Biodegradation of C7 and C8 iso‐alkanes under methanogenic conditions

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.