Lipid characterization of coastal hypersaline cyanobacterial mats from the Camargue (France)

Abstract

A study of the sedimentary lipids of coastal hypersaline microbial mats sampled in the Camargue (France) allowed identification of the predominant inputs of organic matter and provided insights about early diagenetic processes. The biomarker profiles revealed a surprising diversity of organic matter sources. The mat-forming cyanobacterium Microcoleus chthonoplastes leaves few distinguishing features in the extractable lipid sedimentary record although a contribution to the fatty acids and hydrocarbons can be recognized. The high bis-O-phytanylglycerol content of the mats attests to the importance of halophilic archaebacteria, as does the presence of dihydrophytol, squalene, tetrahydrosqualenes and geranylgeraniol. Unexpectedly, marine microalgae are a major source of extractable lipids. The mats contain high contents of alkenones derived from a haptophyte thought to be related to Chrysotila. Smaller amounts of the corresponding alkenols also derive from this alga. Some mat samples contain much higher contents of alkenols which seem to result from microbial reduction of the alkenones. A diverse range of sterols from eukaryotic sources (haptophytes, diatoms, dinoflagellates and green microalgae) is present. Evidence for higher plant input (probably via aeolian input) is provided by the presence of long chain alcohols and fatty acids with an even-carbon predominance, long-chain n-alkanes with an odd-carbon predominance, 24-ethylcholesterol and small contents of triterpenoid alcohols such as α- and β-amyrin. The distributions also show evidence for the rapid degradation of organic matter in this type of environment. Due to the high solar irradiance and presence of phototrophic organisms, visible light-induced photoprocesses intensively degrade the organic matter. The high amounts of stanols and stanones in the mats are also indicative of strong microbial degradation by aerobic and sulfate-reducing bacteria. However, despite the high sulfate-reducing activity, there is little evidence for sulfurization of the organic matter presumably due to the high content of iron present.