Evolution of the molybdenum and vanadium cycles through time and their impact on ancient nitrogen fixation

Biological nitrogen (N ₂ ) fixation is crucial for incorporating atmospheric N ₂ into the biosphere. However, the evolutionary trajectory of this process remains enigmatic, particularly regarding the paradoxical relationship between the presumed scarcity of molybdenum (Mo) in the Archean ocean—before oxidative weathering on land—and the apparent early emergence of Mo-based nitrogenase over the vanadium (V)-based alternative. Here, we integrate partitioning experiments with an extensive analysis of iron (Fe)-rich sediments through time to elucidate the behavior of Mo and V in ancient seawater. Our findings suggest that in the Archean to Paleoproterozoic, dissolved V was more efficiently removed from the water column than Mo, primarily due to the preferential incorporation of V into banded iron formations (BIF). Our data also suggest that dissolved Mo was more available in seawater than commonly assumed and that the ratio Mo/V was higher in the Archean than it is today. Higher Mo abundances and greater availability of Mo relative to V offers a compelling explanation for the earlier evolution of Mo-based nitrogenase over the alternative nitrogenases.