274-2 Microbial Dark Oxygen Production in Freshwater Aquifers

Session: Advances in Ecohydrogeology

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Abstract:

Microbial dark oxygen production has recently been observed in marine and groundwater environments, carried out by ammonium-oxidizing archaea (AOA). These archaea oxidize ammonium to nitrite to meet their energy needs and are abundant in environments with high, very low, or depleted oxygen levels. Recent studies have shown that the ammonium-oxidizer Nitrosopumilus maritimus can produce oxygen and dinitrogen in the dark and oxygen anomalies in deep aquifers might be explained by microbial dark oxygen production. In this study, we examine the microbiology, hydrology, and geochemistry of both a chlorinated solvent-impacted sandstone site and a fresh dolomite aquifer. The contaminated site exhibits varying levels of contamination with chlorinated compounds. Electrical Resistivity Imaging (ERI) indicated low bioactivity within the contaminated groundwater plume, which aligned with total cell counts ranging from 1.1×10^⁶ to 9.1×10^⁷cells/mL, similar to those in the non-contaminated control well (3.7×10^⁷ cells/mL). 16S rRNA sequencing and quantitative PCR (qPCR) targeting bacterial, archaeal, and dehalogenase genes revealed that dehalogenating bacteria belonging to the phyla Chloroflexi and Deltaproteobacteria are present at very low abundances, below 1% of the overall microbial community. They are likely primarily inactive due to high oxygen levels and the redox potential of the groundwater. Surprisingly, we detected an aerobic or facultative anaerobic microbial community consisting of bacteria and archaea, predominantly ammonium-oxidizing archaea (AOA). The microbial communities were dominated by species such as Nitrosopumilus and Nitrososphaera, with relative abundances of up to 65% and 21% of the entire community, consistent with high abundances of ammonium-oxidizing genes (amoA genes). In the dolomite aquifer, seven different bacterial and archaeal species capable of dark oxygen production were identified within the aquifer. This finding is significant because the presence of these microorganisms is both new and understudied in such environments. Although the three known pathways for dark oxygen production—chlorite dismutation, detoxification of reactive oxygen species, and nitric oxide dismutation—are not yet fully understood, these results suggest that these microbes could raise the aquifer's redox potential and affect biogeochemical model results. This has important implications for the development of new electrode-monitoring systems and overall aquifer assessment strategies. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10037

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