Insights from a four year in-situ incubation experiment at the Indian Ridge: How microbes impact seafloor massive sulfide weathering

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. SMS deposits are primarily comprised of pyrite and chalcopyrite, and serve as source for valuable metals such as copper. Once hydrothermal activity ceases these SMS deposits undergo abiotic oxidative weathering when penetrated by oxygenated seawater. Microbial activity considerably accelerates this process transforming sulfide minerals, enhancing metal transport and mineral dissolution. Under certain circumstances the formation of iron-rich silica caps can create low oxygen conditions below the caprock, shielding SMS deposits from both abiotic and biotic oxidative weathering likely extending the lifetime of these deposits. Here we will examine the impact of microbial activity for SMS transformation and dissolution of minerals under oxic and low oxygen conditions. For this we incubated sulfide minerals for four years on the seafloor at an active and an inactive venting side on the Indian Ridge and used hydrothermal vent material for laboratory experiments. We employed a combination of molecular biological techniques, microbial enrichment experiments, physiological studies, microscopy, and geochemistry, to identify key microbial actors responsible for mineral transformation and metal mobilization. SEM images reveal diverse mineral structures which are indicative of the presence of various microorganisms involved in Fe-cycling, including twisted stalks and nanowires. By determining turnover rates and mineral modifications, we aim to predict how microbial activity influences the lifetime of SMS deposits under different oxygen conditions.