ON THE APPEARANCE OF CYANOBACTERIAL CALCIFICATION IN MODERN STROMATOLITES

In August 1991 calcified (magnesian calcite) filamentous cyanobacteria (Scytonema mirabile) appeared on the surface of a modern lacustrine s tromatolite (Lake R2, Rangiroa Atoll, Tuamotu Archipelago, French Poly nesia). Previously, the topmost photosynthetic layer of this sediment was devoid of such microbial biomineralisers. The calcified cyanobacte ria are accompanied by magnesian calcite grains which are evocative of bacterial precipitates. Although the appearance of cyanobacterial cal cification might be related to environmental changes in the lake (in p articular a decrease in water salinity), the persistence of non-calcif ied specimens of a filamentous cyanobacteria (Phormidium crossbyanum), which was once a dominant species of the lake benthos, shows that cal cification and environmental conditions might be only indirectly relat ed. Thus, attempts to use the fossil record of calcareous cyanobacteri al skeletons to infer changes in the chemical composition of past ocea ns cannot be made without due regard of biological processes like the appearance of new competitors.