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It has been suggested that the recent light history of phytoplankton and the kinetics of photoadaptation can be used to provide information about the vertical mixing processes in the upper mixed layer. To be useful as a parameter in a model of photoadaptation and vertical mixing, the response of a photoadaptive variable to changes in growth irradiance must be monotonic, significant, and comparable in time rate scale to mixing processes. Previous studies of photoadaptation kinetics have focused on the response of phytoplankton to changes in light intensity under continuous illumination. This dissertation attempts to elucidate the effects of light:dark cycle, nutrient concentration, growth rate and species difference on the photoadaptation kinetics of marine phytoplankton in a series of light transition experiments. The study found that: (1) Hysteresis of photoadaptive response exists between the two reciprocal (low-to-high vs high-to-low) light transitions. The increase in photoadaptive cellular properties following a light shift is better described by a logistic model, whereas the decrease in the same properties following the reverse light shift is better described by first order kinetics model. Shift-up and shift-down are not simply mechanistically reverse processes. (2) Phytoplankton do not shade-adapt at night; however, cellular photoadaptive variables, including pigment components and chemical composition (C, N), undergo diurnal variations, causing the light response curve of these variables to be non-monotonic under natural light:dark cycles. (3) Different algal species are different in their responsiveness to changes in growth irradiance. The difference exists both between and within taxonomic groups, and may be related to the growth characteristics or selective strategies of individual species. (4) Phytoplankton growth, as controlled by nutrient-limitation, has a positive effect on photoadaptation. As growth rate increases, so does the rate of photoadaptation. However, the increase is not homogeneous among different photoadaptive variables and is not proportional to the increase in growth rate, suggesting that biosynthesis and cell division are uncoupled and that growth is unbalanced during photoadaptation. These results revealed that factors other than light intensity can cause large variabilities in the rate and pattern of photoadaptation in marine phytoplankton. Hence, previous photoadaptation-vertical mixing model based on simple first order kinetic light response of photoadaptive variables should be re-evaluated and refined. |
