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

The effect of CO2 on growth and production of sulphated cell wall polysacch aride in the microalga Porphyridium sp. (Rhodophyta) was investigated by su pplying cultures with CO2 at concentrations of about 0.03% (air), 3%. and 5 %. At the two elevated CO2 concentrations. the specific growth rate was abo ut 1.7-fold and the growth yield about fivefold higher than in the culture supplied with air alone. Carbonic anhydrase activity could be detected sole ly in disrupted cells grown without the addition of CO2. The highest total polysaccharide content (soluble and bound fractions) was found in the cultu re aerated with 5% CO2, which was about six times greater than that in the air-sparged culture. However, the fraction of soluble polysaccharide (per c ell) 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 s oluble polysaccharide in the culture sparged with air alone was about twice that found in the CO2-enriched cultures (0.75 vs 0.40). However. relativel y insignificant changes were found in the content of the other sugars, sulp hur, and glycoprotein in the cell wall polysaccharide. These results sugges t that CO2 concentration affects polysaccharide composition by altering the partitioning of the fixed carbon, probably by modification of the biosynth etic pathway.