Calcareous Algae
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CALCAREOUS ALGAE
CALCIFIED CYANOBACTERIA
The term “calcareous algae” refers to various kinds of benthic and planktonic algae whose thalli contain biochemically precipitated calcium carbonate (CaCO3) as skeletal material (Wray, 1977; Braga and Riding, 2005). Precipitation of CaCO3 (as calcite and/or aragonite) may occur within or on the algal bodies. The term may also include mechanically accreted deposits of calcium carbonate caused by algae, usually as an interaction of biological and physical processes. Calcareous algae are a highly artificial group that constitutes calcifying members of the Chlorophyta (green algae), Rhodophyta (red algae), and Phaeophyta (brown algae) and is sometimes also used for Cyanobacteria. At present, calcareous algae are one of the most important reef builders (see “Carbonate Environments”). For a detailed reading, please refer to “Algae (Eukaryotic).”
Robert Riding University of Tennessee, Knoxville, TN, USA
Bibliography Braga, J. C., and Riding, R., 2005. Calcareous algae. In Selley, R., Cocks, L. R. M., and Plimer, I. R. (eds.), Encyclopedia of Geology. Amsterdam: Elsevier, pp. 428–436. Wray, J. L., 1977. Calcareous Algae. Developments in Palaeontology and Stratigraphy. Amsterdam: Elsevier, Vol. 4, 185 pp.
CALCIFICATION See entries “Animal Biocalcification, Evolution,” “Biofilms and Fossilization,” “Calcified Cyanobacteria,” “Calcite Precipitation, Microbially Induced,” “Calcium Biogeochemistry,” “Carbonate Environments,” “Dolomite, Microbial,” “Microbialites, Modern,” “Microbialites, Stromatolites, and Thrombolites,” and “Pedogenic Carbonates.”
Definition Cyanobacteria are alga-like bacteria that can perform oxygenic photosynthesis and nitrogen fixation (Whitton and Potts, 2000; Herrero and Flores, 2008). They have a long history and are diverse and widespread in marine, freshwater, and terrestrial environments at the present-day, where they are key primary producers in both microbial mat and planktic ecosystems. As the initiators of plastids, they have played a fundamental role in algal and plant evolution (Raven, 2002). Cyanobacteria occupy benthic substrates and can also drift in the water column. Their photosynthetic uptake of inorganic carbon can stimulate CaCO3 precipitation. This calcification can produce filamentous microfossils in benthic mats that are preserved as stromatolites and thrombolites, and can also cause water column precipitation of carbonate mud that settles to lake and sea floors. Introduction However, cyanobacterial calcification is not obligatory and is directly dependent on environmental conditions. This accounts for apparent discrepancies between the geological ranges of organic-walled and calcified cyanobacterial fossils. Calcified cyanobacteria have not been recognized in marine sediments older than 1200 Ma ago (Kah and Riding, 2007), whereas there is evidence that cyanobacteria appeared in the late Archaean or Palaeoproterozoic, in the range 2900–2150 Ma ago (Cavalier-Smith et al., 2006; Hofmann, 1976). This midProterozoic appearance of sheath-c
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