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Aquifers are rich in microbial diversity. However, there is a lack of information about sessile communities in these environments because of the difficulty in sampling fresh in situ rock surfaces. Thus, this study's objective was to better understand the sessile community in a fractured aquifer. Additionally, the impact of the rock mineral composition on microbial community composition during colonization was explored. Using a system of bioreactors, we recreated the environmental conditions of a 1.5 m deep aquifer in Covey Hill (QC, Canada) using groundwater samples collected from the site. We carried out 16S/18S rRNA amplicon sequencing of the water and sessile communities after 24 days of incubation. Our data showed that many microbial taxa overlapped between the sessile and planktonic communities, indicating colonization of the solid surfaces. Quartz and feldspar had a significant impact on bacterial community structure. Sessile communities were dominated by Gaillonella, Alkanindiges, unclassified Acetobacteraceae, Apoikiales, Glissomonadida, and Synurales. We could not detect any Archaea in the sessile community. The sessile communities contained bacterial genera involved in iron cycling and adapted to acidic and low-carbon-concentration environments. Eukaryotic predators dominated the sessile community. [ABSTRACT FROM AUTHOR]

Groundwater recharge and discharge rates and zones are important hydrogeological characteristics of aquifer systems, yet their impact on the formation of both subterranean and surface microbiomes remains largely unknown. In this study, we used 16S rRNA gene sequencing to characterize and compare the microbial community of seven different aquifers, including the recharge and discharge areas of each system. The connectivity between subsurface and surface microbiomes was evaluated at each site, and the temporal succession of groundwater microbial communities was further assessed at one of the sites. Bacterial and archaeal community composition varied between the different sites, reflecting different geological characteristics, with communities from unconsolidated aquifers being distinct from those of consolidated aquifers. Our results also revealed very little to no contribution of surface recharge microbial communities to groundwater communities as well as little to no contribution of groundwater microbial communities to surface discharge communities. Temporal succession suggests seasonal shifts in composition for both bacterial and archaeal communities. This study demonstrates the highly diverse communities of prokaryotes living in aquifer systems, including zones of groundwater recharge and discharge, and highlights the need for further temporal studies with higher resolution to better understand the connectivity between surface and subsurface microbiomes. [ABSTRACT FROM AUTHOR]

Groundwater environments provide habitats for diverse microbial communities, and although Archaea usually represent a minor fraction of communities, they are involved in key biogeochemical cycles. We analysed the archaeal diversity within a mixed carbonate-rock/siliciclastic-rock aquifer system, vertically from surface soils to subsurface groundwater including aquifer and aquitard rocks. Archaeal diversity was also characterized along a monitoring well transect that spanned surface land uses from forest/woodland to grassland and cropland. Sequencing of 16S rRNA genes showed that only a few surface soil-inhabiting Archaea were present in the groundwater suggesting a restricted input from the surface. Dominant groups in the groundwater belonged to the marine group I (MG-I) Thaumarchaeota and the Woesearchaeota. Most of the groups detected in the aquitard and aquifer rock samples belonged to either cultured or predicted lithoautotrophs (e.g., Thaumarchaeota or Hadesarchaea). Furthermore, to target autotrophs, a series of 13 CO 2 stable isotope-probing experiments were conducted using filter pieces obtained after filtration of 10,000 L of groundwater to concentrate cells. These incubations identified the SAGMCG Thaumarchaeota and Bathyarchaeota as groundwater autotrophs. Overall, the results suggest that the majority of Archaea on rocks are fixing CO 2 , while archaeal autotrophy seems to be limited in the groundwater.