Arsenic contamination in areas surrounding mines and selection of potential As-resistant purple nonsulfur bacteria for use in bioremediation based on their detoxification mechanisms

The objectives of this study were to investigate arsenic (As) contamination in areas surrounding mines in Thailand and to isolate As-resistant purple nonsulfur bacteria (PNSB) for their potential application in microbial As bioremediation. Total As in water and soil samples collected from two As-contaminated sites in Thailand exceeded the standard guidelines. Four strains of As-resistant PNSB were selected from 348 isolates based on their As detoxification mechanisms, including biofilm formation, As redox transformations, glutathione (GSH) production, and production of methylcobalamin (vitamin B12) and S-adenosylmethionine (SAM) for As biomethylation. GSH production in these strains followed the order AB3 > L28 > C1. Strains AB3, C31, and L28 accumulated higher levels of intracellular vitamin B12 than strain C1. However, only strain C1 produced SAM and showed As biovolatilization activity. When strain C1 was incubated under microaerobic light conditions, it produced arsenobetaine [(AsB(V)] in medium with As(III) and monomethylarsonic acid [MMA(V)] in medium with As(V), which were found in the culture supernatants. Volatile methylated As compounds, such as dimethylarsenic acid [DMA(V)] and MMA(V)], were also detected in strain C1 during 30 days of incubation with As(III) and As(V). Strains AB3, C1, and L28 were identified as Rhodopseudomonas palustris and strain C31 as Rubrivivax benzoatilyticus. The resistance of these strains to As [As(III) and As(V)] based on minimum inhibition concentration values under aerobic dark and microaerobic light conditions, respectively, was in the order of C1 > AB3 > C31 > L28. The overall results demonstrate that all four selected strains of PNSB have a great potential for future application in the remediation of As-contaminated areas although R. palustris C1 was the most effective candidate.