drawdown under conditions of expanded euxinia. Furthermore, the strongly positive δ¹⁵N values recorded in these strata are potentially the result of partial denitrification or partial ammonium oxidation, suggesting operation of an aerobic N-cycle, at least in the shallowest parts of the water column.