Alkylsuccinate Synthase in Hydrocarbon-Impacted Environments: Look High, Look Low.... Look Lower

Biotransformation of n-alkanes by anaerobic microorganisms plays a significant role in the bioremediation of polluted ecosystems, energy recovery in deep subsurface environments, and biogeochemical cycling. However, the genetic machinery involved in anaerobic n-alkane degradation is not well elucidated.  To date, there are three proposed mechanisms of non-methane alkane oxidation: 1) alkane addition across the double bond of fumarate to form alkyl-substituted succinates, 2) intra-aerobic hydroxylation via dismutation of nitrogen species under denitrifying conditions, and 3) anaerobic hydroxylation followed by carboxylation. The first mechanism has been well described and is presumably catalyzed by the glycyl radical enzyme, alkylsuccinate synthase.  The discovery of alkylsuccinate synthase has allowed the interrogation of contaminated sites as well as uncharacterized enrichment cultures/isolates, thus expanding the molecular toolbox for in situ studies. In recent studies, targeted gene assays, microarrays, and metagenomics have been exploited to investigate the presence of alkylsuccinate synthase in hydrocarbon-impacted ecosystems including estuaries, rivers, groundwater systems, coal-bed formations, hydrothermal sediments, and deep-sea sediments. In conjunction with metabolite profiling, alkylsuccinate synthase has been shown to serve as a valuable biomarker of in situ alkane degradation.  Given significant advances in our understanding of the biochemistry and genetics that govern anaerobic hydrocarbon oxidation, future directions are aimed toward reconciling different “omic” methodologies to unearth new lineages of microorganisms and genes involved in anaerobic hydrocarbon metabolism.