Individual and combined inhibitory effects of methanol and toluene on acetyl-CoA synthetase expression level of acetoclastic methanogen, Methanosaeta concilii

• Individual and combined effects of methanol and toluene on the expression level of Acs1 were investigated.
• Singular methanol and methanol + toluene mixtures did not cause any significant effect on the Acs1 expression level.
• Singular toluene-exposed reactors were completely inhibited after 3rd exposure at all concentrations.
• Methanol and toluene exposures did not change the relative abundance of active Methanosaeta spp. cells significantly.

Organic solvents found in waste streams can be inhibitory to microbial communities in wastewater treatment plants. In this study, possible inhibitory effects of commonly used organic solvents, methanol and toluene, were investigated in batch anaerobic digestion tests. Anaerobic sludges were fed with acetate repeatedly and exposed to various concentrations of toluene, methanol and mixture of them. Expression level of the key enzyme of acetoclastic methanogenesis, acetyl-CoA synthetase 1 (Acs1), was determined by reverse transcriptase real time PCR. Additionally, active populations of Methanosaeta spp. were monitored and quantified by fluorescent in situ hybridization (FISH). Transcript abundance of Acs1 was 1.31 × 106 mRNAs ml−1 in the control reactors; whereas, singular methanol exposure of 0.1 M, 0.3 M, 0.5 M and 1.0 M and methanol + toluene combination of 1.0 M + 0.5 mM and 1.0 M + 1.5 mM did not cause any significant effect on the acetyl-CoA expression level. However, singular toluene-exposed serum bottle reactors were completely inhibited after 3rd exposure at all concentrations from 0.5 mM to 4.0 mM. FISH results for singular methanol and toluene additions as well as their combination in the ranges studied showed no particular effect on the relative abundance of Methanosaeta spp. cells.