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UBC Theses and Dissertations

Effects of nutrient limitation on biochemical composition of three microalgae and their food value to oyster larvae, Crassostrea gigas Calderwood, Gail Sibbald


Three microalgae commonly used in bivalve aquaculture were grown in unialgal batch culture and harvested for chemical analyses at rigorously monitored stages of nitrate, phosphate, or silicate limitation. Two series of cultures were analysed; the first was light-limited. Light-saturated cultures of the second series were used in oyster larvae growth trials. Cellular caloric value was reduced by nutrient starvation of Tahitian Isochrysis galbana (clone T-Iso) (nitrate or phosphate) and Chaetoceros calcitrans (nitrate or silicate) due to depletion of cellular protein and lipid, while carbohydrate content increased. Conversely, Thalassiosira pseudonana (clone 3H) increased cellular protein and lipid and decreased carbohydrate under silicate starvation resulting in an increase in final caloric content. Fatty acid composition (percent of the total fatty acids) of light-saturated, nutrient-limited diatoms was remarkably stable through 6 hours of starvation. I. galbana (T-Iso) polyunsaturated:monounsaturated fatty acid ratios dropped dramatically by the second day of nutrient starvation in series 2. Changes observed in series 1 could either be attributed to light limitation or long starvation periods. Polyunsaturated:monounsaturated fatty acid ratios declined with time in both diatoms in series 1. The most decisive effect on fatty acid composition among all three algal species occurred under phosphate limitation of T. pseudonana during which 20:5n3 decreased from 22% to 6% and 16:0 increased two-fold from 19% to 37%. I. galbana (T-Iso) was a poor diet for C. gigas larvae compared to both diatom diets regardless of nutrient status, supporting the belief that the fatty acid 20:5n3 may be a more significant nutritional factor than 22:6n3. The latter is found in high concentration in I. galbana (T-Iso) but not in the diatoms. C. gigas larvae grew fastest when fed the most energy rich diatom treatments. When caloric values were similar, increased carbohydrate may have provided energy which spared essential proteins and lipids, thereby permitting faster growth. It is recognized that ration levels were sub-optimal in the diet trials.

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