Microbiome Composition and Dynamics of a Reductive/Oxidative Bioelectrochemical System for Perchloroethylene Removal: Effect of the Feeding Composition

Tensions the paper itself points to. Each side quotes the passage it comes from.

• Within the paper tceA gene abundance does not correlate with PCE removal efficiency across feeding regimes

  One side

  MM: tceA 2.46E+07 copies/L, PCE removal 97%

  “tceA was found as the most abundant reductive dehalogenase gene in all the conditions explored (up to 2.46 E + 07 gene copies/L in MM)”

  Other side

  SG: tceA 1.87E+05 copies/L (132-fold lower), PCE removal 95% (only 2% lower)

  “the utilization of RG in the reactor yielded a low RD rate (0.51 ± 0.04 µeq/Ld) mostly due to the complexity of RG in terms of chemical composition”

  Authors’ reading. Authors attribute the tceA suppression in SG to competition from sulfate/nitrate reduction (3.2 meq/Ld sulfate reduction vs. 0.3 meq/Ld nitrate reduction), which diverts electrons away from dechlorination but does not prevent it. The system maintains high PCE removal because the cathode potential (−450 mV vs. SHE) still drives dechlorination, albeit at a lower rate (68 vs. 99 µeq/Ld). This is not a true contradiction but a trade-off: competing reactions suppress tceA expression without eliminating dechlorination.

Specific gaps in this paper, with an experiment that would close each.

• High No systematic variation of cathode potential between −550 and −450 mV vs. SHE in the presence of nitrate/sulfate to identify the threshold potential at which dechlorination is outcompeted by sulfate/nitrate reduction

  RD rate dropped 31% (99 to 68 µeq/Ld) when potential shifted from −550 to −450 mV and sulfate/nitrate were introduced, but the paper does not isolate the effect of potential alone. A potential sweep under constant SG composition would reveal the operating window.

  Try. Vary cathode potential from −600 to −350 mV vs. SHE in SG (30 mg NO3−/L, 200 mg SO42−/L); measure RD rate, sulfate reduction rate, nitrate reduction rate, and tceA abundance at each potential.

• High No measurement of N2O or nitrite accumulation during nitrate reduction in the SG feeding condition

  Nitrate reduction was quantified at 0.3 meq/Ld in SG, but no speciation of products (NO2−, N2O, N2) was reported. Incomplete denitrification could produce N2O, a potent greenhouse gas, or accumulate toxic NO2−.

  Try. Measure NO3−, NO2−, N2O, and N2 in the reductive reactor outlet during SG feeding using ion chromatography and gas chromatography; vary cathode potential and HRT to optimize complete denitrification to N2.

• Medium No direct measurement of cathode potential or current density in the reductive reactor during RG feeding; only applied potential (−450 mV vs. SHE) is reported

  RG contains 31 mg/L Fe total and 20 mg/L Fe3+, which may form a biofilm or precipitate on the electrode, changing the actual potential experienced by the biofilm. Iron(III) reduction dominated (945 meq/Ld), suggesting the electrode surface chemistry differs from MM/SG conditions.

  Try. Measure electrode impedance and surface potential using electrochemical impedance spectroscopy (EIS) during RG feeding; correlate with biofilm composition and iron(III) reduction rate.

• Medium No variation of HRT in the oxidative reactor to determine the minimum residence time for VC removal under different feeding regimes

  HRT was 5.1 d (MM), 3.7 d (SG), and 1.8 d (RG), but these were not systematically varied. The 1.8 d HRT in RG achieved 92% VC removal, suggesting further reduction may be possible, which would improve throughput.

  Try. Reduce HRT from 1.8 to 0.9 d in RG feeding while maintaining galvanostatic control at +15 mA; measure VC and cis-DCE removal efficiency and etnC/etnE gene abundance.

• Medium No measurement of dissolved oxygen (DO) in the oxidative reactor outlet or control of aeration to optimize ethenotroph activity

  etnC and etnE genes encode aerobic VC oxidation enzymes, but the paper does not report DO levels or aeration strategy. RG feeding achieved 100-fold higher etnC/etnE abundance, possibly due to higher DO from natural organic matter oxidation or electrode-driven oxygen production.

  Try. Measure DO in the oxidative reactor outlet under MM, SG, and RG feeding; vary aeration rate (0–5 L/min) and measure VC removal efficiency and etnC/etnE abundance.

• Low No measurement of catholyte pH in the reductive reactor, which affects electrode potential and microbial metabolism

  pH is critical for electrode kinetics and denitrification (optimal pH 6.5–8.5), but only the applied potential is reported. MM contains 2.5 g/L NaHCO3 (buffering), while SG and RG may have different pH profiles.

  Try. Monitor pH in the reductive reactor outlet during MM, SG, and RG feeding; correlate with RD rate, sulfate reduction rate, and D. mccartyi abundance.