Calcium carbonate formation by calcareous algae –Its mechanisms and productivity in ocean

Abstract

It was suggseted that a very delicate carbon balance between atmosphere, hydrosphere and geosphere on the present earth was attained by CO₂ fixation by photosynthesis and by biological calcification in the ocean. The "calcareous algae", which deposit massive calcium carbonate on the thallus, are mostly marine and are widely scattered among Cyanophyta, Rhodophyta, Chlorophyta, Chlomophyta and Haptophyta. The coral reef is especially important for standing crops of macro-calcareous algae and for their productive environment of calcium carbonate. Calcification in the macro-algae takes place extracellularly in the specially separated sites from the outer seawater, i. e., in intercellular space (Halimeda in Chlorophyta) or thickened cell walls (Corallinaceae in Rhodophyta). It seems to be coupled with an increase in pH in the same sites which is caused by CO₂ fixation by photosynthesis. Calcium carbonte is deposited as aragonite (Halimeda) or magnesian calcite (Corallinaceae). Unicellular algae, coccolithophorids in Haptophyta, produce disc-shaped scales of calcite called "coccoliths". The calcified scales are formed intracellularly and then extruded to the outer surface of the cell. The morphology of coccoliths is under genetic control, and an involvement of specific calcium-binding acid polysaccharides is strongly suggested in the formation of fine structure of coccoliths. The coccolith formation is intimately associated with photosynthesis. Emiliania hyxleyi, a species in coccolithophorids, is a cosmopolitan species and a main producer of pelagic carbonate. Annual production of calcium carbonate by calcareous algae are not known in detail, but it may be estimated as sevelal ten parcent of the total annual production of carbonate (2.0x10⁹ ton as CO₂) in the ocean. Calcium carbonate formation in the ocean accompanies CO₂ evolution as follows; 2HCO₃^-+Ca²⁺→CaCO₃+H₂O+CO₂, although CO₂ fixation by photosynthesis acts as a "driving force" for CaCO₃ precipitation. Therefore, for the evaluation of calcification in the effect on atmospheric CO₂, it must be estimated how photosynthesis overcomes the calcification in rate and what portion of organic matter synthesized in photosynthesis is remained without oxidation back to CO₂.