Non-random processes determine the colonization of groundwater sediments by microbial communities in a pristine porous aquifer.

Sediments accommodate the dominating share of groundwater microbiomes, however the processes that govern the assembly and succession of sediment-attached microbial communities in groundwater aquifers are not well understood. To elucidate these processes, we followed the microbial colonization of sterile sediments in in situ microcosms that were exposed to groundwater for almost 1 year at two distant but hydrologically connected sites of a pristine, shallow, porous aquifer. Our results revealed intriguing similarities between the community succession on the newly-colonized sediments and succession patterns previously observed for biofilms in other more dynamic aquatic environments, indicating that the assembly of microbial communities on surfaces may be governed by similar underlying mechanisms across a wide range of different habitats. Null model simulations on spatiotemporally resolved 16S rRNA amplicon sequencing data further indicated selection of specific OTUs rather than random colonization as the main driver of community assembly. A small fraction of persistent OTUs that had established on the sediments during the first 115 days dominated the final communities (68%-85%), suggesting a key role of these early-colonizing organisms, in particular specific genera within the Comamonadaceae and Oxalobacteraceae, for community assembly and succession during the colonization of the sediments. Overall, our study suggests that differences between planktonic and sediment-attached communities often reported for groundwater environments are not the result of purely stochastic events, but that sediment surfaces select for specific groups of microorganisms that assemble over time in a reproducible, non-random way.