Photosynthetic activity of two life history stages of Costaria costata (Laminariales, Phaeophyceae) in response to PAR and temperature gradient

:Photosynthetic responses to photosynthetically active radiation (PAR) and temperature were investigated in the macroscopic sporophyte (SPO) and microscopic gametophyte (GAM) stages of the brown alga Costaria costata from Hokkaido, Japan. Measurements of net photosynthesis, respiration rates and photochemical efficiency were carried out using dissolved oxygen sensors and a pulse amplitude–modulated fluorometer. Net photosynthesis–irradiance curves of the two life history stages revealed a similarity in their light-saturated photosynthesis rates but a difference in initial slopes, with lower efficiency in oxygenic evolution for SPO than for GAM. Consequently, compensation PAR (Ec = 9 μmol photons m−2 s−1) and saturation PAR (Ek = 243 μmol photons m−2 s−1) of the sporophyte were higher than for the gametophyte (Ec = 1 μmol photons m−2 s−1; Ek = 44 μmol photons m−2 s−1). This reflects the low-PAR adaptation of the microscope stage that is commonly found on the underside of rocks or in crevices where light exposure is limited. Continuous exposures to low (6 h, 100 μmol photons m−2 s−1) and high PAR (1000 μmol photons m−2 s−1) induced photoinhibition of photosynthesis, with larger declines in gametophyte effective quantum yields (ΦPSII). Gametophyte maximum quantum yields (Fv/Fm) also did not fully recover even after 12 h of dark acclimation, indicating photodamage. As for the temperature response in C. costata, gross photosynthesis and Fv/Fm characteristics were similar for both developmental stages, with temperature optima ranging from 14°C to 22°C. Their Fv/Fm and oxygenic net photosynthesis were relatively insensitive to low temperatures but declined above 24°C. This may explain the disappearance of C. costata sporophytes from their habitat during summer as well as their southern distributional limit in the western Pacific.