The effect of light intensity on the quantities of chlorophyll and carotenoid pigments in a natural marine diatom community has been observed. Marked daily fluctuations in the quantities of these pigments, which showed highest concentrations around mid-day and at night, resulted from changes in quantities within the cells. Rates of synthesis and decomposition throughout the day have been computed. Highest cellular concentrations of chlorophyll and carotenoid pigments corresponded to optimal light intensities for photosynthesis....