Effect of PAR and temperature on the photosynthesis of the Japanese alga, Ecklonia radicosa (Laminariales), based on field and laboratory measurements

The effect of photosynthetically active radiation (PAR) and temperature on the photosynthesis of a Japanese brown alga, Ecklonia radicosa, was determined by using dissolved oxygen sensors and pulse-amplitude modulated (PAM) fluorometry. The effective quantum yield (Phi(PSII)) of the sporophytes, in a natural population in shallow water, showed a characteristic noontime decline that indicated the influence of high PAR. However, Phi(PSII) of sporophytes in deeper water (-10 to -20 m) did not appear to be affected and maintained relatively high values of Phi(PSII). In laboratory measurements of UPSII, after 12 h of continuous low and high PAR (100 and 1000 mu mol photon m(-2) s(-1)), a relatively greater decrease in UPSII in the high PAR treatment was observed. Furthermore, despite an overnight dark acclimation, the maximum quantum yield (F-v/F-m) under the high PAR treatment did not fully recover. A net photosynthesis-irradiance (P-E) curve was generated at 20 degrees C, which revealed low compensation and saturation irradiances (E-c and E-k) of 14 and 45 mu mol photon m(-2) s(-1), suggesting adaptation to a low PAR environment. Results from experiments to determine the net/gross photosynthesis and dark respiration rates revealed that the maximal gross photosynthetic rates occurred at 27.5 degrees C and were 5.18 mu g O-2 g(ww)(-1) min(-1); whereas, the dark respiration rate increased exponentially with a mean value of 1.35 mu g O-2 g(ww)(-1) min(-1) at 228 degrees C. F-v/F-m was relatively stable at low temperature, and the highest value (0.73) occurred at 16.5 degrees C. The field survey and laboratory experiments revealed that E. radicosa is well adapted to relatively low PAR and water temperatures typical of warm temperate coastal regions in Japan. The poor response to high PAR and the inability of sporophytes to fully recover overnight from high PAR exposure indicates that the vertical distribution of this species is driven by the PAR environment.