Iron and sulfate reduction structure microbial communities in (sub-)Antarctic sediments

Permanently cold marine sediments are heavily influenced by increased input of iron as a result of accelerated glacial melt, weathering, and erosion. The impact of such environmental changes on microbial communities in coastal sediments is poorly understood. We investigated geochemical parameters that shape microbial community compositions in anoxic surface sediments of four geochemically differing sites (Annenkov Trough, Church Trough, Cumberland Bay, Drygalski Trough) around South Georgia, Southern Ocean. Sulfate reduction prevails in Church Trough and iron reduction at the other sites, correlating with differing local microbial communities. Within the order Desulfuromonadales, the family Sva1033, not previously recognized for being capable of dissimilatory iron reduction, was detected at rather high relative abundances (up to 5%) while other members of Desulfuromonadales were less abundant (<0.6%). We propose that Sva1033 is capable of performing dissimilatory iron reduction in sediment incubations based on RNA stable isotope probing. Sulfate reducers, who maintain a high relative abundance of up to 30% of bacterial 16S rRNA genes at the iron reduction sites, were also active during iron reduction in the incubations. Thus, concurrent sulfate reduction is possibly masked by cryptic sulfur cycling, i.e., reoxidation or precipitation of produced sulfide at a small or undetectable pool size. Our results show the importance of iron and sulfate reduction, indicated by ferrous iron and sulfide, as processes that shape microbial communities and provide evidence for one of Sva1033’s metabolic capabilities in permanently cold marine sediments

Pore water geochemistry of surface sediments around South Georgia (Annenkov Trough GeoB22054-2, Church Trough GeoB22031-1, Cumberland Bay GeoB22046-1, Drygalski Trough GeoB22015-1)

The permanently cold sediments of South Georgia were investigated in this study to identify the geochemical parameters that are linked to microbial community composition. To identify these parameters, pore water profiles of sulfate, sulfide, dissolved iron, ammonium, silicate, dissolved inorganic carbon, and phosphate were analysed. The results presented here were obtained during the RV METEOR M134 expedition (January - March, 2017) and contribute to our understanding of microbial community composition in surface sediments of South Georgia