The microbial lipid signature in sediments and chimneys within a back-arc basin hydrothermal system south of the Antarctic Polar Front

The impact of hydrothermal systems on surrounding sedimentary microbial communities is not well understood and previous work has been limited to high temperature vent sites at slow or ultraslow spreading oceanic centres. To build on the current understanding of hydrothermal systems, we explore for the first time the organic geochemistry of the only known back-arc basin hydrothermal system outside the Pacific Ocean: the East Scotia Ridge (ESR), which belongs to the South Georgia and South Sandwich Islands Marine Protected Area. Lipid biomarkers contained in sediments and hydrothermal sulphides along two hydrothermal vent fields north and south of the ESR, respectively, revealed the impact of hydrothermal activity, including both high temperature and low temperature diffusive venting, on sedimentary microbial communities. In the vent field north of the ESR, elevated ring indices of glycerol dialkyl glycerol tetraethers (GDGTs) and proportions of monoalkyl glycerol tetraethers (GMGTs), and a high ratio of total fatty acids (FAs; free plus polar lipids) to putative phytoplankton biomarkers in sediments suggest that high-temperature hydrothermalism has a local impact on surrounding sediments through the input of plume dwelling archaea and bacteria. This impact seems to be restricted to the periphery of the vent source, in agreement with the limited dataset available from slow or ultraslow spreading centres. Likewise, elevated FA to phytoplankton biomarker ratios within a diffusive hydrothermal field south of the ESR suggest an additional input of bacterial biomass relative to background sediments. Our results indicate that low temperature diffusive venting might have a higher impact than previously thought, being locally important in supporting the food chain in deep-sea environments. The distribution of tetraether lipids suggests that a higher proportion of thermophilic archaea thrive in the interior of sulphide chimneys, whereas total FA concentrations and distributions suggest that most bacteria inhabit the exterior chimney layers, where temperature is cooler than the innermost layer in contact with the hydrothermal fluid. Furthermore, differences in total FA concentrations suggest that chimney wall thickness is a control on bacterial abundance through the availability of a higher or lower diversity (and volume) of microhabitats. Our results also indicate that bacteria adapt to increasing temperatures by decreasing their degree of unsaturation. By comparison to GDGT data from other settings, it seems that overall ring indices in hydrothermal deposits are governed by growth temperature, although they might also reflect ecological factors. Our results suggest that hydrothermalism shapes microbial communities within chimneys and surrounding sediments following broadly similar patterns regardless of the type of spreading centre they are located at.