The stability of microbial communities is an important aspect of biodegradation strategies, as it can often decide the success or failure of biological treatment technologies (BTT). Recent studies on the dynamics of microbial communities within bioreactors have demonstrated that functional stability is not necessarily correlated to community stability [1]. However, in order to associate functional perturbations to community changes, the community has to be stable under constant operating conditions. Otherwise, it would not be reasonable to assume a cause-effect relation, as the community could change independently of functional perturbations. We have identified bacterial strains which remain stable during the biodegradation of halogenated compounds, which suggests that xenobiotic compounds can induce a strong selective pressure in bacterial communities and enhance strain stability [2].

We have then used these strains to further study the relationships between microbial strain dynamics and bioreactor system performance, using an Oil-Absorber-Bioscrubber as a model system. This bioreactor configuration was developed recently within our group to control fluctuating loads of dichloroethane DCE [3]. This configuration consisted of a bioscrubber combined with a packed oil-absorber column containing vegetal oil placed upstream, where the waste gas was bubbled through. This set-up prevented the contact between the liquid medium, containing the biomass, and the organic phase. Industrial (BTT) are often exposed to random variations in pollutant concentrations profiles and alternating pollutant compositions. These dynamic conditions can inhibit microbial activity and induce long re-acclimation periods, which can in turn compromise the treatment performance. In order to prevent bacterial inhibition and improve BTT robustness, it is interesting to combine the study of microbial dynamics with the bioreactor engineering development.

The aims of our study were therefore to: I) assess the stability of bacterial strains with specific metabolic pathways for the degradation of halogenated organic compounds, through the application of biomolecular techniques; ii) apply these stable strains to the treatment of waste gas under a sequentially alternating pollutant (SAP) regime, using the oil-absorber bioscrubber configuration.

The long-term stability of a microbial strain able to degrade chlorobenzene (CB) was a monitored over a full year, under constant operating conditions and a non-sterile environment. Fluorescence in situ hybridization (FISH) and denaturing gradient gel electrophoresis (DGGE) were applied to monitor the specific degrader and the overall community changes. It was observed that the non-sterile environment led to the natural out competition of this initial strain for a more competitive species within the initial weeks of operation. The succession was attributed to the competitive kinetic behaviour of the novel strain belonging to the Pandoraea genus, which exhibited faster growth and higher substrate affinity than the initial strain [4]. This strain was further applied to the treatment of SAP in an oil-absorber bioscrubber system.

The performance of a combined absorber-bioscrubber system, degrading a gas stream contaminated with fluorobenzene (FB) and chlorobenzene (CB), was studied under different cyclic feeding regimens, simulating industrial conditions. The absorber was applied as a strategy to buffer pollutant inlet concentrations into the bioscrubber, and as well, to provide a maintenance feed of compounds being desorbed from the oil to the bacterial culture [3]. In order to analyze the performance of the oil-absorber-bioscrubber system (OAB), it was compared to a bioscrubber-only (BO) configuration, operated under the same functional conditions. In both configurations, the bacterial communities were closely monitored by FISH and DGGE. The bioscrubber performance results have shown that the total organic discharged (TOD) was significantly lower in the OAB system when compared to the BO configuration. In the case of FB, the OAB configuration prevented the discharge of 1165 g m-3 to the environment, while in the CB case it prevented the release of 300 g m-3. This result has shown that the oil-absorber had a positive effect on the removal of FB and CB, and that it could be used as a strategy to deal with sequentially alternating polluting and fluctuating loads. The FISH and DGGE results contributed to the understanding of the OAB strategy success, revealing that this configuration sustained a more active community, which responded quickly to pollutant changes.

Overall, this study demonstrated the remarkable long-term stability of a novel CB degrading strain and highlighted the worth of the application of biomolecular techniques to follow microbial dynamics in bioreactors. Additionally, this study has shown that the combined oil-absorber bioscrubber system is a robust technology, offering an effective solution to the biological treatment of waste-gas streams during SAP treatment conditions.

References:

1. Fernandez A, Huang S, Seston S, Xing J, Hickey R, Criddle C, Tiedje JM. 1999. How stable is stable? Function versus community composition. Appl Environ Microbiol 65:3697-3704.

2. Baptista IIR, Peeva L, Zhou N-Y, Leak DJ, Mantalaris A, Livingston AG. 2006. Stability and performance of Xanthobacter autotrophicus GJ10 during 1,2-dichloroethane biodegradation. Appl Environ Microbiol 72:4411-4418.

3. Koutinas M, Martin J, Peeva LG, Mantalaris A, Livingston AG. 2006. An oil-absorber-bioscrubber system to stabilize biotreatment of pollutants present in waste gas. Fluctuating loads of 1,2-dichloroethane. Environ Sci Technol 40:595-602.

4. Baptista IIR, Zhou N-Y, Emanuelsson EAC, Peeva LG, Leak DJ, Mantalaris A, Livingston AG. 2008. Evidence of species succession during chlorobenzene biodegradation. Biotech Bioeng 99:68-74.