Photosynthetic performance of Saccharina angustata (Laminariales, Phaeophyceae) at the southern boundary of subarctic kelp distribution in Japan

Effects of photosynthetically active radiation (PAR) and temperature on the photosynthesis of a subarctic edible brown alga, Saccharina angustata (Laminariaceae, Laminariales), from Hokkaido, Japan, were determined using a pulse amplitude modulation (PAM)-chlorophyll fluorometer and optical dissolved oxygen sensors. Photosynthesis-PAR experiments at 8, 16, and 20 degrees C revealed highest net photosynthetic rates at 16 degrees C; whereas, the temperature effect on maximum quantum yield (F-v/F-m) showed the highest F-v/F-m at 7.0 to 8.4 degrees C. These temperatures corresponded to the optimum temperatures for growth of Japanese Saccharina species. The maximum gross photosynthetic rate from gross photosynthesis-temperature curve was at 21.2 degrees C. Though consistent with the highest seawater temperature in the study site, it is close to the limit for thermal inhibition, because F-v/F-m above 20 degrees C was compromised after 48-72h of incubation. Continuous 6-h exposures to PAR of 200 mu mol (low) and 1000 mu mol (high) photons m(-2)s(-1) at 8, 16, and 20 degrees C caused chronic photoinhibition in the alga, with greater declines in effective quantum yields of photosystem II (Phi(PSII)) and failure of recovery in post-dark acclimation F-v/F-m under high PAR. This physiological performance provides a basis for understanding the persistence of S. angustata near its southern boundary in the western Pacific.