PREDATION BY A DINOFLAGELLATE ON A RED MICROALGA WITH A CELL-WALL MODIFIED BY SULFATE AND NITRATE STARVATION

The dinoflagellate Crypthecodinium cohnii was found to prey specifically on the unicellular red alga Porphyridium sp. and to contain enzymes that degrade its cell wall. Cell wall production and composition 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 Polphyridium 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.