Impact of redox condition and co-contaminant exposure history on anaerobic microbial communities during toluene and benzene biodegradation in laboratory soil columns
Aromatic compounds are present as highly toxic and persistent contaminants in both aquatic and terrestrial environments, demanding effective remediation of contaminated areas. In this study, laboratory column experiments were performed with aquifer sediments from a contaminated former gasworks site. First, toluene and benzene biodegradation as a sole source of carbon was assessed under different redox conditions. Results of single compound experiments showed that toluene biodegradation was more efficient under sulfate-reducing conditions (80%) compared to nitrate-reducing conditions (55%) during initial exposure. Subsequently to toluene biodegradation, benzene showed an opposite effect where removal efficiencies were 44.5% and 59.5% under sulfate and nitrate-reducing conditions, respectively. These findings led to the conclusion of compound-specific redox-condition efficiency. The relatively rapid removal of the compounds under anaerobic conditions (within one to two months) was attributed to the alternating flow-batch approach applied to the columns where selective enrichment of specific microorganisms was stimulated by alternating biomass washout at high flow rate and growth at batch mode. Additionally, to single compound experiments, different substrate combinations were tested, within different exposure order to investigate the effect of substrate composition and pollution history on the biodegradation process of the aromatic hydrocarbons. Toluene biodegradation was not affected by the presence of benzene, but slightly inhibited by preincubation with a complex mixture of various aromatic contaminants (BTEX, indene, indane and naphthalene). On the contrary, benzene removal efficiencies were reduced to 8% by the presence of toluene, but not affected (40-50% removal) by the complex mixture. When the columns were pre-exposed to either toluene or benzene, the biodegradation of these two chemicals continued unaffectedly when subsequently fed with a more complex mixture. This indicates that once the microbial community had been adapted to biodegrade toluene or benzene under single compound conditions, co-contaminants appear not to interfere the biodegrading community. Beta-diversity analysis revealed distinct microbial community clustering between sulfate and nitrate-reducing columns, emphasizing the greater influence of redox conditions on microbial community composition compared to substrate composition or pollution history. Despite certain columns receiving the same substrate mixture at specific times, differences among the microbial consortia were observed in each column, implying the influence of the substrate exposure order (pollution history) on microbial community dynamics. It is essential to consider all the findings derived from this study when developing effective strategies for the bioremediation of groundwater contaminated with complex mixtures containing various aromatic hydrocarbons.