Patterns of (trace) metals and microorganisms in the Rainbow hydrothermal vent plume at the Mid-Atlantic Ridge

Hydrothermal vent fields found at mid-ocean ridges emit hydrothermal fluids which disperse as neutrally buoyant plumes. From these fluids seafloor massive sulfides (SMS) deposits are formed which are being explored as possible new mining sites for (trace) metals and rare earth elements (REE). It has been suggested that during mining activities large amounts of suspended matter will appear in the water column due to excavation processes, and due to discharge of mining waste from the surface vessel. Understanding how hydrothermal plumes can be characterised by means of geochemistry and microbiology as they spread away from their source and how they affect their surrounding environment may help in characterising the behaviour of the dilute distal part of chemically enriched mining plumes. This study on the extensive Rainbow hydrothermal plume, observed up to 25 km downstream from the vent site, enabled us to investigate how microbial communities and (trace) metal composition change in a natural plume with distance. The (trace) metal and REE content of suspended particulate matter (SPM) was determined using HR-ICP mass spectrometry and the microbial communities of the neutrally buoyant plume, above plume-, below plume-, and near-bottom water and sediment were characterised by using 16S rRNA amplicon sequencing methods. Both vertically in the water column and horizontally along the neutrally buoyant plume, geochemical and biological changes were evident as the neutrally buoyant plume stood out by its enrichments in (trace) metals and REEs as e.g. Fe, Cu, V, Mn and REE were enriched by factors of up to ~80, ~90, ~52, ~2.5 and ~40 respectively, compared to above plume water samples taken at 1000 m water depth. The concentrations of these elements changed as the plume aged shown by the decrease of element/Fe molar ratios of chalcophile elements (Cu, Co, Zn), indicative of rapid removal from the hydrothermal plume or removal from the solid phase. Conversely, increasing REE/Fe molar ratios imply uptake of REE from the ambient seawater onto Fe-oxyhydroxides. This was also reflected in the background pelagic system as Epsilonproteobacteria started to dominate and univariate microbial biodiversity declined with distance away from the Rainbow hydrothermal vent field. The Rainbow hydrothermal plume provides a geochemically enriched natural environment, which is a heterogeneous, dynamic habitat that is conducive to ecological changes in a short time span. This study of a hydrothermal plume provides a baseline study to characterize the natural plume before the interference of deep-sea mining.