New Insights into SMS Deposits: How Microbial Activity and Oxygen Levels Shape Metal Preservation

Seafloor massive sulfide (SMS) deposits form on the modern ocean seafloor at active hydrothermal vent systems through mixing of mineral-rich, hydrothermal fluids with ambient oxygenated seawater. Once hydrothermal activity ceases, oxygenated seawater infiltrates these deposits, fostering to abiotic oxidative weathering. Microbial activity considerably accelerates this transformation, driving sulfide mineral breakdown, thus enhancing metal transport. Under conditions, restricting oxygen entrainment, low-oxygen zones form below the surface, shielding SMS deposits from oxidative weathering, potentially extending their preservation. SMS deposits are valuable sources of metals governing the interest of their lifespan.
In this study, we explore the impact of microbial activity on SMS transformation and mineral dissolution under oxic and low-oxygen conditions. We incubated sulfide minerals, i.e. pyrite and chalcopyrite for four years on the seafloor at active and inactive vent sites along the Indian Ridge. These sulfide minerals were then used for metagenomics, microscopy, microbial enrichment experiments, physiological studies, and geochemistry to identify the key microbial agents driving mineral transformation and metal release. Scanning electron microscopy (SEM) reveals diverse mineral structures, such as twisted stalks and nanowires, suggesting various Fe-oxidizing microbes as well as those involved in extracellular electron transfer. Preliminary metagenomic analyses provide insights into the presence of genes associated with iron oxidation and reduction. Laboratory cultivation experiments mimicked different temperature, oxygen, and pH conditions of hydrothermal vent fluids admixed to distinct degrees with ambient seawater and suggest faster microbially mediated mineral dissolution under oxic conditions and of pyrite as opposed to chalcopyrite. By assessing turnover rates of mineral transformations, we aim to predict how microbial activity affects SMS deposit longevity under varying oxygen conditions.