Changes in buoyancy and chemical composition during growth of a coastal marine diatom: Ecological and biogeochemical consequences

Growth and sinking of the coastal marine diatom Thalassiosira weissflogii was studied during experiments in a 2.1 m tall water column. Under nutrient-replete conditions, T. weissflogii grew rapidly in the upper layer of the tank and cells exhibited nearly neutral buoyancy. Cells sank, however, after depletion of ambient nitrate. The growth and sinking of cells were well described by a simple model with nitrate-dependent growth and sinking terms. The best fit was obtained by describing the sinking response with a sigmoid function, where the time scale for increased sinking was 25 h after nitrate depletion. Smaller-scale batch culture experiments showed changes that were consistent with increased sinking upon depletion of nitrate: carbohydrate per mi of culture increased 2- to 5-fold over 3.5 d, while protein per mi did not change significantly. After re-introduction of nitrate into the medium, carbohydrate:protein ratios reverted to their original values. Calculations show that intracellular density and cell sinking rate should change substantially due to fluctuations in carbohydrate and that increased ballast of carbohydrate was sufficient to cause the sinking observed in the tank. Changes in the buoyancy of small (15 mu m) diatoms such as T. weissflogii in response to changes in nutrient status have important ecological and biogeochemical implications. Nutrient-dependent changes in sinking rates can result in increased residence time of cells in the mixed layer of the ocean, and in enhanced transport of deep nutrients to the euphotic zone uncoupled from inputs of inorganic carbon.