Anaerobic bacterial degradation of pristenes and phytenes in marine sediments does not lead to pristane and phytane during early diagenesis

Anaerobic degradation of mixtures of isomeric pristenes and phytenes by sedimentary bacterial communities was investigated. These isoprenoid alkenes were quickly metabolized without formation of the corresponding alkanes. Identification of several bacterial metabolites allowed us to confirm the key role played by hydration in the metabolism of alkenes. Despite the increasing production of 5α-stanols during incubation, attesting to the involvement of biohydrogenation, this process did not act significantly on the double bond of pristenes and phytenes. Their resistance is attributed to the lack of binding polar group(s) to anchor the substrate to the enzyme and thus allow the double bond to reach the reductive catalytic site. It therefore seems that microbial hydrogenation of pristenes and phytenes does not account for the presence of pristane and phytane in recent sediments during early diagenesis.