COMPARING COMMUNITY COMPOSITION BETWEEN ULTRA-SMALL AND LARGER SIZED BACTERIA AND ARCHAEA IN AQUIFERS

Aquifers are extreme oligotrophic environments populated by a vast diversity of microorganism well adapted to life in such conditions. The study of these microbial communities has led to the discovery of ultra-small bacteria and archaea, capable of passing through a 0.2 μm pore size sample filter. Recent studies suggest that these ultra-small microbes are omnipresent and can numerically dominate in aquatic ecosystems. Here we investigated how community composition differs between these ultra-small communities (USC) and larger-sized communities (LSC) in two shallow aquifers located in Rigaud, Québec. Both aquifers differ in geological formation (sand/clay) and physicochemical groundwater parameters. We analyzed the USC and LSC using Illumina amplicon sequencing (16S rRNA genes), in both aquifers. Community composition between USC and LSC is statistically different (PERMANOVA, p < 0.05). The Shannon diversity index of the USC was lower than the diversity index of LSC for both bacteria and archaea. Most bacteria from the USC belonged to the Firmicute phylum while Proteobacteria dominated the LSC, and for archaea, the main phylum present were Nanoarchaeota and Crenarchaeota for USC and LSC respectively. The USC community composition varied greatly over the course of four months (fall season) suggesting these communities might be more susceptible to hydrogeochemical changes. The clustering of both groups at the phylum level revealed an overlapping between USC and LSC which confirms previous studies suggesting cell size varies considerably within a phylum. Understanding the dynamics of USC composition and the role of these organism in groundwater ecosystems is crucial to protect groundwater resources as the ubiquitous presence of ultra-small bacteria and archaea may suggest that some may play a critical role in subsurface geochemical cycling and the degradation of anthropic pollutant.