The combined effects of photosynthetically active radiation (PAR) and temperature on the photosynthesis of a temperate Japanese brown alga, Sargassum patens (Fucales), were determined by field and laboratory measurements. Underwater measurements of the natural population of this alga in Kagoshima, Japan, revealed that the effective quantum yield (I broken vertical bar (PSII) ) declined with increasing incident PAR, with minimum I broken vertical bar (PSII) occurring during noon to early afternoon. I broken vertical bar (PSII) recovered in the evening, indicating dynamic photoinhibition. In laboratory experiments, I broken vertical bar (PSII) was also negatively correlated with PAR, which decreased after 12 h of continuous exposure to 200 (low) and 1000 (high) mu mol photons m(-2) s(-1) at 8, 20, and 28 A degrees C. Maximum quantum yield (F (v) /F (m) ) at 8 A degrees C with low PAR failed to recover after 12 h of dark acclimation, suggesting the influence of low temperature in chronic photoinhibition. Photosynthesis-irradiance (P-E) curves likewise revealed lower net photosynthetic rates and photoinhibition at 8 A degrees C. Gross photosynthesis and dark respiration experiments determined over a range of temperatures (8-40 A degrees C) revealed that the maximum gross photosynthetic rate (GP (max)) occurred at 26.9 A degrees C. F (v) /F (m) after 72 h of temperature exposures was stable at 8-32 A degrees C; but it was deactivated at 36 and 40 A degrees C. This species is well-adapted to the current range of temperature in the temperate region of Japan (7-28 A degrees C). However, the combined effects of low temperature and PAR may influence algal photosynthetic efficiency and so may be limiting at the marginal region of northern distribution of this temperate species.