Photosynthetic responses of Pyropia yezoensis f. narawaensis (Bangiales, Rhodophyta) to a thermal and PAR gradient vary with the life-history stage

The effect of photosynthetically active radiation (PAR) and temperature on photosynthesis of the cultivated red alga, Pyropia yezoensis f. narawaensis (Saga-# 5 Strain, Bangiales), was determined for microscopic sporophytes and macroscopic gametophytes with pulse-amplitude fluorometry and dissolved oxygen sensors. A clear difference in the temperature response of the maximum quantum yield (F-v/F-m) and oxygenic gross photosynthesis (GP) was revealed between the two life-history stages. The sporophyte Fv/Fm was not sensitive to temperature, although the model revealed an F-v/F-m maximum (0.60) at 16.78 degrees C. In contrast, the gametophyte F-v/F-m was clearly sensitive to temperature and attained a maximum mean value of 0.55 at 14.78 degrees C. The relationship between GP and temperature was also different. The maximum rates of GP of the sporophyte occurred at 30.78 degrees C and was 17.1 mu g O-2 mg(chl)-a(-1) min(-1) (15.4-19.4, 95% Bayesian credible interval); however, maximum rates of GP for the gametophyte were much higher (110 mu mol O-2 mg(chl)-a(-1) min(-1)) and occurred at a lower temperature (14.48 degrees C). The response of oxygenic net photosynthesis to PAR also varied, and the initial slope (alpha), the saturation PAR (E-k) and the maximum photosynthetic rate (P-max) of the gametophyte were much higher than the sporophyte. Therefore, we hypothesised that in species with a heteromorphic life history, such as those in the genus Pyropia, physiological responses to environmental gradients would be dissimilar.