The temperature and light responses on the photosynthesis of two freshwater red algae, Virescentia helminthosa and Sheathia arcuata (Batrachospermaceae), from Japan

The temperature and light responses of photosynthesis in two freshwater red algae, Virescentia helminthosa and Sheathia arcuata (Batrachospermaceae), were determined by a pulse amplitude modulation (PAM) chlorophyll fluorometer and dissolved oxygen sensors. Net oxygenic photosynthesis–photosynthetically active radiation (PAR) models of V. helminthosa and S. arcuata revealed similar low PAR-adapted responses, with a compensation PAR (Ec) of 6.95 and 11.5 μmol photons m−2 s−1 (5.58–8.42 and 9.10–11.42, 95% Bayesian prediction interval, BPI) and saturating PAR (Ek) of 18.8 and 17.7 μmol photons m−2 s−1 (14.5–24.7 and 13.0–23.90, BPI), respectively. A temperature-dependent model of net photosynthesis and dark respiration for two species also showed similar temperature responses, and the gross photosynthetic rate (GPmax), 1.79 and 1.19 μg O2 gww−1 min−1 (1.62–1.96 and 1.08–1.29, BPI), was highest at 26.4 and 30.3 °C (23.9–28.7 and 28.3–32.1, BPI). The maximum quantum yields (Fv/Fm) for two species also had similar responses with respect to temperature; however, it was generally stable at low temperatures (8–20 °C) with the highest value of 0.52 and 0.56 (0.49–0.54 and 0.54–0.58, BPI) occurring at 18.5 and 20.9 °C (17.1–19.7 and 19.8–21.9, BPI). Continuous exposure (12 h) to PAR of 100 (low) and 1000 (high) μmol photons m−2 s−1 at 12, 16, and 24 °C revealed greater declines in their effective quantum yield (ΦPSII) in the two species under high PAR. Nevertheless, their Fv/Fm mostly recovered after a subsequent 12-h dim-light acclimation, suggesting the potential of recovery from daytime chronic photoinhibition.