Résumé
A study on the predominant sources of organic matter and the main diagenetic processes in two different cyanobacterial mats from evaporite-controlled environments was performed. Fatty acids, hydrocarbons, alcohols, ketones and aldehydes were anlyzed in selected millimetre and submillimetre core sections. The changes in lipid composition were evaluated by comparison with the vertical distributions of the populations observed by optic microscopy and with the lipid patterns of enrichment cultures of species such as cyanobacteria, diatoms, purple bacteria, sulphate-reducers and methanogens obtained from the mats.
The cyanobacteria
Phormidium valderianum and
Microcoleus chthonoplastes are the predominant primary producers, and occur almost as monocultures in the respective top layers. However, these mat-forming organisms only leave minor features in the solvent-extractable lipid sedimentary record. The predominant fatty acid distributions parallel the composition observed in the enrichment cultures of purple bacteria and appear mixed with acids characteristic of heterotrophic eubacteria such as sulphate-reducers. The concentrations of these lipids are, however, 5–10 times lower than the cyanobacterial acids from the top layer.
De novo heterotrophic eubacterial synthesis is also observed in cases such as the highly branched isoprenoid eicosenes, the major hydrocarbon in the deep layers (>2 mm) of the
Phormidium mat. Other major diagenetic changes involve dehydration and hydrogenation. These two processes take place concurrently under anoxic conditions and have been observed among the sterols and the isoprenoid alcohols. Significant amounts of 5ß(H)-stanols were observed in the more reducing sections where molecular indicators of methanogenic bacteria were also found.