Algae. 1992; 7(1): 147-154.

Algal Calcification: Its Contribution to the CO sub(2) Problem

Megumi Okazaki

Department of Biology, Tokyo Gakugei University

The recent increase in atmospheric CO sub(2) is of serious concern because CO sub(2) in air causes global warming by the greenhouse effect. It was suggested that a very delicate carbon balance between atmosphere, hydrosphere and geosphere on the present earth was attained by photosynthesis and biological calcification in the ocean. Most calcareous algae are marine and the coral reef is especially important and productive environment for standing crops of calcareous macroalgae. The crustose genus Lithophyllum (Corallinaceae, Rhodophyta) and Halimeda (Chlorophyta, Caulerpaceae) Play a very important role in formation of coral reefs. The latter is remarkable as a CaCo sub(3) production in coral reefs in the oceans is estimated to be 0.62 - 6.2 × 10 super(9) t as CO sub(2). Unicellular calcareous algae belonging to the coccolithophorid such as Emiliania huxleyi (Haptophyta) produce calcified scales coccoliths. They have been responsible for most of the pelagic carbonate over the past 100 million years, including the present oceans. Algal calcification takes place by a following reaction: 2 HCO sub(3) super(-) + Ca super(2+) → CO sub(2) + CaCO sub(3) + H sub(2) O. Thus, it should be noted that CaCo sub(3) formation itself accompanies CO sub(2) formation from dissolved HCO sub(3) super(-) in seawater. However, no release of Co sub(2) fixation. But, if their photosynthetic products could be completely oxidized back to CO sub(2) in ocean, algal calcification leads to CO sub(2) release as a final reaction. This is clearly shown on a Deffeyes composition diagram. Recent field study on coral reefs revaled the effective net fixation of CO sub(2), and the concept of a coral reef eco-factory was proposed to fix the excess CO sub(2) in the atmosphere. It was also proposed to use the coccolithophorid algae, Pleurochrysis carterae and Gephyrocapsa oceanica, for fixation of CO sub(2) in exhaust gas into CaCo sub(3) because they were grown easily and rapidly in the laboratory.

Keywords : gal calcification, atmospheric carbon dioxide, calcareous algae, calcium carbonate deposition, climate change, coccolithophorid, global carbon cycle, global warming