Abstract
Microorganisms found in terrestrial subsurface environments make up a large proportion of the Earth’s biomass. Biogeochemical cycles catalyzed by subsurface microbes have the potential to influence the speciation and transport of radionuclides managed in geological repositories. To gain insight on factors that constrain microbial processes within a formation with restricted groundwater flow we performed a meta-community analysis on groundwater collected from multiple discrete fractures underlying the Chalk River Laboratories site (located in Ontario, Canada). Bacterial taxa were numerically dominant in the groundwater. Although these were mainly uncultured, the closest cultivated representatives were from the phenotypically diverse Betaproteobacteria, Deltaproteobacteria, Bacteroidetes, Actinobacteria, Nitrospirae, and Firmicutes. Hundreds of taxa were identified but only a few were found in abundance (>1%) across all assemblages. The remainder of the taxa were low abundance. Within an ecological framework of selection, dispersal and drift, the local and regional diversity revealed fewer taxa within each assemblage relative to the meta-community, but the taxa that were present were more related than predicted by chance. The combination of dispersion at one phylogenetic depth and clustering at another phylogenetic depth suggest both niche (dispersion) and filtering (clustering) as drivers of local assembly. Distance decay of similarity reveals apparent biogeography of 1.5 km. Beta diversity revealed greater influence of selection at shallow sampling locations while the influences of dispersal limitation and randomness were greater at deeper sampling locations. Although selection has shaped each assemblage, the spatial scale of groundwater sampling favored detection of neutral processes over selective processes. Dispersal limitation between assemblages combined with local selection means the meta-community is subject to drift, and therefore, likely reflects the differential historical events that have influenced the current bacterial composition. Categorizing the study site into smaller regions of interest of more closely spaced fractures, or of potentially hydraulically connected fractures, might improve the resolution of an analysis to reveal environmental influences that have shaped these bacterial communities.
Highlights
Biogeochemical cycles of the subsurface have the potential to influence the speciation and transport of radionuclides managed in geological repositories
We found that at the phylogenetic depth for mean pair wise distance (MPD) (Figure 4, middle panel), the observed MPD values for all the assemblages were within null distribution, which indicates the assemblages were dispersed evenly relative to the meta-community
We posit that the microbial assemblages within the terrestrial subsurface provide additional evidence that microbial diversity follows an apparent pattern of biogeography on scales of a few millimeters to thousands of kilometers (Martiny et al, 2006), challenging the once long-standing assumption of unlimited dispersal of microorganisms that are selected by the environment alone
Summary
Biogeochemical cycles of the subsurface have the potential to influence the speciation and transport of radionuclides managed in geological repositories. There is limited knowledge on how microbial diversity relates to biogeochemical processes (Griebler and Lueders, 2009) such as the flow of energy through an ecosystem, and how elements like carbon are recycled These processes involve metabolism and competition, where microorganisms act as catalysts and the available free energy supports a community sensing and responding to environmental changes (Lever, 2011; Jorgensen et al, 2012; Flynn et al, 2013; Algora et al, 2015). While these interactions can connect the supply of electron donor and acceptor compounds to abundant taxa, when viewed at multiple spatial scales, random factors like dispersal, speciation and extinction influence diversity (Hubbell, 2001). Taxa distributions between sampling locations, can stem from a combination of competition, environmental constraints, differences in dispersal among a regional pool of taxa and drift due to dispersal limitation
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