Aquifers are inhabited by microorganisms from the three major domains of life: Archaea, Eukaryotes and Bacteria. Although interest in the processes that govern the assembly of these microbial communities is growing, their study is almost systematically limited to one of the three domains of life. Archaea, Bacteria and Eukaryotes are however interconnected and essential to understand the functioning of their living ecosystems. We, therefore, conducted a spatial study of the distribution of microorganisms by sampling 35 wells spread over an area of 10,000 km2 in the Quebec region (Canada). The obtained data allowed us to define the impact of geographic distance and geochemical water composition on the microbial communities. A null model approach was used to infer the relative influence of stochastic and determinist ecological processes on the assembly of the microbial community from all three domains. We found that the organisms from these three groups are mainly governed by stochastic mechanisms. However, this apparent similarity does not reflect the differences in the processes that govern the phyla assembly. The results obtained highlight the importance of considering all the microorganisms without neglecting their individual specificities.
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