Profile of Aliphatic Hydrocarbons in a Recent Polynesian Microbial Mat

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Abstract
The extractable aliphatic hydrocarbon composition of four layers of a microbial mat from Hao atoll, French Polynesia, have been examined by GS-MS. These hydrocarbon distributions illustrate both the imprint of microbial populations on the lipid profiles and the selective diagenetic transformation of specific microbial lipids. In the uppermost layer, principally composed of cyanobacteria, the n-alkane distribution is typical of such microbial origins, being dominated by C17. With depth this n-alkane pattern is progressively changed by selective degradation of lower homologues. Thus, n-C31, presumably derived from vegetation surrounding the pond, becomes the dominant component in the deeper layers. Also, C18 and C19 methyl mid-chain branched alkanes appear to persist with depth relative to their linear analogues, indicating their preferential survival, or liberation from bound lipids. A number of alkenes (e.g. C17, C29, C31) appear to be associated with either the upper cyanobacterial, or underlying photosynthetic bacterial, horizons. They decrease markedly in concentration in the deeper layers, showing an enhanced degradation relative to n-alkanes of the same carbon number.