Mats of colourless sulphur bacteria .2. Structure, composition of biota and successional patterns

Mats of colourless sulphur bacteria from a relatively exposed shallow water sediment and from a deeper, more protected sediment were studied. In the exposed habitat, patches of white sulphur bacteria had a limited lifetime and displayed a characteristic succession in which small sulphur bacteria (Thiospira, Macromonas) dominated at first followed by Thiovulum, which was eventually displaced by Beggiatoa; during summer phototrophic organisms dominated. Thiovulum films were only similar to 0.2 mm thick and the projected area of the cells covered only 7 to 36% of the sediment surface while mats of filamentous bacteria were thicker and represented a higher biomass. In the protected locality the Beggiatoa mats were similar to 0.6 mm thick and represented a 10-fold higher biovolume than those from the exposed habitat. The projected area of the filaments covered 2 to 3 x the surface area of the sediment and had a combined length of 10 to 70 m cm(-2). The mats had a porosity of 90 to 95% and did not contain a mucous matrix. Phototrophs (mainly filamentous cyanobacteria and diatoms) were always present. Protozoan biota included more than 80 species and represented a high cell density (corresponding to 10(4)-10(5) ciliates ml(-1) in addition to flagellates and lobose amoebae). Many species depended on sulphur bacteria for food. The mats sometimes also harboured several representatives of meio- and macrofauna.