Effect of light intensity on growth and polysaccharide production in red and blue-green rhodophyta unicells

Light utilization efficiency was studied in two species of the unicellular red algae, Porphyridum sp., a marine species, and P. aerugineum, a fresh water species, having as their main accessory pigments phycoerythrin and phycocyanin, respectively. When cultivated under three photon flux densities of 75 (low), 150 (medium) and 300 (high) μmole quanta m^-2 s^-1, the growth rate for the two algae was one doubling per day under medium and high photon flux but only one doubling per every 2 days under the low photon flux. Under all photon fluxes the maximum cell concentration was higher for Porphyridium sp. than for P. aerugineum, and the total cell polysaccharide was higher for P. aerugineum than for Porphyridium sp. In P. aerugineum cell polysaccharides increased with increasing photon flux, but cell number remained almost constant. In contrast, Porphyridium sp. responded to varying photon flux by changing cell concentration, and the differences in cell polysaccharides were much smaller at the different light intensitie. The chlorophyll a content of P. aerugineum was much higher than that of Porphyridium sp. The ratio of phycobiliproteins to chlorophyll a was higher in Porphyridium sp., and the changes in this ratio with increasing light intensities were more marked in P. aerugineum. The photosynthetic rates in general and the differences in the rates between the high and low photon flux were higher in Porphyridium sp. than in P. aerugineum. The differences in growth characteristics between the two algae seem to result from differences in light absorbance of the cultures resulting from the differences in cellular pigment ratio and concentration.