DOES CARBOHYDRATE CONTENT AFFECT THE SINKING RATES OF MARINE DIATOMS? 1

We tested the hypothesis that positive relationships between sinking rate and irradiance were due to increases in cell density caused by accumulations of carbohydrate. In semicontinuous batch cultures of Thalassiosira weissflogii (Gru.) Fryxell el Hasle and Ditylum brightwellii (t. West) Grunow in Van Huerk, carbohydrate content was varied by growing cells under diel cycles of high or low light. Sinking rate was measured at the end of the light period and the end of the dark period, on live and heat‐killed cells. No positive correlations were found between sinking rate (which varied from – 0.060 to 0.13 m·d−1) and carbohydrate content (which varied from 10 to 950 pg · cell−1), indicating that accumulations of carbohydrate did not significantly affect sinking rate. There were no large diel variations in the sinking rate of T. weissflogii, but sinking rates of D. brightwellii grown under high light ranged from being negative (i.e. cells were floating) at the end of the light period to positive at the end of the dark period. This is the first report of positive buoyancy in vegetative D. brightwellii, a phenomenon that may only occur in D. brightwellii grown under diel cycles.

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