Insights into the Depth-resolved Geochemical Constraints on Microbial Community Structure and Metabolic Potential for Carbon Cycling in Shallow Subsurface Sediment

Subsurface microorganisms are strongly constrained by geochemistry of sediment. In this study, we aim to study depth-resolved geochemical influence on microbial community structure and metabolic potential for carbon cycling in sediment. The sediment core was sampled near a groundwater well FW306 at Oak Ridge Reservation Field Research Center, Oak Ridge, Tennessee, containing both surface soil and subsurface sediments from vadose, capillary fringe, and saturated zones.

We measured geochemical parameters including pH, moisture, nitrate, sulfate, seven metals, total organic and inorganic carbon in the core. We also applied ultrahigh-resolution mass spectrometry (FT-ICR MS) to characterize dissolved organic carbon (DOC) which is usually considered as bioavailable fraction of natural organic carbon for microbes. Metagenomics was used to survey the structure and functional genes of indigenous microbial communities. A correlation analysis shows that organic carbon is one of the environmental constraints on microbial community structure. The molecular composition and property of DOC change significantly along the depth of the core. Labile carbon such as carbohydrate and tannin are abundant in surface soil and vadose zone, while in deeper sediments from capillary fringe and saturated zones, recalcitrant carbon such as lipid and condense aromatic become the dominant DOC forms. As the property of the DOC pool transitions towards recalcitrant C along the core, the dominant bacterial species shifted accordingly from copiotrophs to oligotrophs. Fresh organic C associated (or preferred) microbial populations such as members of Cyanobacteria, Acidobacteria, and Gammaproteobacteria are dominant in surface soil and vadose zone while members of Chloroflexi and Dehalococcoidia which are usually linked to degradation of more recalcitrant, aromatic compounds and detrital proteins, are dominant in deeper sediments. The microbial metabolic potential for carbon cycling and its correlation with DOC property are still under investigation based on metagenomics data.

Results from this study may provide insights into biogeochemical carbon cycle in subsurface sediment, and highlight the classes of DOC and microbial members that are vital in this process.