Microbial hydrocarbon degradation at coal gasification plants.

Abstract

Almost every bigger city harbors a former coal gasification plant, where sometimes huge amounts of contaminants such as tar oil have leaked into the subsurface. This continuous release of monoaromatic hydrocarbons such as benzene, toluene, ethylbenzene, xylene (BTEX), as well as polycyclic aromatic hydrocarbons (PAH) such as naphthalene has resulted in considerable contaminant plumes. After decades of deposition, contaminant dissolution from the source is often in equilibrium with biodegradation processes. However, owing to slow dissolution and limited attenuation potentials, these sites are expected to remain impacted for hundreds and thousands of years. Today, novel means allow assessing biodegradation at former gasification sites by, e.g., metabolite analysis, fingerprinting of substrate spectra, or mass balances based on electron acceptor depletion. Biodegradation can even be quantified by stable isotope fractionation analysis. Also, knowledge on aerobic microbial hydrocarbon-degrading microorganisms is quite elaborate, which helps to understand degradation in unsaturated zones. However, these heavily contaminated aquifers usually turn anoxic, and apart from the degradation of toluene, ethylbenzene, and methylnaphthalene, the biochemistry of most anaerobic degradation pathways is still elusive. Hence, the controls of in situ biodegradation processes are still poorly understood. Recently, it has become apparent that the spatial separation of electron acceptors and contaminants in contaminant plumes caused by limited mixing in aquifers may be one of the most important factors limiting biodegradation in these systems. Electron acceptors are depleted in the center of plumes, restricting degradation activities to the fringe zones, where electron acceptors and contaminants meet in steep geochemical counter-gradients. Microbial communities in water and sediment samples from gasification plants generally exhibit abundance orders of magnitude higher than in uncontaminated references. At the same time, total community composition can be similarly diverse, albeit significant structural distinctions have been reported for the populations found in strongly or less impacted zones. The occurrence of both typical and uncultured degradation key-players seems to be correlated with zones of contamination and the prevailing biogeochemical conditions. Especially in degradation hot-spots at plume fringes, the abundance of specific hydrocarbon degraders can be surprisingly high. We highlight that substantial research efforts still need to be devoted to a better understanding of key aromatics and hydrocarbon degraders under iron- and sulfate-reducing, and methanogenic conditions, as well as to the biogeochemical and ecological controls of their activity in contaminated subsurface systems