Distinct Depth-discrete Profiles of Microbial Communities and Geochemical Insights in the Subsurface Critical Zone

Microbial assembly and metabolic potential in the Subsurface Critical Zone(SCZ) are substantially impacted by subsurface geochemistry andhydrogeology, selecting for microbes distinct from those in surficialsoils. In this study, we integrated metagenomics and geochemistry toelucidate how microbial composition and metabolic potential are shaped andimpacted by vertical variations in geochemistry and hydrogeology interrestrial subsurface sediment. A sediment core from an uncontaminated,pristine well at Oak Ridge Field Research Center in Oak Ridge, Tennessee,including the shallow subsurface, vadose zone, capillary fringe, andsaturated zone, was used in this study. Our results showed that subsurfacemicrobes were highly localized, and communities were rarelyinter-connected. Microbial community composition as well as metabolicpotential in carbon and nitrogen cycling varied even over very shortvertical distances. Further analyses indicated strong depth-relatedco-variation of community composition with a subset of 12 environmentalvariables. Analysis of dissolved organic carbon (DOC) quality viaultrahigh resolution mass spectrometry suggested that the SCZ wasgenerally a low-carbon environment with the relative portion of labile DOCdecreasing and that of recalcitrant DOC increasing along the depth,selecting microbes from copiotrophs to oligotrophs and also impacting themicrobial metabolic potential in C cycle. Our study demonstrates thatsediment geochemistry and hydrogeology are vital in selecting distinctmicrobial population and metabolism in the SCZ.