The effect of nitrogen source on photoinhibition in marine phytoplankton

The marine phytoplankton community represents a diverse group of organisms that display a wide range of metabolic activity. This diversity led to Hutchinson's "Paradox of the Plankton" in which such diversity in the face of (apparent) stability is enigmatic. The range of the diversity of this group has been extended beyond morphological considerations to physiological variation. This thesis addresses the physiological variation of marine phytoplankton in terms of the plasticity of photosynthesis and nitrogen metabolism. It seeks to further understanding of nutrient control of photosynthesis beyond the effect of nutrient depletion. The interaction between nitrogenous nutrients and photosynthesis was addressed by investigating the hypothesis that some phytoplankton are capable of utilizing nitrate to mitigate photoinhibitory effects.; The effect of nitrogen source (nitrate or ammonium) on photosynthetic activity was investigated in a dinoflagellate and three species of diatoms grown under moderate light levels. Estimates of the photoinhibition parameter, {dollar}beta,{dollar} indicated that the dinoflagellate, Pyrocystis lunula, maintained higher rates of oxygen evolution in cultures grown on nitrate compared to cultures grown on ammonium. One of the diatoms, a large clone of Ditylum brightwellii, showed a similar but less marked response. The small diatoms Asterionellopsis glacialis, Skeletonema sp. and a small clone of Ditylum brightwellii showed insignificant differences between {dollar}beta{dollar} as a function of nitrogen source. Results suggested the nitrate mitigation of photoinhibition could be a function of phylogeny or cell size.; To further elucidate the possible interaction of nitrate photoprotection and cell size, internally stored nitrogen was also investigated. While both nitrate photoprotection and internal nitrogen pool sizes tended to increase with cell volume, the two parameters were not directly correlated. This lack of correlation suggested that internally available nitrate is not the proximal cause of differences in the ability to moderate photoinhibition via nitrate metabolism.; While oxygen evolution is a useful means to measure photosynthesis, it may be less reliable at high light intensities due to cell processes that consume oxygen. In order to examine directly the effects of nitrogen source on light harvesting, fluorescence techniques were employed. The saturation pulse technique confirmed results obtained from oxygen evolution experiments for Skeletonema sp. and Pyrocystis lunula. Results for the two clones of Ditylum brightwellii differed indicating that oxygen consuming processes may be employed by this species at high light intensities to maintain active photosynthetic electron transport.