The effect of PAR and temperature on the photosynthetic performance of a freshwater red alga, Thorea gaudichaudii (Thoreales) from Kagoshima, Japan
文摘
The effect of photosynthetically active radiation (PAR) and temperature on the photosynthesis of an endangered freshwater red alga, Thorea gaudichaudii C. Agardh (Thoreales), was determined by laboratory and field measurements. This species was found in a freshwater spring habitat that was mostly shaded from direct sunlight and under almost undetectable seasonal fluctuations in water temperature (23.0 ± 0.31 °C, ±SD). A net oxygenic photosynthesis–PAR model revealed that the net photosynthetic rate quickly saturated, with a saturating PAR (E k) and compensation PAR (E c) of 12 μmol photons m−2 s−1 (9–16, 95 % Bayesian credible interval (BCI)) and 7 μmol photons m−2 s−1 (5–9, 95 % BCI), respectively. A temperature-dependent (i.e., 12–44 °C range) dome-shaped model of gross photosynthesis and dark respiration was fitted to reveal that the gross photosynthetic rate, 190 μg O2 gww −1 min−1 (175–208, 95 % BCI), was highest at 34.9 °C (33.5–36.0, 95 % BCI) and decreased sharply above this temperature. The dark respiration rate was also dome shaped in response to temperature, with a peak of 66 μg O2 gww −1 min−1 (59–76) at 32.2 °C (28.2–40.2) and then slow decay in rates at higher temperatures. In contrast, the effective quantum yield only gradually decreased with increases in PAR and the maximum quantum yield was weakly dome shaped with respect to temperature. Indeed, the highest value of maximum quantum yield of 0.596 (0.592–0.599, 95 % BCI) occurred at 24.1 °C (22.0–26.2, 95 % BCI). This species appears to be well adapted to the heavily shaded and relatively stable temperature conditions of its environment.