May microbial ecological baseline exist in continental groundwater?

Background
Ecological baseline as a fundamental reference with less human interference is of great significance for understanding ecological evolution and global biogeochemical cycles, but the existence of microbial ecological baseline on subsurface groundwater is still an open question so far.

Results
Based on the high-throughput sequencing information derived from a national monitoring of 733 newly constructed wells, we find that the groundwater microbial communities exhibit significant lateral diversity gradient, and gradually approach the topsoil microbial latitudinal diversity gradient with decreasing burial depth of phreatic water. Moreover, Patescibacteria act as keystone taxa that harmonize microbes in shallower aquifers and accelerate decline in bacterial diversity with increasing well-depth, though Proteobacteria are most abundant in the groundwater bacterial community of 74 phyla. Decreasing habitat niche breadths with increasing well-depth suggests a general change in relationship among key microbes from closer cooperation in shallow to stronger competition in deep groundwater. Unlike surface-water microbes, microbial communities in pristine groundwater are predominantly shaped by deterministic processes, potentially associated with nutrient sequestration under dark and anoxic environments in aquifers.

Conclusions
By unveiling the biogeography and ecological driver of microbes in pristine groundwater throughout China, we firstly confirm the existence of groundwater microbial ecological baseline (GMEB) in shallower aquifers, and highlights the importance of GMEB in evaluation of anthropogenic impacts on the unknown subterranean ecosystems.