Irradiance and daylength effects on growth and chemical composition of Gyrodinium aureolum Hulburt in culture

For Gyrodinium aureolum significant irradiance and daylengtheffects were found on the division rate and on the growth-relevantChla-normalized photosynthetic rate (^gP^B). Optimum conditionsof irradiance and daylength were found at 230 µmol m^–2s^–1 and 14 h for the division rate, and at >260 µmolm^–2 s^–1 and <6 h for ^gP^B.^gP^B showed no photoinhibition,while the division rate decreased markedly at irradiances abovesaturation. This difference and the difference in optimum irradiancebetween the division rate and ^gP^B are explained by a decreasein cellular Chla/carbon ratio with increasing irradiance. Thecellular content of carbon and nitrogen decreased significantlywith increasing irradiance. Total phosphorus was independentof irradiance and daylength. Below the saturation irradiancefor ^gP^B the daily Chla-normalized carbon yield may be describedas an exponential function of the daily irradiance (irradiancex daylength).