Effect of dynamic light regimes on the photophysiology of marine diatom Thalassiosira weissflogii with special emphasis on xanthophyll cycling

Static (instantaneous) irradiance cycles are useful in examining the basic physiological responses of phytoplankton such pigment changes and photosynthetic capabilties; however, they reveal little regarding the response to in situ irradiance changes. Understanding such a response is imperative to studies examining photosynthesis and photoacclimation. The response of xanthophyll pigments and photosynthesis to a dynamic (gradually changing) light regime was examined in Thalassiosira weissflogii.; Cultures were grown under 100muEinm-2s -1 (designated as "high" light) and 15muEinm -2s-1 (designated as "low" light) of white light, and 15muEinm-2s-1 of blue filtered light. During exponential growth, cultures were exposed to a dynamic light regime utilizing a specially designed bioreactor that allows for gradual irradiance changes. A dim white light filter and a blue light filter were utilized to represent surface vs. depth irradiance changes, respectively. Cultures were subjected to 45, 90 and 120-minute sinusoidal light curves, representing what a cell might experience in a circulating mixing regime such as a Langmuir cell.; Greater amounts of diadinoxanthin were converted to diatoxanthin with increasing cycle time, indicating that the rate of irradiance change for the 45-minute cycle occurred faster than the xanthophyll cycle could respond. PvsE curves also changed in response to dynamic light changes and were especially dependent on growth irradiance. High light cultures exhibited the greatest change in Pmax between pre- and post-cycle measurements. In previous studies on fluctuating light, greater irradiance shifts led to an increase in Pmax (per chla and per cell basis), alpha and Ek values up to a maximal shift in irradiance. A similar pattern was seen in this study between dim white light and blue cycle shifts in the high light culture, where blue cycle had higher values than dim white light. This further supports suggestions from previous studies that the phytoplankton's light history plays a role in its photosynthesis.; Much debate exists regarding the accuracy of ocean productivity models, many of which do not account for photoacclimation responses. From this study, it is apparent the mixing rates play a role in determining photosynthetic rates even when other photoacclimation responses are negligible.