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 stromatolite (Lake R2, Rangiroa Atoll, Tuamotu Archipelago, French Polynesia). Previously, the topmost photosynthetic layer of this sediment was devoid of such microbial biomineralisers. The calcified cyanobacteria are accompanied by magnesian calcite grains which are evocative of bacterial precipitates. Although the appearance of cyanobacterial calcification might be related to environmental changes in the lake (in particular a decrease in water salinity), the persistence of non-calcified specimens of a filamentous cyanobacteria (Phormidium crossbyanum), which was once a dominant species of the lake benthos, shows that calcification and environmental conditions might be only indirectly related. Thus, attempts to use the fossil record of calcareous cyanobacterial skeletons to infer changes in the chemical composition of past oceans cannot be made without due regard of biological processes like the appearance of new competitors.