| Nonphotochemical quenching (NPQ) of chlorophyll fluorescence occurs when excess irradiance absorbed by photosynthetic pigments is dissipated as heat, thereby preventing cell damage. Although NPQ is a dominant process governing fluorescence yield near the sea surface where irradiance is high, the influence of phytoplankton physiology on NPQ is not well understood. This thesis assesses the impact of light history and nutrient stress on variability in fluorescence vs. incident irradiance for cultures of the diatom Thalassiosira pseudonana grown at 50 and 500 mumol photons m-2 s -1 under N-replete, acclimated N-limited, and N-starved conditions. Changes in the onset and saturation of NPQ were sensitive to N-starvation and correlated with variability observed for saturation irradiance of photosynthesis with light acclimation and N-stress. However, changes in maximum NPQ cannot account for the greater than 10-fold variability observed in sea-surface fluorescence yield, emphasizing the importance of both photoprotective and photoinhibitory processes. |
