The effect of irradiance and temperature on the photosynthesis of a native alga Sargassum fusiforme (Fucales) from Kagoshima, Japan

The effect of irradiance and temperature on the photosynthesis of a Japanese native brown alga, Sargassum fusiforme, was determined by using dissolved oxygen sensors and pulse amplitude-modulated fluorometry. In situ measurements of the effective quantum yield (Phi(PSII)) decreased as irradiance increased at 12: 00 PM and recovered in the late afternoon, indicating that a proportionately greater number of reaction centres were closed. Likewise, laboratory measurement after 6 hours of exposure to low and high (300 and 1000 mu mol photons m(-2) s(-1), respectively) irradiances showed a relatively greater decrease in Phi(PSII) at high irradiance, and both treatments recovered overnight. A model of the net photosynthesis-irradiance relationship determined at 20 degrees C revealed that the net photosynthetic rate quickly increased at irradiances below the estimated saturation irradiance of 391 mu mol photons m(-2) s(-1) [ 95% Bayesian credible interval (BCI) of 299-515 mu mol photons m(-2) s(-1)]. Gross photosynthesis and dark respiration rates determined over a range of temperatures (8-36 degrees C) revealed that the maximal gross photosynthetic rate was 24.1 (BCI: 21.5-26.8) mu g O-2 g(ww)(-1) min(-1) at 22.9 degrees C (BCI: 19.8-25.8), and the dark respiration rate increased nonlinearly from a low of 0.5 mu g O-2 g(ww)(-1) min(-1) at 8 degrees C to a high of 8.9 mu g O-2 g(ww)(-1) min(-1) at 36 degrees C. The highest value of the quantum yield (F-v/F-m = Phi(PSII) at 0 mu mol photons m(-2) s(-1)) was estimated to occur at 22.9 degrees C (BCI: 21.8-23.9) and was 0.72 (BCI: 0.70-0.73). The specimens examined appeared to be well adapted to the natural temperature range of the study site. Although the irradiances on the fronds were strong enough to induce a drop in quantum yield, our evidence clearly suggests that full recovery occurs overnight.