Long chain alkenones in hypersaline and marine coastal microbial mats

Long-chain alkenones in microbial mats from Mediterranean solar salterns and coastal areas from the Mediterranean Sea and North Sea have been studied. These ecosystems exhibit a distinct C37–C40 alkenone composition characterized by a predominance of triunsaturated compounds in all C37–C40 homologues and by a specific correspondence between carbonyl position and chain length. Thus, C37 and C39 homologues are methyl ketones and C38 and C40 homologues are ethyl ketones. The distribution also contains tetraunsaturated alkenones in significant proportion, particularly among the C37 homologues. The double bonds are located at Δ8,15,22,29 and Δ9,16,23,30 in the methyl and ethyl ketones, respectively, indicating that the number of carbon atoms between the carbonyl group and the first double bond remains constant. This distribution is different from C37–C40 alkenone mixtures previously found in open sea waters or freshwater environments. It suggests a biological origin distinct from that previously described for marine algal cultures or recent or ancient open sea sediments. The uniformity of the distributions, despite the strong salinity differences considered in the present study (salinity of 35–200) is remarkable. Depth analysis of the microbial mats from hypersaline environments shows fluctuations in total alkenone concentration, but no defined trends involving selective changes in compounds with a specific degree of unsaturation or chain length.