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The influence of irradiance, nitrogen limitation, and temperature on the biochemical composition of marine phytoplankton and their nutritional value to larval Crassostrea gigas Thompson, Peter Allan


The major objective of this study was to determine how marine planktonic autotrophs responded biochemically and physiologically to physical variation in their environment. The results were interpreted from ecological and evolutionary perspectives. Phytoplankton are the basis for the world's largest food web. If naturally occurring variations in temperature, light, and nutrient availability influence the biochemical, and perhaps nutritional value of the phytoplankton, then these variations may be significant in determining the efficiency of biomass transfer between trophic levels. For 10 species of marine phytoplankton, the cellular changes in carbon quota, cell volume, and nitrogen quota which occurred under different levels of light (energy) limitation were determined. Generally, cell volume, carbon, and nitrogen quotas all decreased with the decreasing availability of light energy. Fatty acid composition also responded to variation in light energy. Frequently the lowest percentage of the fatty acid 16:0 and the greatest percentage of the essential fatty acid 20:5ω3 occurred at the lowest irradiance. An increase in the percentage of the other essential fatty acid, 22:6ω3, occasionally occurred at the highest irradiance, particularity for Pavlova lutheri (Droop). Variation in temperature also influenced fatty acid composition. Most species showed an increase in the fatty acid 16:4ω1, and a decrease in 16:3ω4 at low temperatures. Only 1 of 8 species (Thalassiosira pseudonana, (Hustedt)), showed a significant relationship between temperature and the degree of unsaturation of the fatty acids. Thalassiosira pseudonana cells varied in their biochemical composition in response to variation in irradiance and nitrogen availability. At saturating irradiances, cells were relatively high in carbon, primarily short chain saturated fatty acids and carbohydrate. T. pseudonana cells grown at a saturating irradiance were a better food item for larval osyters (Crassostrea gigas, (Thunberg)) than cells grown at low levels of irradiance. When fed saturating rations of T. pseudonana cells grown at a high irradiance, C. gigas larvae had significantly higher growth rates and significantly lower mortalities.

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