Depth Distribution and Assembly of Sulfate-Reducing Microbial Communities in Marine Sediments of Aarhus Bay

Abstract

Most sulfate-reducing microorganisms (SRMs) present in subsurface marine sediments belong to uncultured groups only distantly related to known SRMs, and it remains unclear how changing geochemical zones and sediment depth influence their community structure. We mapped the community composition and abundance of SRMs by amplicon sequencing and quantifying the gene, which encodes dissimilatory sulfite reductase subunit beta, in sediment samples covering different vertical geochemical zones ranging from the surface sediment to the deep sulfate-depleted subsurface at four locations in Aarhus Bay, Denmark. SRMs were present in all geochemical zones, including sulfate-depleted methanogenic sediment. The biggest shift in SRM community composition and abundance occurred across the transition from bioturbated surface sediments to nonbioturbated sediments below, where redox fluctuations and the input of fresh organic matter due to macrofaunal activity are absent. SRM abundance correlated with sulfate reduction rates determined for the same sediments. Sulfate availability showed a weaker correlation with SRM abundances and no significant correlation with the composition of the SRM community. The overall SRM species diversity decreased with depth, yet we identified a subset of highly abundant community members that persists across all vertical geochemical zones of all stations. We conclude that subsurface SRM communities assemble by the persistence of members of the surface community and that the transition from the bioturbated surface sediment to the unmixed sediment below is a main site of assembly of the subsurface SRM community. Sulfate-reducing microorganisms (SRMs) are key players in the marine carbon and sulfur cycles, especially in coastal sediments, yet little is understood about the environmental factors controlling their depth distribution. Our results suggest that macrofaunal activity is a key driver of SRM abundance and community structure in marine sediments and that a small subset of SRM species of high relative abundance in the subsurface SRM community persists from the sulfate-rich surface sediment to sulfate-depleted methanogenic subsurface sediment. More generally, we conclude that SRM communities inhabiting the subsurface seabed assemble by the selective survival of members of the surface community.

Glacial runoff promotes deep burial of sulfur cycling-associated microorganisms in marine sediments

“…OTU correlation analysis showed that OTU clusters and thus the microbial communities of young and old sediment zones in NGI sediments became ‘disconnected’ at a sediment age of about 300-400 years (Figure 3), corresponding to a sediment depth of approximately 30 cmbsf at NGI stations. This is in line with observations that a considerable shift in microbial community structures occur in marine sediments below the zone of bioturbation, which was suggested to be the main site of assembly of the subsurface community (Jochum et al, 2017; Starnawski et al, 2017). Most OTUs that positively correlated with sediment ages in NGI sediments were affiliated with lineages known to harbor members that selectively persist from the surface into deep subsurface sediments, e.g., Chloroflexi, Aerophobetes, Atribacter (JS1), Aminicenantes (OP8), Alphaproteobacteria and Deltaproteobacteria (Supplementary Figure S5) (Orcutt et al, 2011; Wang et al, 2016).…”

Sub‐seafloor biogeochemical processes and microbial life in the Baltic Sea

“…This corresponded to a drop from about 25% to 5% of the total 16S rRNA gene copy numbers or from 10% to 1% of the total microscopic cell counts, which is rather similar to other data from Aarhus Bay obtained by Jochum et al . (). From these cell numbers, they calculated that the mean cell‐specific sulphate reduction rates dropped from 0.06 to 0.003 fmol cell −1 d −1 .…”

“…It is an important question whether life in the deep biosphere requires mutations relative to the surface microorganisms or unique adaptations to subsist with only the basal power requirement of microbial life (Hoehler and Jørgensen, ). Several studies have shown that deep sub‐seafloor communities assemble at the bottom of the bioturbated sediment zone, which is generally at 10–20 cm depth in coastal sediments and that the strongly energy‐limited life conditions of the deep biosphere start there (Chen et al ., ; Jochum et al ., ; Starnawski et al ., ; Marshall et al ., ) (see below). We therefore conclude that the bottom of the bioturbated zone functionally constitutes an upper boundary of the deep biosphere, rather than a fixed subsurface depth as has been mostly used in the literature (e.g., Whitman et al ., ; Kallmeyer et al ., ).…”

Diversity decoupled from sulfur isotope fractionation in a sulfate‐reducing microbial community

“…If this is indeed the case, we can place lower limits on this threshold of dsrB Shannon diversity at a value of 0.8 and dsrB Chao1 species richness at a value of 15. If most natural environments fall into this category, with diversity above these thresholds (e.g., Jochum et al ()), understanding the specific mechanisms of fractionation within taxa is less important to determining net fractionation and interpreting the sulfur isotope rock record than are environmental conditions. This further justifies examining the relationship between diversity and net fractionation in vitro, on artificial communities of small sizes, that may be representative of the functional diversity present when sulfate reduction appeared in geological time.…”