Influence of light intensity on diel variations in rates of growth, respiration and organic release of a marine diatom: comparison of diurnally constant and fluctuating light

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Abstract
Effects of variations in light intensity on diel patterns of growth, respiration and organic release of Skeletonema costatum (Grev.) Cleve in a cyclostat were evaluated. Light intensity was either constant during the light period at levels from 1500 to 15 .mu.E [einstein] m-2 s-1 or fluctuated throughout the light period from 500 to 10 .mu.E m-2 s-1 at rates of either 1 or 12 cycles day-1. Periodicity in cell division was observed only at light intensities of .gtoreq. 130 .mu.E m-2 s-1 and was decreased under diurnally varying light. Under all light conditions C and pigment growth were maximal during the light period but well coupled throughout the 24-h period. C assimilation during the dark period varied from 19-34% of total daily production and was a linear function of growth rate. Respiratory activity during the light period increased relative to total daily respiration as growth rate increased. The increase in nighttime C assimilation with growth rate interacted with nighttime respiration through the refixation of respired C, thus influencing the pattern of respiratory loss of C. Rates of organic release (Ec) were maximal during the light period and diel variations consistently increased with light intensity. Fluctuating light increased Ec relative to constant light. Net growth efficiency was maximal at 130 .mu.E m-2 s-1 when cell division periodicity was greatest. Under other light conditions relatively higher rates of cell division occurred at night and cell division periodicity was reduced as well as net growth efficiency. Cellular chemical fractionation indicated that under high or variable light conditions fixed C was stored during the light period for subsequent synthesis of protein and pigments, and division at night. Such an uncoupling of photosynthesis and other growth parameters resulted in greater metabolic costs to the cell.

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