Persistence and dispersal of microbial populations in subsurface marine sediments

Microbial subsurface sediments have been shown to remineralize up to 90 % of the inorganic carbon that is deposited on the sediment surface and slowly buried with time (Jorgensen et al., 2012) indicating the significance of the subsurface microbial community. Little is known however on the community assembly of these microbial communities found in the deep biosphere. Four possible influences have been suggested: Selection, diversification, dispersal and drift (Nemergut et al., 2013). In this study the effect of selection as well as diversification on the community assembly was analysed with regard to the role that the bioturbation zone plays as harbouring a ‘seed’ community for the subsurface microbial community. The microbial community of Aarhus Bay station M5 was quantified using qPCR and cell counts. A drop in cell counts in the surface sediment (first 8 cm) was observed with both methods which corresponds to the end of the bioturbation zone. The cell abundance below the bioturbation zone showed less variance. The community composition was analysed with 16S rRNA gene sequencing showing that Proteobacteria dominate the bioturbated zone but decrease in their abundance rapidly in the sediment below. In contrast, Chloroflexi, Planctomycetes and Candidate division OP9 increased in their importance on the community composition below the bioturbation zone. The bottom of the bioturbation zone (7 cm) showed the most pronounced change in the community composition compared to the surface sediment. Below 7 cm, the change of the microbial community composition is gradual, suggesting that the microbial community at the bottom of the bioturbation zone acts as ‘seed’ community for the subsurface microbial community. The decrease in less adapted species (Proteobacteria) to the different geochemical zones and the energy-limited subsurface sediment in combination with an increase in better adapted species (Chloroflexi, Planctomycetes, Candidate division OP9) points towards selection playing a major role in the community assembly of the deep biosphere. The fraction of motile cells of the total community has been analysed using needles filled with pore water that were incubated in sediment from Aarhus Bay station M5 and Marselisborg Havn. Motile cells only made up about 1.8 % of the microbial community in the surface sediment, decreasing in importance to about 0.2 % at 50 cm sediment depth. This suggests a rather small influence of dispersal on the community composition