No abstract

The dlnoflagellate Crypthecodinium cohn~i was found to prey specifically on the unicellular red alga Porphyrjdum sp. and to contain enzymes that degrade its cell wall Cell wall production and conlposition of the alga are affected by nitrate and sulfate deprivation, the main changes being an increase in methylhexose and a decrease in glucose and xylose. When the dinoflagellate was fed with Porphyndium sp. having modified cell walls, fewer cells were ingested. Similarly, in chemosensory experiments it was found that the dinoflagellate was more attracted to the native polysaccharide than to polysaccharides extracted from deprived Porphyridium sp. Polysaccharide-degrading activity was higher when the dinoflagellate was fed with nondeprived algal cells than with deprived cells (82 to 86% and 45 to 53% reduction in viscosity, respectively). Although the dinoflagellate could not survive on polysaccharide alone, the induced enzymatic activity was similar irrespective of whether the polysaccharide was extracted from deprived or nondeprived cells. The results indicate that the process of predation is not linked to polysaccharide-degrading activity, which is affected by the cell-wall composition of the prey, i.e. algal cells.