The temperature and light responses on the photosynthesis of two freshwater red algae, Virescentia helminthosa and Sheathia arcuata (Batrachospermaceae), from Japan Jumpei Kozono 1 & Gregory N. Nishihara 2 & Hikaru Endo 3 & Ryuta Terada 1 Received: 28 August 2019 /Revised and accepted: 14 October 2019 # Springer Nature B.V. 2020 Abstract 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 (E c ) 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 (E k ) 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 (GP max ), 1.79 and 1.19 μgO 2 g ww -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 (F v /F m ) 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 (Φ PSII ) in the two species under high PAR. Nevertheless, their F v /F m mostly recovered after a subsequent 12-h dim-light acclimation, suggesting the potential of recovery from daytime chronic photoinhibition. Keywords Batrachospermaceae . Batrachospermales . Batrachospermum . Rhodophyta . Photochemical efficiency of PSII . Photosynthesis . Pulse amplitude modulation-chlorophyll fluorometry Introduction Riparian vegetation bordering a reach of a stream plays an important role in influencing light and temperature by cre- ating shade (Giller and Malmqvist 1998); therefore, these shaded habitats for biological organisms in the stream or irrigation canal can be maintained by the presence of both underwater and riparian environments. In fact, incident photosynthetically active radiation (PAR) in this shaded environment seems insufficient for higher aquatic plants that can photosynthesize optimally under direct sunlight; however, low PAR adaptation appears to be common in some freshwater red algae of Compsopogonales, Batrachospermales, and Thoreales (Necchi Jr and Zucchi 2001; Necchi Jr 2005; Kusakariba and Necchi Jr 2009; Fujimoto et al. 2014; Terada et al. 2016; Kozono et al. 2018), and it might be a strategy to occupy stable habitat without competition. Two species of the family Batrachospermaceae (Batrachospermales), Virescentia helminthosa (Bory) Necchi, Agostinho et Vis (= Batrachospermum helminthosum Bory) and Sheathia arcuata (Kylin) Salomaki et Vis (= Batrachospermum arcuatum Kylin), which are known as “Kawa-Mozuku (river slimy algae)” Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10811-019-01967-7) contains supplementary material, which is available to authorized users. * Ryuta Terada terada@fish.kagoshima-u.ac.jp 1 United Graduate School of Agricultural Sciences, Kagoshima University, Kagoshima 890-0065, Japan 2 Institute for East China Sea Research, Organization for Marine Science and Technology, Nagasaki University, Nagasaki 851-2213, Japan 3 Faculty of Fisheries, Kagoshima University, Kagoshima 890-0056, Japan Journal of Applied Phycology https://doi.org/10.1007/s10811-019-01967-7