The effects of temperature and irradiance on the photosynthesis of two heteromorphic life history stages of Saccharina japonica (Laminariales) from Japan

This study examined the effects of temperature and irradiance on photosynthetic characteristics of the macroscopic sporophyte (SPO) and microscopic gametophyte (GAM) stages of a subarctic brown alga, Saccharina japonica var. japonica (Laminariales) from Hokkaido, Japan. In vitro measurements under short- and long-term exposures were carried out by using optical dissolved oxygen sensors and the pulse amplitude modulation (PAM)-chlorophyll fluorometer, respectively. The heteromorphic life history stages of S. japonica showed photosynthetic optima at 23–23.3 °C, derived from the gross photosynthesis–temperature model. Maximum quantum yields ( F v / F m ) of SPO and GAM after 72 h of temperature exposures in the dark were reduced to near zero above 24 °C, indicating PSII inactivation. Such similarity in their temperature characteristics suggests the co-occurrence of both generations in the habitat despite the seasonal growth and reproduction of the species. Net photosynthesis–irradiance experiments in the two life history stages at 8, 16, and 24 °C revealed similarity in their light-saturated photosynthetic rates ( NP max = 3.02–4.41 μg O 2 g ww −1 min −1 , SPO; 2.87–3.73 μg O 2 g ww −1 min −1 , GAM), but saturation irradiances of SPO ( E k = 81–102 μmol photons m −2 s −1 ) were higher than those of GAM (48–69 μmol photons m −2 s −1 ). A slight decrease in net photosynthetic rates of GAM above 500 μmol photons m −2 s −1 was likewise observed. This difference may be related to the light regime of their natural habitat, suggesting the low irradiance adaptation of the microscopic stage that settles on rock crevices beneath algal canopies.