||The importance of phytoplankton in oceanic ecosystems and biogeochemical cycles is well recognized, but the ecological role of phytoplankton cell death, the causes, biochemistry, and the quantitative significance of cell death in the ecology of phytoplankton populations and biogeochemical cycles are not well understood. The main objective of this thesis was to determine whether nutrients (N or Si) starvation, silicate resupply and darkness promote cell death in the marine diatom, Thalassiosira weissflogii, and to quantify phytoplankton cell death by using a combination of physiological, biochemical and molecular techniques. Si deficiency has been identified as an important factor that is able to cause phytoplankton cell death and cessation of growth. This work shows that silicate resupply to a Si-starved diatom may also cause cell death which depends on the duration of Si starvation and the concentration of silicate that was resupplied. Short term (24 h) and long term (12 days) recovery from the silicate starvation in stationary or senescent phases was investigated. Both the specific algal cell death ( δb ) and growth ( μb ) rates increased and were positively correlated with concentrations of silicate resupply 24 h or two weeks after silicate was added in the stationary phase, while prolonged Si starvation resulted in a loss of cellular physiological capacity to respond to silicate resupply as there were few differences between cultures with and without silicate resupply in the senescent phase. The resupply of 100 μM silicate showed a significant increase in δb compared to that with no Si supply and the other lower Si additions (p<0.05, one-way ANOVA), which indicated that a resupply of a high silicate concentration might be detrimental or “toxic” to the cell's metabolism due to enhanced protease activities. The effect of darkness on phytoplankton cell growth and death was also addressed, with silicate resupply in the dark during stationary or senescent phases. In this study, the hypothesis that darkness and silicate resupply are important factors in determining the survival and succession of diatoms, and may protect Si-starved cells from cell death and prevent the degradation of various cellular components was experimentally examined. δb of the cultures with the 20 and 120 μM silicate resupply in the stationary phase were ~3-fold higher than those cultures with no silicate supply. However, when silicate was added in the senescent phase, δb showed no obvious difference from the no silicate supply cultures. Cultures with silicate resupply showed a higher level of the caspase-1 and -3 activities in the senescent phase than in the stationary phase. Additionally, cell death that was induced by darkness was also examined when T. weissflogii was under N or Si starvation and silicate resupply. N starvation was a more severe stress than Si starvation for the survival of diatoms. Si-starved diatom cells resulted in a higher cellular capacity for organic carbon and nitrogen, a more gradual decline in Fv/Fm, higher LAP activities and lower caspase-1 activities than N-starved cells. In darkness, nutrient-sufficient cultures showed a 3.8-fold increase in LAP activity, and a 4.2- and 1.6-fold increase in caspase-1 and -3 like activities, respectively. Darkness under the N or Si starvation enabled diatoms to prolong their viability during nutrient stress, but darkness also induced higher caspase activities.