Main effects and interactions of light and temperature on rates of growth (), net photosynthesis (P
s), and dark respiration (R) of the red seaweed
Gracilaria tikvahiae were investigated in outdoor, nutrient-replete continuous-flow seawater culture chambers. Below 15°C,
G. tikvahiae did not grow and between 15° and 30°C, both main effects and interactions of light and temperature on and P
s were significant, which explains the occurrence of this alga as a summer annual in its northern range. Temperature interacted with light (I) through its influence on the vs I and P
s vs I curves. The initial slope of the vs I curve, , the light saturation intensity, I
s, and maximum growth rate,
max, were all significantly lowerat 15°C compared to 20°, 25°, or 30°C. Maximum values of
max, the P
s:R ratio and the net photosynthesis:gross photosynthesis ratio (P
s:P
g) all occurred at 25°C, suggesting that this is the best temperature for growth of
G. tikvahiae. Values for P
max increased up to 30°C, indicating that the temperature for maximum growth and net photosynthesis are not the same for
G. tikvahiae. Significant photoinhibition of growth and photosynthesis at full incident sunlight (I
0) occurred at 15°C but not at 20°, 25°, or 30°C. Steele's equation fit the 15°C vs I data best, whereas the hyperbolic tangent function fit the 20°, 25°, and 30°C data best. Main effects and interactionof light intensity and temperature on rates of R were also significant (
P<0.001). R was highly intercorrelated with and P
s (0.86
r0.94), indicating that R in
G. tikvahiae is primarily regulated by growth rate and not temperature
per se. Environmental factors that regulate growth, such as light intensity, exert a great influence on R in
G. tikvahiae.
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