Effect of temperature and PAR on photosynthesis of an endangered freshwater red alga, Thorea okadae, from Kagoshima, Japan JUMPEI KOZONO 1 ,GREGORY N. NISHIHARA 2 ,HIKARU ENDO 3 AND RYUTA TERADA 1 * 1 United Graduate School of Agricultural Sciences, Kagoshima University, Korimoto 1-21-24, Kagoshima 890-0065, Japan 2 Institute for East China Sea Research, Organization for Marine Science and Technology, Nagasaki University, Taira-machi 1551-7, Nagasaki 851-2213, Japan 3 Faculty of Fisheries, Kagoshima University, Shimoarata 4-50-20, Kagoshima, 890-0056, 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 lmol 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 lmol photon m 2 s 1 (10.3–16.3, 95% Bayesian prediction interval, BPI) and 55.2 lmol photon m 2 s 1 (42.2–72.9, 95% BPI), respectively. A temperature-dependent (i.e., 8–368C range) model of net photosynthesis and dark respiration was fitted and showed that the gross photosynthetic rate, 17.3 lgO 2 g ww 1 min 1 (16.1–18.6, 95% BPI), was highest at 30.88C (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 18.48C (17.0–19.8, 95% BPI). The effective quantum yield (U PSII ) in the PSII was also negatively correlated with PAR, which decreased after 12 hours of continuous exposure to 100 (low) and 1000 (high) lmol photons m 2 s 1 at 128C and 248C. Large declines of U PSII and subsequent failure of F v / F m recovery were particularly enhanced at 128C 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. KEY WORDS: Algae, Endangered species, Freshwater, Photosynthesis, Pulse-amplitude modulation (PAM)–chlorophyll fluorometry INTRODUCTION A freshwater benthic red alga, Thorea okadae Yamada (Thoreaceae, Thoreales), was originally described by Yama- da (1949) based on a specimen collected from Hishikari (Sendai-gawa River), Kagoshima Prefecture, Kyushu Island, Japan. This species can be found on pebbles and cobbles in creeks and rivers (Kumano 2002; Higa et al. 2007) and is known as an endemic species in Japanese fresh waters that is distributed in the temperate regions of Kyushu and Honshu Islands. However, confirmed habitats are quite limited, with fewer than 10 creeks and rivers in the whole of Japan (Seto et al. 1993). In fact, this species is listed in the Red List as an endangered species (category VU, Vulnerable) by the Japanese Ministry of Environment (2012) (Terada 2015). This species has a heteromorphic life history (Yoshizaki 1986; Kumano 2002) that alternates between a macroscopic life history stage (gametophyte; Fig. S1) and a microscopic life history stage (sporophyte; Fig. S2). The macroscopic life history stage can generally be found on substrata during early winter through late spring (December–May). However, occurrence of this species in the type locality is unstable, and it is periodically unobservable in its habitat, suggesting that biotic or abiotic factors influence the incidence of the species (Sabater et al. 2016). A fundamental understanding of the life history, ecology, and physiology of Thorea okadae is essential toward the conservation of this species. After the confirmation of sexual reproduction in the macroscopic gametophyte (Yoshizaki 1986; Necchi 1987), seasonal changes in the two life history stages, including the determination of the maturation period of this species, were reported (Higa et al 2007; Sato et al. 2013). However, despite past studies of its phenology (Higa et al 2007), ecophysiological processes, such as the response of photosynthesis along a gradient of temperature and photosynthetically active radiation (PAR), remain to be examined in detail. Recent studies suggest that freshwater algae, including Thorea gaudichaudii C. Agardh and Nemalionopsis tortuosa Yoneda & Yagi (Thoreaceae, Thoreales), may be well adapted to an environment with low PAR (Necchi & Zucchi 2001; Necchi 2005; Fujimoto et al. 2014; Terada et al. 2016). Hence, these species are often observed in shaded areas, such as those found in the irrigation canals of this region. However, unlike these two species, Thorea okadae can be observed on shallow riverbeds under direct sunlight (Higa et * Corresponding author (terada@fish.kagoshima-u.ac.jp). DOI: 10.2216/18-26.1 Ó 2018 International Phycological Society Phycologia Volume 57 (6), 619–629 Published 21 September 2018 619