Bioavailability of molybdenite to support nitrogen fixation on early Earth by an anoxygenic phototroph

Biological nitrogen fixation, which converts atmospheric dinitrogen to ammonia, is catalyzed mostly by Mo-nitrogenase and is a primary contributor to bioavailable nitrogen on early Earth. Mo-nitrogenase is believed to have evolved during the Archean, despite the extremely low concentration of dissolved Mo. However, it remains unclear whether Mo minerals could serve as a source of Mo to support the prevalence of Mo-nitrogenase on early Earth. Here we investigated the bioavailability of molybdenite by incubating it with a metabolically ancient anoxygenic phototroph (Rhodopseudomonas palustris) under anoxic conditions. In the laboratory, R. palustris utilized molybdenum from molybdenite as a cofactor for nitrogen fixation. This bacterium extracted Mo from molybdenite by secreting molybdophores rhodopetrobactin A and B and by expressing Mo transport proteins. Surface-sensitive techniques demonstrated significant changes in surface chemistry of molybdenite after its interaction with cells. These findings provide novel explanations for the prevalence of Mo-nitrogenase on early Earth, with significant implications for nitrogen fixation in modern Mo-deficient environments.