Biodegradation of C₇ and C₈iso‐alkanes under methanogenic conditions

Abstract

Iso-alkanes comprise a substantial proportion of petroleum and refined products that impact the environment, but their fate is cryptic under methanogenic conditions. We investigated methanogenic biodegradation of C7 and C8 iso-alkanes found in naphtha, specifically 2-methylhexane, 3-methylhexane, 2-methylheptane, 4-methylheptane and 3-ethylhexane. These were incubated as a mixture or individually with enrichment cultures derived from oil sands tailings ponds that generate methane from naphtha components; substrate depletion and methane production were monitored for up to 663 days. 3-Methylhexane and 4-methylheptane were degraded both singly and in the mixture, whereas 2-methylhexane and 2-methylheptane resisted degradation as single substrates but were depleted in the iso-alkane mixture, suggesting co-metabolism. 3-Ethylhexane was degraded neither singly nor with co-substrates. Putative metabolites consistent with succinylated C7 and C8 were detected, suggesting activation by addition of iso-alkanes to fumarate and corresponding to detection of alkylsuccinate synthase-like genes. 454 pyrotag sequencing, cloning and terminal restriction fragment length polymorphism of 16S rRNA genes revealed predominance of a novel member of the family Peptococcaceae (order Clostridiales) and Archaea affiliated with Methanoregula and Methanosaeta. We report here isomer-specific metabolism of C7 -C8 iso-alkanes under methanogenic conditions and propose their activation by a novel Peptococcaceae via addition to fumarate.

Methanogenic Hydrocarbon Degradation: Evidence from Field and Laboratory Studies

“…Recently, methanogenic biodegradation of C 7 and C 8 iso-alkanes (methylhexanes and heptanes) was demonstrated [Abu Laban et al, 2014] . The biodegradation was found to be isomer specific, consistent with some previous in situ observations [Jiménez et al, 2012], and some of the compounds could only be degraded (probably cometabolically) when in iso-alkane mixtures.…”

Co-occurrence of methanogenesis and N2 fixation in oil sands tailings

“…Previous laboratory-scale studies conducted using mature fine tailings (MFT) collected from different tailings ponds have revealed metabolism of different solvent hydrocarbons into CH 4 . These hydrocarbons include n-alkanes (C 5 -C 18 ) (Mohamad Shahimin et al, 2016;Siddique et al, 2006Siddique et al, , 2011, monoaromatics (toluene and isomers of xylene) (Siddique et al, 2007), and cyclic and branched alkanes (C 5 -C 8 ) (Abu Laban et al, 2014;Siddique et al, 2015;Tan et al, 2015). In MFT, biodegradation is carried out by a cascade of microorganisms involved in the initial biodegradation of hydrocarbons to simple substrates such as acetate, H 2 , and CO 2 , which are further utilized to produce CH 4 through acetoclastic and hydrogenotrophic pathways (Penner and Foght, 2010;Siddique et al, 2011;Siddique et al, 2012;Siddique et al, 2015).…”

Microbial metagenomics of oil sands tailings ponds: small bugs, big data

“…For example, among naphtha components, straight-chain alkanes and mono-aromatics are the easiest to degrade (Siddique et al 2006(Siddique et al , 2007, whereas branched-chain and cyclic alkanes are less easily metabolized (Abu Laban et al 2014;Tan et al 2013Tan et al , 2015b. Biodegradation of the latter two hydrocarbon classes may entail a lag phase of months or years in the laboratory before biodegradation begins, and some highly branched or fused-ring hydrocarbons appear to resist biodegradation completely.…”

“…However, almost none of the microbes inferred to be responsible for this "distributed metabolism" have been grown in pure culture, few have been named or characterized, and their metabolic pathways are mostly cryptic. Only recently have some details begun to emerge about these novel microbes (e.g., Abu Laban et al 2014;Tan et al 2015a). Identifying the key species and understanding their inter-connected metabolisms is important because hydrocarbon degradation in situ not only has deleterious environmental consequences (e.g., production of greenhouse gases), but also beneficial effects in the ponds (e.g., depleting solvent concentrations to reduce the toxicity of tailings prior to reclamation, and accelerating settling of the clay minerals; Siddique et al 2014aSiddique et al , 2014b.…”

“…This revealed dynamic selection processes: certain Bacteria dominated when aromatics were present and other species flourished when alkanes were abundant (Abu Laban et al 2014Tan et al 2013); some Bacteria dominated under sulfate-reducing conditions and others under methanogenic (Abu Laban et al 2015a;Tan et al 2015a); community structure shifted in response to the composition of the residual naphtha as its hundreds of components were selectively and sequentially degraded in order of microbial preference. Thus, the huge biodiversity in the ponds likely makes the indigenous communities resilient to change (through functional redundancy) as well as adaptive (able to draw on different capabilities), allowing them to respond to changes in the solvent composition from process variations or as a result of selective biodegradation in situ.…”