Influence of irradiance and spectral quality on the carbon metabolism of phytoplankton

Within the euphotic zone of oceans and lakes, planktonic algae are exposed to concurrent changes in irradiance and spectral quality Most models of photoadaptations, however, consider only the effects of machance. Using a factorial expenmental design, this study evaluated the influence of irradiance and spectral quality on the chemical composition, and rates of carbon uptake, polymer synthesis and cell division of 2 species of marlne algae. Both the irradiance and spectral quality of light significantly @ S 0.05) influenced the chemical composition and the patterns and rates of carbon uptake and polymer synthesis. Cell division rates were not affected by light quality. Responses of the algae were complex and taxon-specific; however, there were several consistent patterns for all spectral qualities, chlorophyll a, proteins and polysaccharides and the proportion of photosynthate incorporated into proteins were lower at high than low irradiances. At all irradiances, chlorophyll a was greater in white and blue than red light. Rates of protein synthesis, carbon uptake and respiration were greater in blue and red than white hght of equal energy. Since there were distinct effects of irradiance and spectral quality on the metabolism and carbon uptake of marine phytoplankton, simulated in situ incubations which adjust irradiance but not spectral quality to in situ levels may not accurately estimate in situ rates of carbon uptake and protein metabolism. These experiments clearly show the importance of considering spectral quality in models of photoadaptation and primary production.