Abstract
Clay layers are common in subsurface where microbial activities play an important role in impacting the biogeochemical properties of adjacent aquifers. In this study, we analyzed the community structure and abundance of bacteria and archaea in response to geochemical properties of six clay sediments at different depths in a borehole (112°34′0″E, 30°36′21″N) of Jianghan Plain (JHP), China. Our results suggested that the top two clay layers were oxic, while the remaining bottom four clay layers were anoxic. Both high-throughput sequencing and qPCR of 16S rRNA gene showed relatively high abundance of archaea (up to 60%) in three of the anoxic clay layers. Furthermore, microbial communities in these clay sediments showed distinct vertical stratification, which may be impacted by changes in concentrations of sulfate, HCl-extractable Fe2+ and total organic carbon (TOC) in the sediments. In the upper two oxic clay layers, identification of phyla Thaumarchaeota (11.2%) and Nitrosporales (1.2%) implied nitrification in these layers. In the two anoxic clay layers beneath the oxic zone, high abundances of Anaeromyxobacter, Chloroflexi bacterium RBG 16_58_14 and Deltaproteobacteria, suggested the reductions of nitrate, iron and sulfate. Remarkably, a significant portion of Bathyarchaeota (∼25%) inhabited in the bottom two anoxic clay layers, which may indicate archaeal anaerobic degradation of TOC by these organisms. The results of this study provide the first systematic understandings of microbial activities in subsurface clay layers at JHP, which may help develop microorganism-based solutions for mitigating subsurface contaminations.
Highlights
Clays and clay minerals are abundant in soils, sediments, and subsurface (Dong et al, 2009; Dong, 2012)
Interactions between and among variables only explained
Microbial community composition residing in different subsurface clay layers of JHP generally consisted of 67.5% bacterial phyla, 29.5% archaeal phyla, and 3% unclassified phyla
Summary
Clays and clay minerals are abundant in soils, sediments, and subsurface (Dong et al, 2009; Dong, 2012). The clays rich in aluminum oxides (Al2O3), inhibit the growth of sulfate-reducing bacteria (SRB) (Wong et al, 2004) This is consistent with the observations that microbial sulfate reduction activity in subsurface clay layers is lower compared to that in the adjacent sand layers (McMahon and Chapelle, 1991; Ulrich et al, 1998). It was suggested that microorganisms in the subsurface clay layers converted complex organic matters into low-molecular-mass fatty acids and hydrogen gas (H2) via fermentation These fermentation products diffused into the surrounding environments (e.g., aquifers) where they were used by SRB as electron donors to reduce sulfate (McMahon and Chapelle, 1991; McMahon et al, 1992; Krumholz et al, 1997; Fredrickson and Balkwill, 2006). Microbial activity in subsurface clay layers influences the diagenesis of subsurface sediments, such as aquitard (McMahon et al, 1992; Hesse and Schacht, 2011)
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