The effect of irradiance and temperature on the photosynthesis of a native alga Sargassum fusiforme (Fucales) from Kagoshima, Japan

Abstract:

The effect of irradiance and temperature on the photosynthesis of a Japanese native brown alga, Sargassum fusiforme, was determined by using dissolved oxygen sensors and pulse amplitude–modulated fluorometry. In situ measurements of the effective quantum yield (Φ_PSII) decreased as irradiance increased at 12:00 PM and recovered in the late afternoon, indicating that a proportionately greater number of reaction centres were closed. Likewise, laboratory measurement after 6 hours of exposure to low and high (300 and 1000 μmol photons m⁻² s⁻¹, respectively) irradiances showed a relatively greater decrease in Φ_PSII at high irradiance, and both treatments recovered overnight. A model of the net photosynthesis–irradiance relationship determined at 20°C revealed that the net photosynthetic rate quickly increased at irradiances below the estimated saturation irradiance of 391 μmol photons m⁻² s⁻¹ [95% Bayesian credible interval (BCI) of 299–515 μmol photons m⁻² s⁻¹]. Gross photosynthesis and dark respiration rates determined over a range of temperatures (8–36°C) revealed that the maximal gross photosynthetic rate was 24.1 (BCI: 21.5–26.8) μg O₂ g_ww⁻¹ min⁻¹ at 22.9°C (BCI: 19.8–25.8), and the dark respiration rate increased nonlinearly from a low of 0.5 μg O₂ g_ww⁻¹ min⁻¹ at 8°C to a high of 8.9 μg O₂ g_ww⁻¹ min⁻¹ at 36°C. The highest value of the quantum yield (F_v/F_m = Φ_PSII at 0 μmol photons m⁻² s⁻¹) was estimated to occur at 22.9°C (BCI: 21.8–23.9) and was 0.72 (BCI: 0.70–0.73). The specimens examined appeared to be well adapted to the natural temperature range of the study site. Although the irradiances on the fronds were strong enough to induce a drop in quantum yield, our evidence clearly suggests that full recovery occurs overnight.

Key Words::

• Algae
• Diurnal change
• Hijikia fusiformis
• Photosynthesis
• Pulse amplitude modulation–chlorophyll fluorometry
• Sargassum fusiforme

ACKNOWLEDGEMENTS

This research was supported in part by the Grant-in-Aid for Scientific Research (#22510033, #25340012 and #25450260) from the Japanese Ministry of Education, Culture, Sport and Technology (R.T. and G.N.N.).