Abstract
South Africa has numerous thermal springs that represent topographically driven meteoric water migrating along major fracture zones. The temperature (40–70°C) and pH (8–9) of the thermal springs in the Limpopo Province are very similar to those of the low salinity fracture water encountered in the South African mines at depths ranging from 1.0 to 3.1 km. The major cation and anion composition of these thermal springs are very similar to that of the deep fracture water with the exception of the dissolved inorganic carbon and dissolved O2, both of which are typically higher in the springs than in the deep fracture water. The in situ biological relatedness of such thermal springs and the subsurface fracture fluids that feed them has not previously been evaluated. In this study, we evaluated the microbial diversity of six thermal spring and six subsurface sites in South Africa using high-throughput sequencing of 16S rRNA gene hypervariable regions. Proteobacteria were identified as the dominant phylum within both subsurface and thermal spring environments, but only one genera, Rheinheimera, was identified among all samples. Using Morisita similarity indices as a metric for pairwise comparisons between sites, we found that the communities of thermal springs are highly distinct from subsurface datasets. Although the Limpopo thermal springs do not appear to provide a new window for viewing subsurface bacterial communities, we report that the taxonomic compositions of the subsurface sites studied are more similar than previous results would indicate and provide evidence that the microbial communities sampled at depth are more correlated to subsurface conditions than geographical distance.
Highlights
Whitman et al (1998) estimated that the terrestrial subsurface biosphere comprises 40–50% of the world’s biomass; comprehensive surveys of its phylogenetic diversity and distribution are geographically sparse relative to the immense volume the subsurface encompasses
The lack of overlap in the microbial communities observed between the Limpopo thermal springs and South African subsurface sites, despite their physical-chemical similarities, suggest that thermal springs arising from gravity-driven meteoric water flow may not provide the clearest windows to the terrestrial subsurface
The geographic distance between the thermal springs and the subsurface sites and the fact that four of the springs occur within the Karoo Supergroup, where no subsurface samples have been studied, may account for this lack of sequence overlap
Summary
Whitman et al (1998) estimated that the terrestrial subsurface biosphere comprises 40–50% of the world’s biomass; comprehensive surveys of its phylogenetic diversity and distribution are geographically sparse relative to the immense volume the subsurface encompasses. Deming and Baross (1993) were among the first to propose that deep-sea hydrothermal vents could provide “windows” to the deep biosphere of the oceanic crust This was based upon the reasoning that only subsurface hyperthermophiles could survive within the hydrothermal vent fluids and that these fluids are representative of the sub-seafloor ocean crust at ridges and, by extension, the deep ocean crust globally. The reasoning is that the extremely high pH of these fluids (pH 10–12) can only support subsurface alkaliphilic microorganisms and that these fluids are representative of the ultrabasic serpentinite at depth These springs provide an attractive target for sampling the terrestrial subsurface at lower cost; only a handful of terrestrial serpentinite sites are known and studied. In order to obtain a global picture of the deep terrestrial subsurface, more surficial, terrestrial portals need to be studied and the means for identifying which microorganisms are truly “denizens”
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