Abstract
Analyses of microbial diversity in marine sediments have identified a core set of taxa unique to the marine deep biosphere. Previous studies have suggested that these specialized communities are shaped by processes in the surface seabed, in particular that their assembly is associated with the transition from the bioturbated upper zone to the nonbioturbated zone below. To test this hypothesis, we performed a fine-scale analysis of the distribution and activity of microbial populations within the upper 50 cm of sediment from Aarhus Bay (Denmark). Sequencing and qPCR were combined to determine the depth distributions of bacterial and archaeal taxa (16S rRNA genes) and sulfate-reducing microorganisms (SRM) (dsrB gene). Mapping of radionuclides throughout the sediment revealed a region of intense bioturbation at 0–6 cm depth. The transition from bioturbated sediment to the subsurface below (7 cm depth) was marked by a shift from dominant surface populations to common deep biosphere taxa (e.g., Chloroflexi and Atribacteria). Changes in community composition occurred in parallel to drops in microbial activity and abundance caused by reduced energy availability below the mixed sediment surface. These results offer direct evidence for the hypothesis that deep subsurface microbial communities present in Aarhus Bay mainly assemble already centimeters below the sediment surface, below the bioturbation zone.
Highlights
Marine sediments are a massive microbial habitat, harboring as many as 5.39 × 1029 prokaryotic cells on a global scale and 0.18–3.6% of Earth’s total living biomass (Kallmeyer et al, 2012; Parkes et al, 2014)
By converting operational taxonomic units (OTUs) relative abundances to absolute abundances using quantitative PCR (qPCR) data (Starnawski et al, 2017), we found that several OTUs increased in absolute abundance with depth, running counter to the drop in absolute abundance observed for the total microbial community and the majority of dominant surface OTUs (Figure 4)
Four processes—diversification, dispersal, selection, and drift, have been proposed as major drivers of microbial community assembly (Nemergut et al, 2013). These processes have been examined within the context of the marine subsurface environment, suggesting that subsurface microbial communities predominantly assemble by selective survival of buried taxa (Jochum et al, 2017; Petro et al, 2017; Starnawski et al, 2017)
Summary
Marine sediments are a massive microbial habitat, harboring as many as 5.39 × 1029 prokaryotic cells on a global scale and 0.18–3.6% of Earth’s total living biomass (Kallmeyer et al, 2012; Parkes et al, 2014). This burial can isolate microorganisms from the surface world for hundreds to millions of years, cutting populations off from fresh detrital organic matter and subjecting them to severe energetic limitations that increase with sediment depth (Middelburg, 1989; Langerhuus et al, 2012; Lomstein et al, 2012) Despite these energy limitations, deep marine sediments are populated by living microbial communities. It was suggested that the assembly of subsurface communities is associated with the transition from the bioturbated surface zone of the sediment to the unmixed sediment zones below (Jochum et al, 2017; Starnawski et al, 2017) To test this hypothesis, we performed a fine-scale analysis of the distribution and activity of microbial populations present within the upper 50 cm of sediment from Aarhus Bay (Denmark). Sulfate reduction rates were measured as a proxy for microbial activity and overall organic carbon mineralization in the sediment and the vertical extent of the bioturbation zone was mapped by determining radionuclide distributions
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