How nitrate affects perchlorate reduction in a methane-based biofilm batch reactor

Area of Science:

• Environmental Microbiology
• Bioremediation Engineering

Background:

• Perchlorate (ClO₄^-) is a widespread environmental contaminant.
• Methane (CH₄) can serve as an electron donor for perchlorate reduction.
• Nitrate (NO₃^-) co-occurrence can affect bioremediation processes.

Purpose of the Study:

• To investigate the impact of nitrate on methane-driven perchlorate reduction in a membrane batch biofilm reactor (MBBR).
• To identify microbial communities involved in co-metabolism of nitrate and perchlorate.
• To elucidate the molecular mechanisms and energetic preferences governing the competition between nitrate and perchlorate reduction.

Main Methods:

• Utilized a membrane batch biofilm reactor (MBBR) with methane as the electron donor.
• Measured perchlorate and nitrate reduction rates under varying conditions.
• Employed reverse transcription-polymerase chain reaction (RT-PCR) to quantify reductase gene expression.
• Applied Density Functional Theory (DFT) to analyze electron transfer energy barriers.

Main Results:

• Nitrate significantly inhibited perchlorate reduction rates, which recovered upon nitrate depletion.
• Genera Denitratisoma and Azospirillum were identified as key players in both nitrate and perchlorate reduction.
• Perchlorate reductase (pcrA) gene expression correlated with the combined concentration of nitrate and perchlorate.
• DFT analysis indicated a lower energy barrier for nitrate reduction compared to perchlorate reduction.

Conclusions:

• Nitrate acts as a competitive electron acceptor, hindering perchlorate bioremediation in MBBRs.
• Bacterial preference for nitrate over perchlorate is energetically driven, impacting contaminant removal efficiency.
• Understanding microbial competition is crucial for optimizing bioremediation strategies for co-contaminated sites.