Effect of PAR and temperature on the photosynthesis of the Japanese alga, Ecklonia radicosa (Laminariales), based on field and laboratory measurements

Abstract:

The effect of photosynthetically active radiation (PAR) and temperature on the photosynthesis of a Japanese brown alga, Ecklonia radicosa, was determined by using dissolved oxygen sensors and pulse-amplitude modulated (PAM) fluorometry. The effective quantum yield (Φ_PSII) of the sporophytes, in a natural population in shallow water, showed a characteristic noontime decline that indicated the influence of high PAR. However, Φ_PSII of sporophytes in deeper water (−10 to −20 m) did not appear to be affected and maintained relatively high values of Φ_PSII. In laboratory measurements of Φ_PSII, after 12 h of continuous low and high PAR (100 and 1000 μmol photon m⁻² s⁻¹), a relatively greater decrease in Φ_PSII in the high PAR treatment was observed. Furthermore, despite an overnight dark acclimation, the maximum quantum yield (F_v/F_m) under the high PAR treatment did not fully recover. A net photosynthesis–irradiance (P-E) curve was generated at 20°C, which revealed low compensation and saturation irradiances (E_c and E_k) of 14 and 45 μmol photon m⁻² s⁻¹, suggesting adaptation to a low PAR environment. Results from experiments to determine the net/gross photosynthesis and dark respiration rates revealed that the maximal gross photosynthetic rates occurred at 27.5°C and were 5.18 μg O₂ g_ww⁻¹ min⁻¹; whereas, the dark respiration rate increased exponentially with a mean value of 1.35 μg O₂ g_ww⁻¹ min⁻¹ at 22°C. F_v/F_m was relatively stable at low temperature, and the highest value (0.73) occurred at 16.5°C. The field survey and laboratory experiments revealed that E. radicosa is well adapted to relatively low PAR and water temperatures typical of warm temperate coastal regions in Japan. The poor response to high PAR and the inability of sporophytes to fully recover overnight from high PAR exposure indicates that the vertical distribution of this species is driven by the PAR environment.

Key Words::

• Algae
• Ecklonia radicosa
• Eckloniopsis
• Kelp
• Photoinhibition
• Photosynthesis
• Pulse amplitude modulation (PAM)–chlorophyll fluorometry

ACKNOWLEDGEMENTS

This research had been started since 2008 as the preliminary study, and was supported in part by the Grant-in-Aid for Scientific Research (22510033, 25340012, and 25450260; RT and GNN) from the Japanese Ministry of Education, Culture, Sport and Technology and the Grant-in-Aid for JSPS Fellows (115J02533; YW) from the Japan Society for the Promotion of Science (JSPS). All authors have provided consent.