Abstract
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.
Highlights
IntroductionOrganic matter degradation is the main source of electron donors and carbon for microbial metabolism in marine
Organic matter degradation is the main source of electron donors and carbon for microbial metabolism in marineThese authors contributed : Lea C
In the sediments at the iron-rich sites, SO42− concentrations stayed stable with depth (~28 mM) with only minor decreases observed in the surface sediments collected in Cumberland Bay
Summary
Organic matter degradation is the main source of electron donors and carbon for microbial metabolism in marine. Sulfate, which is generally present in high concentrations in the water column (~28 mM), is supplied to the sediment by downward diffusion, accelerated by bio-irrigation and other advective processes [7, 8, 17]. It is the final product of reoxidation of sulfide [18, 19], which itself is produced by sulfate reduction [20], potentially resulting in a cryptic sulfur cycle [18, 19, 21]. While iron reduction is favored in certain marine settings [23, 24], organic matter oxidation by sulfate reduction is often more important than iron reduction in marine sediments [7, 8], a competition shown to be regulated by the availability and reactivity of organic matter and ferric iron [22, 25]
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