Calcareous Algae and Stromatolites

Abstract

Calcified Cyanobacteria Calcification affects the sheath of some coccoid and filamentous cyanobacteria during their life, under suitable environmental conditions. There is probably a link between sheath character, the environment and nucleation of CaCO3 within or upon the sheath. Calcification is thus extracellular and is influenced, but not fully controlled, by the cyanobacterium. It is promoted by thick sheaths, which provide favourable sites for CaCO3 nucleation, and by conditions favouring the physicochemical precipitation of CaCO3. In the Recent, calcification is widespread in freshwater lakes and streams in limestone areas, but is virtually unknown in marine environments. However, calcification occurred quite widely in marine cyanobacteria during the Palaeozoic and Mesozoic. This could reflect temporal changes in oceanic carbonate chemistry. Calcification results in (1) discrete skeletons; (2) skeletal stromatolites; (3) dendrolites and thrombolites; and (4) tufa stromatolites. Where calcification is uncommon the mucilaginous sheaths and felted masses ofcyanobacterial mats trap particulate sediment to form stromatolites which may be calcareous in composition, but which do not consist of calcified cyanobacteria in the sense of precipitation of CaCO3 on or within the sheath. The main groups of fossil calcified cyanobacteria may be exemplified by the following genera: Angulocellularia, Epiphyton, Girvanella, Hedstroemia, Renalcis and Garwoodia/ Mitcheldeania, although some of these are not definitely known to by cyanobacteria. In addition there is a variety of fossil calcareous microproblematica which have been widely attributed to the cyanobacteria. Well-defined and diverse calcified cyanobacteria first appear near the Precambrian-Cambrian boundary. Dendrolites and thrombolites composed of probable cyanobacteria are common in the Cambrian and Lower Ordovician. Subsequently, calcified cyanobacteria are generally less common in marine environments, although they reappear in relative abundance in the Upper Devonian and Lower Carboniferous, and in the Middle Triassic to mid-Cretaceous. A number of these fossils may be compared with modern analogues in freshwater lakes and streams, but the precise affinities of others are still uncertain.