Using Microbial Community Gene Expression to Highlight Key Biogeochemical Processes
A study of gene expression in an aquifer reveals unexpectedly diverse microbial metabolism in biogeochemical hot spots.
Researchers conducted a study of naturally reduced zones (NRZs)—biogeochemical hot spots—in the Rifle, Colo., aquifer, a legacy Department of Energy uranium mill site. They performed a state-of-the-art analysis of gene expression in the aquifer’s microbial communities, elucidating metabolic pathways and organisms underlying observed biogeochemical phases as well as revealing unexpected metabolic activities.
NRZs, organic-rich deposits heterogeneously distributed in alluvial aquifers, modulate aquifer redox status and influence the speciation and mobility of metals. Overall, NRZs have an outsized effect on groundwater geochemistry. This study’s results highlight the complex nature of organic matter transformation in NRZs and the microbial metabolic pathways that interact to mediate redox status and elemental cycling.
Organic matter deposits in alluvial aquifers have been shown to result in the formation of NRZs, which can modulate aquifer redox status and influence the speciation and mobility of metals, significantly affecting groundwater geochemistry. In this study, researchers sought to better understand how natural organic matter fuels microbial communities within anoxic biogeochemical hot spots (or NRZs) in a shallow alluvial aquifer at the Rifle site. The researchers conducted an anaerobic microcosm experiment in which NRZ sediments served as the sole source of electron donors and microorganisms. Biogeochemical data indicated that native organic matter decomposition occurred in different phases, beginning with the mineralization of dissolved organic matter (DOM) to carbon dioxide (CO2) during the first week of incubation. This was followed by a pulse of acetogenesis that dominated carbon flux after two weeks. DOM depletion over time was strongly correlated with increases in the expression of many genes associated with heterotrophy (e.g., amino acid, fatty acid, and carbohydrate metabolism) belonging to a Hydrogenophaga strain that accounted for a relatively large percentage (roughly 8%) of the metatranscriptome. This Hydrogenophaga strain also expressed genes indicative of chemolithoautotrophy, including CO2 fixation, dihydrogen (H2) oxidation, sulfur compound oxidation, and denitrification. The acetogenesis pulse appeared to have been collectively catalyzed by a number of different organisms and metabolisms, most prominently pyruvate:ferredoxin oxidoreductase. Unexpected genes were identified among the most highly expressed (more than 98th percentile) transcripts, including acetone carboxylase and cell-wall-associated hydrolases with unknown substrates. Many of the most highly expressed hydrolases belonged to a Ca. Bathyarchaeota strain and may have been associated with recycling of bacterial biomass. Overall, these results highlight the complex nature of organic matter transformation in NRZs and the microbial metabolic pathways that interact to mediate redox status and elemental cycling.
Harry R. Beller
Senior Scientist, Lawrence Berkeley National Laboratory
This work was supported as part of the Subsurface Biogeochemical Research Scientific Focus Area funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research under award number DE-AC02-05CH11231. This work used the Vincent J. Coates Genomics Sequencing Laboratory at the University of California, Berkeley, supported by the National Institutes of Health S10 instrumentation grants S10RR029668 and S10RR027303.
Jewell, T. N. M., U. Karaoz, M. Bill, R. Chakraborty, E. L. Brodie, K. H. Williams, and H. R. Beller. 2017. “Metatranscriptomic Analysis Reveals Unexpectedly Diverse Microbial Metabolism in a Biogeochemical Hot Spot in an Alluvial Aquifer,” Frontiers in Microbiology, DOI: 10.3389/fmicb.2017.00040.