Insights into growth kinetics and roles of enzymes of Krebs' cycle and sulfur oxidation during exochemolithoheterotrophic growth of Achromobacter aegrifaciens NCCB 38021 on succinate with thiosulfate as the auxiliary electron donor

Multi-approach assessment of groundwater biogeochemistry: Implications for the site characterization of prospective spent nuclear fuel repository sites

The disposal of spent nuclear fuel in deep subsurface repositories using multi-barrier systems is considered to be the most promising method for preventing radionuclide leakage. However, the stability of the barriers can be affected by the activities of diverse microbes in subsurface environments. Therefore, this study investigated groundwater geochemistry and microbial populations, activities, and community structures at three potential spent nuclear fuel repository construction sites. The microbial analysis involved a multi-approach including both culture-dependent, culture-independent, and sequence-based methods for a comprehensive understanding of groundwater biogeochemistry. The results from all three sites showed that geochemical properties were closely related to microbial population and activities. Total number of cells estimates were strongly correlated to high dissolved organic carbon; while the ratio of adenosine-triphosphate:total number of cells indicated substantial activities of sulfate reducing bacteria. The 16S rRNA gene sequencing revealed that the microbial communities differed across the three sites, with each featuring microbes performing distinctive functions. In addition, our multi-approach provided some intriguing findings: a site with a low relative abundance of sulfate reducing bacteria based on the 16S rRNA gene sequencing showed high populations during most probable number incubation, implying that despite their low abundance, sulfate reducing bacteria still played an important role in sulfate reduction within the groundwater. Moreover, a redundancy analysis indicated a significant correlation between uranium concentrations and microbial community compositions, which suggests a potential impact of uranium on microbial community. These findings together highlight the importance of multi-methodological assessments in better characterizing groundwater biogeochemical properties for the selection of potential spent nuclear fuel disposal sites.

Hydrogen sulfide affects the performance of a methanogenic bioelectrochemical system used for biogas upgrading

Methanogenic bioelectrochemical systems (BESs) can convert carbon dioxide (CO₂) to methane (CH₄) and may be used for anaerobic digester biogas upgrading. However, the effect of hydrogen sulfide (H₂S), a common biogas component, on BES performance is unknown. Thus, the objective of this study was to assess the effect of H₂S addition to the cathode headspace on BES performance at a range of initial gas-phase H₂S concentrations (0-6% v/v), as well as its effect on the anode and cathode microbial communities. As the initial cathode headspace H₂S increased from 0 to 2% (v/v), biocathodic CH₄ production increased by two-fold to 3.56 ± 0.36 mmol/L-d, due to dissolved H₂S transport from the cathode to the anode where H₂S was oxidized. Elemental sulfur and sulfate were H₂S oxidation products detected in the anode. Above 3% initial cathode headspace H₂S, biocathodic CH₄ production declined due to inhibition. A phylotype most closely related to Methanobrevibacter arboriphilus dominated the cathode archaeal communities. In the sulfide-amended BES, a phylotype similar to the exoelectrogen Ochrobactrum anthropi was enriched in both the anode and cathode, whereas phylotypes related to sulfate-reducing and sulfur oxidizing Bacteria were detected in the bioanode. Thus, sulfide transport and oxidation in the anode play an important role in methanogenic BESs treating sulfide-bearing biogas.