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

Abstract:

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 F_v/F_m was not sensitive to temperature, although the model revealed an F_v/F_m maximum (0.60) at 16.7°C. In contrast, the gametophyte F_v/F_m was clearly sensitive to temperature and attained a maximum mean value of 0.55 at 14.7°C. The relationship between GP and temperature was also different. The maximum rates of GP of the sporophyte occurred at 30.7°C and was 17.1 μg O₂ 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 μmol O₂ mg_chl-a⁻¹ min⁻¹) and occurred at a lower temperature (14.4°C). The response of oxygenic net photosynthesis to PAR also varied, and the initial slope (α), 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.

Key Words::

• Algae
• Heteromorphic life histories
• Nori
• Photosynthesis
• Pulse amplitude–modulated (PAM)-chlorophyll fluorometry
• Temperature tolerance

ACKNOWLEDGEMENTS

This research was supported in part by the Grant-in-Aid for Scientific Research (#25340012 and #25450260; R.T. and G.N.N.) from the Japanese Ministry of Education, Culture, Sport and Technology and the Grant-in-Aid for JSPS Fellows (#115J02533; Y.W.) from the Japan Society for the Promotion of Science (JSPS). This research was a part of dissertation submitted by the first author in partial fulfillment of the PhD degree. All authors have provided consent.