Hydrogen concentrations in sulfate-reducing estuarine sediments during PCE dehalogenation

Prior research has shown hydrogen plays a role in microbial electron transfer under anaerobic conditions. It was already known that sulfate-reducing bacteria and dehalogenating microbes compete for hydrogen in subsurface environments. However, no prior work had resolved how hydrogen thresholds differ between sulfate reduction and chloroethene dehalogenation. This gap motivated a closer look at hydrogen dynamics in estuarine sediments. Existing studies have not fully characterized hydrogen concentrations during perchloroethylene (PCE) transformation. That uncertainty drove a need to compare hydrogen availability in systems with and without PCE. No prior work had resolved how sulfate reduction and PCE dehalogenation partition hydrogen resources. This uncertainty highlights the need for microcosm experiments to track hydrogen thresholds in sulfidogenic environments.

The aim of this study was to evaluate hydrogen concentrations during PCE dehalogenation in sulfate-reducing estuarine sediments. The specific problem addressed is the competition for hydrogen among microbial populations in anoxic environments. The motivation stems from the lack of data on hydrogen thresholds in systems where PCE serves as a terminal electron acceptor. Researchers sought to determine how hydrogen availability shifts when PCE is present versus absent. The study also aimed to assess how sulfate reduction and PCE dehalogenation partition reducing equivalents. By measuring hydrogen concentrations, the authors aimed to clarify the energetic hierarchy of electron-accepting processes. The goal was to test whether PCE dehalogenation lowers hydrogen thresholds compared to sulfate reduction. This approach allows for a better understanding of microbial competition in sulfidogenic estuarine systems.

The study used anoxic estuarine sediment slurries in microcosm experiments. Sediment samples were collected from estuarine environments and prepared as slurries. The slurries were amended with PCE to initiate dehalogenation processes. Hydrogen concentrations were measured using sensitive analytical techniques. Sulfate reduction was monitored through sulfide production measurements. Molybdate was used as an inhibitor to assess methanogen activity in the system. The experiments tracked hydrogen thresholds over time in both PCE-amended and non-amended slurries. The data were analyzed statistically to compare hydrogen concentrations between treatments.

PCE was reductively transformed to trichloroethylene (TCE) after a 13-day lag period. Hydrogen concentrations in PCE-amended slurries were significantly lower (0.5 nM) than in non-amended slurries (0.8 nM). Sulfate reduction occurred in all slurries, but PCE-amended systems directed half the reducing equivalents to sulfate reduction. These findings suggest a lower hydrogen threshold exists when PCE is present. Molybdate inhibition showed methanogens could compete with dechlorinators for hydrogen. The presence of PCE appears to shift hydrogen availability dynamics in the system. The results indicate that PCE dehalogenation is energetically more favorable than sulfate reduction. These data provide direct evidence of hydrogen partitioning among microbial processes.

The authors propose that a lower hydrogen threshold exists in systems where PCE serves as a terminal electron acceptor. These findings suggest that PCE dehalogenation may outcompete sulfate reduction for hydrogen in estuarine sediments. The results indicate that hydrogen availability is significantly lower during active PCE transformation. The study supports the hypothesis that more energetically favorable processes lower hydrogen thresholds. Methanogens may effectively compete with dechlorinators when sulfate reduction is inhibited. The observed hydrogen concentrations suggest a hierarchy of electron-accepting processes. The authors suggest that microbial competition for hydrogen is influenced by the energetic favorability of available processes. These conclusions are based on the observed hydrogen concentrations and microbial activity patterns.