Inhibition of anaerobic microbial o-xylene degradation by toluene in sulfidogenic sediment columns and pure cultures

It is frequently observed in aromatic hydrocarbons such as benzene, toluene, ethylbenzene, xylene (BTEX)-contaminated aquifers that toluene degrades faster than xylenes and benzene. In sediment column experiments which were run with a mixture of BTEX compounds toluene degradation started after a lag period of several weeks. When we omitted toluene from the culture medium o-xylene degradation started. Xylene degradation could be inhibited by adding toluene back to the medium and could be recovered when toluene was omitted again. This was observed repeatedly when toluene concentrations higher than 20 μM were added. Two sulphate-reducing bacterial species, isolated from the column material, were used to investigate the degradation behaviour in detail. Strain TRM1 degraded exclusively toluene, strain OX39 degraded preferentially o-xylene and toluene only after an adaptation period of more than 90 days when added as the sole substrate. Growth and o-xylene degradation of strain OX39 were inhibited by toluene concentrations as low as 40 μM, whereas, in contrast, toluene degradation by strain TRM1 was not inhibited by o-xylene concentrations up to 0.5 mM. Both the column data and the batch experiments indicated that two organisms were responsible for the toluene/xylene degradation in the sediment column. One strain degraded only toluene and was not effected by xylene and the second degraded xylene and was inhibited by toluene. Our findings offer an explanation that the observed differential degradation of BTEX compounds in contaminated aquifers could originate from a partial metabolic inhibition of xylene-degrading organisms by toluene.