S52: Methanogenic hydrocarbon biodegradation

Methanogenesis is an important terminal electron accepting process in environments often devoid of alternate electron acceptors such as hydrocarbon-contaminated groundwater aquifers and  subsurface petroleum reservoirs.   Understanding how methanogenic communities thrive in such fuel-laden ecosystems is ecologically and economically important for bioremediation efforts and for the development of biotechnology related to enhanced energy recovery from marginal reservoirs.  However, relatively little is understood about how syntrophic, methanogenic consortia bioconvert hydrocarbons to methane.

In order to determine the key microbes involved in methanogenic hydrocarbon biodegradation and the associated metabolic pathways, we established methanogenic enrichments on the model aromatic hydrocarbons toluene, 2-methylnaphthalene (2-MN) or 2, 6-dimethylnaphthalene (2, 6-diMN).  All enrichments produced stoichiometric amounts of methane based on starting substrate concentrations. 16S rRNA gene pyrosequencing of these cultures revealed the dominance of bacterial members affiliating within the Firmicutes and the presence of both acetotrophic and hydrogenotrophic methanogens.  RNA-based stable isotope probing conducted with the toluene-degrading culture revealed that a Deltaproteobacterium and a member of the Clostridiaceae were involved in the early steps of toluene metabolism.  The detection of benzylsuccinate in culture fluids along with a BssA gene fragment within this community indicated that fumarate addition was a key mechanism of initial toluene activation.  In the 2-MN and 2, 6-diMN-degrading enrichments, 2-naphthoic acid and 6-methyl-2-naphthoic acid were the key metabolites detected, respectively.  The cultures were further shown to have a broader substrate range than the original enrichment substrates that included other monoaromatics (for the toluene culture) and up to three-ringed aromatic compounds (for the 2-MN culture).