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Production and composition of the sulphated cell wall polysaccharide of Porphyridium (Rhodophyta) as affected by CO2 concentration

Abstract The effect of CO2 on growth and production of sulphated cell wall polysaccharide in the microalga Porphyridium sp. (Rhodophyta) was investigated by supplying cultures with CO2 at concentrations of about 0.03% (air), 3%, and 5%. At the two elevated CO2 concentrations, the specific growth rate was about 1.7-fold and the growth yield about fivefold higher than in the culture supplied with air alone. Carbonic anhydrase activity could be detected solely in disrupted cells grown without the addition of CO2, The highest total polysaccharide content (soluble and bound fractions) was found in the culture aerated with 5% CO2, which was about six times greater than that in the air-sparged culture. However, the fraction of soluble polysaccharide (per cell) and the ratio of soluble: bound polysaccharide were highest in the air-sparged culture. CO2 concentration also influenced the chemical composition of the cell wall polysaccharide: the ratio of galactose to xylose in the soluble polysaccharide in the culture sparged with air alone was about twice that found in the CO2-enriched cultures (0.75 vs 0.40). However, relatively insignificant changes were found in the content of the other sugars, sulphur, and glycoprotein in the cell wall polysaccharide. These results suggest that CO2 concentration affects polysaccharide composition by altering the partitioning of the fixed carbon, probably by modification of the biosynthetic pathway.