Evidence of molybdenum association with particulate organic matter under sulfidic conditions.

The geochemical behavior of molybdenum (Mo) in the oceans is closely linked to the presence of sulfide species in anoxic environments, where Fe availability may play a key role in the Mo scavenging. Here, we show that Mo(VI) is reduced in the presence of particulate organic matter (represented by sulfate-reducing bacteria). Molybdenum was immobilized at the surface of both living cells and dead/lysed cells, but not in cell-free control experiments. Experiments were carried out at four different Mo concentrations (0.1 to 2 mm) to yield cell-associated Mo precipitates with little or no Fe, consisting of mainly Mo(IV)-sulfide compounds with molecular structures similar to Mo enzymes and to those found in natural euxinic sediments. Therefore, we propose that Mo removal in natural sulfidic waters can proceed via a non-Fe-assisted pathway that requires particulate organic matter (dead or living sulfate-reducing bacteria). This pathway has implications for global marine Mo cycling and the current use of Mo-based proxies for paleo-environmental investigations.