Effects of Variations in Daylength and Temperature on Net Rates of Photosynthesis, Dark Respiration, and Excretion by Isochrysis galbana Parke

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Abstract
The effects of variable daylength and temperature on net rates of photosynthesis, dark respiration and excretion of a unicellular marine haptophyte, I. galbana Parke, were examined and related to division rates. Six combinations of daylength (18:6, 12:12, 6:18 light:dark, LD) and temperature (20, 25.degree. C) were used. Daily rates of net photosynthesis were closely correlated to division rates, suggesting a direct relationship and were maximal when cells were grown at 12:12 LD at both temperatures and 18:6 LD at 20.degree. C. A daylength of 6 h decreased daily rates by decreasing the time for C uptake. Cells grown with this daylength had maximal chlorophyll a contents, suggesting a physiological adaptation by photosynthetic units to short light periods. A photoperiod of 18:6 LD at 25.degree. C decreased daily rates of net photosynthesis by reducing the hourly rate of net photosynthesis via an unidentified mechanism. The importance of rates of net dark respiration in controlling daily net photosynthesis was small, with C lost during periods varying between 4 and 14% of that gained during light periods. The influence of net excretion was small, varying betwen 1.0 and 5.5% of daily net photosynthesis.