Résumé
Laminated microbial ecosystems (microbial mats) on the island of Schiermonnikoog (The Netherlands) were studied with respect to variation in oxygen and sulphide profiles, depth distributions of photopigments and viable number and cell volume of purple sulphur bacteria. Cyanobacteria occurred in the top 2 mm, the dominant species being
Microcoleus chthonoplastes. The blooming of purple sulphur bacteria below the cyanobacterial layer was observed in autumn, the dominant species being the immotile
Thiocapsa roseopersicina. Cell volume of this species is indicative of its growth rate.
In situ measurements showed strong diel fluctuations in oxygen and sulphide profiles. Frequently, cyanobacteria and purple sulphur bacteria were exposed to oxygen during the day, and to anoxic conditions at night. Sulphide sometimes reached the layer of the cyanobacteria. The cyanobacteria and the purple sulphur bacteria both are very well adapted to these diel fluctuations. In addition, strong seasonal variations were observed, whereas short-term fluctuations of oxygen occurred due to changing light-climate and rainfall.
Attention was paid to the unusual occurrence of microbial mats on the North Sea beach during the autumn of 1987.