An effort to understand and improve the anaerobic biodegradation of petroleum hydrocarbons: A literature review

Oil and fuel spills occur regularly in terrestrial and aquatic environments and substances such as crude oil can contain many compounds that are highly resistant to degradation. Among these constituents are alkanes and monoaromatic and polycyclic aromatic hydrocarbons (PAHs), which are not only toxic but also carcinogenic and/or mutagenic. Provided there are sufficient nutrient levels and proper growth conditions, many complex hydrocarbons, including PAHs, readily biodegrade under aerobic conditions (i.e., in the presence of oxygen). However, oxygen-depleted environments are ubiquitous, e.g., deep subsurfaces or general aerobic environments where biological oxygen consumption exceeds replenishment. Anaerobic bacteria and archaea in such anaerobic environments are the dominant catalysts to initiate and complete degradation, including most PAHs. It is therefore imperative to understand the biochemical reactions and mechanisms by which anaerobic degradation takes place though much slower compared with those under aerobic conditions. For alkanes and aromatics, new biochemical mechanisms carried out by the genes and enzymes responsible have been reported to advance and enrich the knowledge on anaerobic transformation. However, validation of these biochemical reactions have not been fully rectified with convincing results of the expression of the genes and also the chemical signature degradation intermediates in oil production system or oil contaminated sites. In order to better mitigate the contaminated sites, biostimulation and bioaugmentation are discussed for accelerated and effective removal of petroleum oil or specific constituents.