Key factors controlling microbial distribution on a DNAPL source area

Abstract

Chlorinated solvents are among the common groundwater contaminants that show high complexity in their distribution in the subsoil. Microorganisms play a vital role in the natural attenuation of chlorinated solvents. Thus far, how the in situ soil microbial community responds to chlorinated solvent contamination has remained unclear. In this study, the microbial community distribution within two boreholes located in the source area of perchloroethene (PCE) was investigated via terminal restriction fragment length polymorphism (T-RFLP) and clone library analysis. Microbial data were related to the lithological and geochemical data and the concentration and isotopic composition of chloroethenes to determine the key factors controlling the distribution of the microbial communities. The results indicated that Proteobacteria, Actinobacteria, and Firmicutes were the most abundant phylums in the sediment. The statistical correlation with the environmental data proved that fine granulometry, oxygen tolerance, terminal electron-acceptor processes, and toxicity control microbial structure. This study improves our understanding of how the microbial community in the subsoil responds to high concentrations of chlorinated solvents.

Morphology and Size of Bacterial Colonies Control Anoxic Microenvironment Formation in Porous Media

“…Processes using less thermodynamically favorable electron acceptors than O 2 (such as nitrates, iron oxides, sulfates, and CO 2 ) are typically confined to deeper and reducing environments. ,, Surprisingly, recent investigations have shown the occurrence of nonaerobic metabolisms in well-oxygenated regions of the subsurface. − For example, oxygenated and well-drained superficial soils largely contribute to methane and NO X emissions generated by facultative methanotrophs and denitrifiers . Also, Mn reduction and reductive dehalogenation are active processes in oxic aquifers and are crucial players in the natural attenuation and bioremediation of recalcitrant pollutants that accumulate in groundwater. , …”

“…Anoxic microenvironments have been observed to occupy no more than 10% of the pore space, are generally smaller than a few millimeters, and persist from hours to days. ,, Despite the small portion of pore volume occupied, anoxic microenvironments are nowadays considered fundamental to explain the dynamics of many macroscale ecological processes such as element cycling (e.g., soil carbon stabilization and sulfur reduction and precipitation), greenhouse gas production (e.g., methanogenesis , and NO x emissions ,, ), heavy metal mobilization, , and natural attenuation of recalcitrant pollutants that accumulate in groundwater. , …”

“…9 Also, Mn reduction and reductive dehalogenation are active processes in oxic aquifers and are crucial players in the natural attenuation and bioremediation of recalcitrant pollutants that accumulate in groundwater. 14,15 This apparent paradox results from our limited capacity to fully characterize the pore-scale heterogeneity of the geochemical environment experienced by bacteria in the subsurface. 16 Depending on the sampling technique, bulk O 2 measurements typically capture the average oxygenation level over the centimeter−meter scale.…”

“…4,10,29 Despite the small portion of pore volume occupied, anoxic microenvironments are nowadays considered fundamental to explain the dynamics of many macroscale ecological processes such as element cycling (e.g., soil carbon stabilization 13 and sulfur reduction and precipitation 30 ), greenhouse gas production (e.g., methanogenesis 31,32 and NO x emissions 17,29,33 ), heavy metal mobilization, 34,35 and natural attenuation of recalcitrant pollutants that accumulate in groundwater. 14,15 Yet, a quantitative understanding and predictability of anoxic microenvironment formation in oxic subsurface porous environments are missing due to major methodological limitations. Indeed, direct in situ monitoring of biomass growth and O 2 distribution at the colony scale remains challenging due to (i) the opacity of porous matrices of soils and sediments and (ii) the difficulties in simultaneously capturing the different spatial and temporal scales of interest for anoxic microenvironment formation.…”