The effect of irradiance and temperature on the photosynthesis and growth of a cultivated red alga Kappaphycus alvarezii (Solieriaceae) from Vietnam, based on in situ and in vitro measurements

The effect of irradiance and temperature on the photosynthesis and growth of a Vietnamese-cultivated red alga, Kappaphycus alvarezii, was determined by laboratory and field measurements. Dissolved oxygen sensors and pulse amplitude modulated (PAM) fluorometry were used for the measurements of photosynthetic efficiency. A Diving-PAM revealed that in situ measurements of the effective quantum yield (ΦPSII) were negatively correlated with incident irradiance. ΦPSII decreased as irradiance increased at noon and recovered in the evening, suggesting photo-inhibition. A model of the net photosynthesis–irradiance (P-E) relationship determined in a laboratory experiment at 26 °C revealed that the net photosynthetic rate quickly increased at irradiances below the estimated saturation irradiance of 154 μmol photons m−2 s−1 (95 % Bayesian credible interval (BCI) of 117–203 μmol photons m−2 s−1). Gross photosynthesis and dark respiration under the laboratory measurements were also determined over a range of temperatures (8–42 °C), revealing that the gross photosynthetic rates of 8.0 (BCI 7.03–9.59) μg O2 gww−1 min−1 at 31.1 (BCI 30.2–32.0)°C, and the dark respiration rate increased exponentially from a low of 0.22 μg O2 gww−1 min−1 at 10 °C to a high of 1.88 μg O2 gww−1 min−1 at 42 °C. The estimated highest value of the maximum quantum yield (ΦPSII at 0 μmol photons m−2 s−1 = Fv/Fm) determined with the Imaging-PAM and occurred at 22.2 (BCI 20.8–23.5)°C and was 0.51 (BCI 0.48–0.54). Relative growth rate (RGR) and the survival rate after 2 weeks of laboratory culture showed that the relatively high growth rates (ca. 1.5 %) occurred at 28 and 32 °C; however, mortality was 100 % at 36 °C. This species is considered to be well-adapted to the natural temperature range of study site; however, it is likely close to the marginal temperature conditions. Furthermore, daytime incident irradiance on the frond under fine clear sky might be too strong for the effective photosynthesis of this species, suggesting that optimum irradiance for gross photosynthesis with the minimum decline of ΦPSII seems to be much lower.