Response and Recovery of Thalassiosira weissflogii to Nitrogen and Silicon Starvation: Growth Rates, Nutrient Uptake, and C, Si, and N per Cell

Understanding the response of diatoms to nutrient stress is important, both on its own terms and for the accurate portrayal of this key group of organisms in ecological and biogeochemical models. We therefore examined the growth and elemental composition of Thalassiosira weissflogii grown into nutrient starvation and then recovery following nutrient readdition. During nitrate starvation, T. weissflogii continued dividing, producing cells with low quotas of N, P, and Si. Following nitrate readdition, cells immediately began taking up nitrate at relatively low net cell specific rates and to rebuild cellular stores of all nutrients. More than 30 hours elapsed before there was a visible increase in cell numbers. Cellular C/N and C/P ratios remained high and N/P ratios remained low for the remaining 45 hours of the experiment. Cells in the silicon starvation experiment abruptly ceased dividing at 1-3 µM dissolved silicon (DSi) and immediately resumed dividing upon resupply of DSi. Growth rates and net cell specific rates of silicon uptake recovered to maximal values within 3 to 7 hours, but net cell specific rates of nitrate and phosphate never did, driving a decline in N and P per cell during the whole experiment. Ratios of C/N and N/P remained close to Redfield values. While cell specific rates of silicon uptake scaled strongly with DSi concentrations during all portions of the silicon starvation experiment, during the nitrogen starvation experiment, they were directly related to cellular growth rates, suggesting that during nitrate starvation, silicon acquisition was acting as the rate limiting step for cell division.