The effect of photosynthetically active radiation (PAR) and temperature on the photosynthesis of an endangered Japanese freshwater red alga, Thorea okadae (Thoreaceae, Thoreales), was determined by using pulseamplitude modulation (PAM) fluorometry and dissolved oxygen sensors. The macroscopic life history stage (gametophyte) was observed on pebbles and cobbles in a shallow riverbed from early winter to late spring (December-May), in direct sunlight (similar to 800 mu mol photons m(-2) s(-1)) during winter noon time with a clear sky. A net oxygenic photosynthesis-PAR model revealed that the net photosynthetic rate increased linearly and saturated, with a compensation (E-c) and saturating PAR (E-k) of 13.3 mu mol photon m(-2) s(-1) (10.3-16.3, 95% Bayesian prediction interval, BPI) and 55.2 mu mol photon m(-2) s(-1) (42.2-72.9, 95% BPI), respectively. A temperature-dependent (i.e., 8-36 degrees C range) model of net photosynthesis and dark respiration was fitted and showed that the gross photosynthetic rate, 17.3 mu g O-2 g(ww)(-1) min(-1) (16.1-18.6, 95% BPI), was highest at 30.8 degrees C (30.0-31.7, 95% BPI) and decreased sharply above this temperature. The dark respiration rate exponentially increased in response to temperature. The maximum quantum yield (F-v/F-m) in photosystem II (PSII) was dome-shaped with respect to temperature; however, it was somewhat stable at low temperatures with the highest value of 0.47 (0.46-0.48, 95% BPI) at 18.4 degrees C (17.0-19.8, 95% BPI). The effective quantum yield (Phi(PSII)) in the PSII was also negatively correlated with PAR, which decreased after 12 hours of continuous exposure to 100 (low) and 1000 (high)mu mol photons m(-2) s(-1) at 12 degrees C and 24 degrees C. Large declines of Phi(PSII) and subsequent failure of F-v/F-m recovery were particularly enhanced at 12 degrees C with high PAR, signifying cold-temperature induced photoinhibition. However, based on the E-c and E-k of this species and the incident PAR in the habitat, this species seems to be relatively better adapted to higher PAR environments compared with other Japanese species of Thoreaceae, Thorea gaudichaudii and Nemalionopsis tortuosa.
