Effect of temperature and PAR on photosynthesis of an endangered freshwater red alga, Thorea okadae, from Kagoshima, Japan

Abstract 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 pulse-amplitude 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 (∼800 μmol 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 (Ec) and saturating PAR (Ek) of 13.3 μmol photon m−2 s−1 (10.3–16.3, 95% Bayesian prediction interval, BPI) and 55.2 μmol photon m−2 s−1 (42.2–72.9, 95% BPI), respectively. A temperature-dependent (i.e., 8–36°C range) model of net photosynthesis and dark respiration was fitted and showed that the gross photosynthetic rate, 17.3 μg O2 gww−1 min−1 (16...