Comparison of Mechanisms of Alkane Metabolism under Sulfate-Reducing Conditions among Two Bacterial Isolates and a Bacterial Consortium

Abstract

Recent studies have demonstrated that fumarate addition and carboxylation are two possible mechanisms of anaerobic alkane degradation. In the present study, we surveyed metabolites formed during growth on hexadecane by the sulfate-reducing isolates AK-01 and Hxd3 and by a mixed sulfate-reducing consortium. The cultures were incubated with either protonated or fully deuterated hexadecane; the sulfate-reducing consortium was also incubated with [1,2-13 C 2 ]hexadecane. All cultures were extracted, silylated, and analyzed by gas chromatography-mass spectrometry. We detected a suite of metabolites that support a fumarate addition mechanism for hexadecane degradation by AK-01, including methylpentadecylsuccinic acid, 4-methyloctadecanoic acid, 4-methyloctadec-2,3-enoic acid, 2-methylhexadecanoic acid, and tetradecanoic acid. By using d 34 -hexadecane, mass spectral evidence strongly supporting a carbon skeleton rearrangement of the first intermediate, methylpentadecylsuccinic acid, was demonstrated for AK-01. Evidence indicating hexadecane carboxylation was not found in AK-01 extracts but was observed in Hxd3 extracts. In the mixed sulfatereducing culture, however, metabolites consistent with both fumarate addition and carboxylation mechanisms of hexadecane degradation were detected, which demonstrates that multiple alkane degradation pathways can occur simultaneously within distinct anaerobic communities. Collectively, these findings underscore that fumarate addition and carboxylation are important alkane degradation mechanisms that may be widespread among phylogenetically and/or physiologically distinct microorganisms.

Anaerobic phenanthrene mineralization by a carboxylating sulfate-reducing bacterial enrichment

“…One clone (Phe4C) from the phenanthrene-degrading enrichment was 99% similar to the 16S rDNA sequence of the alkane-degrading sulfate-reducing bacterium strain Hxd3 (proposed new species: Desulfococcus oleovorans). The distinguishing feature of the strain Hxd3 is its ability to attack n-alkanes by an initial carboxylation reaction (Aeckersberg et al, 1991;So et al, 2003;Callaghan et al, 2006). In contrast, Phe4A is phylogenetically distant from other known sulfate-reducing bacteria, but most similar (91%) to Desulfofrigus oceanense (not shown).…”

Carboxylases in Natural and Synthetic Microbial Pathways

“…However, whether these genes encode true benzene or naphthalene carboxylases remains to be demonstrated. Finally, the degradation of medium-to long-chain alkanes in the strictly anaerobic Hxd3 sulfate-reducer strain was also suggested to follow an initial carboxylation reaction (25,111). Overcoming incomplete ␤-oxidation: propionate and leucine assimilation.…”

Anaerobic Activation of p -Cymene in Denitrifying Betaproteobacteria: Methyl Group Hydroxylation versus Addition to Fumarate

“…strain T (12). This type of reaction proved to be archetypical among diverse facultatively and obligately anaerobic bacteria for the initial activation of the benzylic methyl group in toluene, all three xylene isomers (13)(14)(15), or 2-methylnaphthalene (16,17), as well as the subterminal methylene group in n-alkanes (18,19) and the benzylic methylene group in ethylbenzene (20). In the case of the latter, addition to fumarate is only known for energy-limited sulfate-reducing bacteria, while denitrifying "Aromatoleum aromaticum" strains EbN1 and EB1 activate ethylbenzene via hydroxylation to (S)-1-phenylethanol (21)(22)(23).…”