310
Morphology and Ecological Physiology of Corals
.5
7,6' ,-----::----------------,
1,2
0,8
Fig. 8.4. The light absorption spectra of zooxanthellae from the gorgonaceans
Rhigidogorgia flabellum (1) and Antillogorgia turgida (2); S optical density (relative
units); € wavelength. (Data by Burkholder and Burkholder 1960)
Fig. 8.5. Light dependence curves of photosynthesis in isolated zooxanthellae from
colonies of the coral Pocillopora verrucosa, taken from well-illuminated (1) and from
shadowed (2) reef habitats; P photosynthesis rate, ~l O2 h- 1 10- 6 cells of zooxanthellae; I
light intensity, wt m- 2 . (After Titlyanov 1981)
within 20-30wtm- 2 (0.5-0.7Em- 2 h- l ) (Fig. 8.5). In the light-adapted
corals the zooxanthellae may have even much lower light saturation
threshold - within 10-15 wt m -2. Zooxanthellae also exhibit light inhibition
in excessive illumination, while intact corals do not. The light saturation
threshold in zooxanthellae is lower even than in free-living phytoplankton
(Scott and Jitts 1977).
The rate of photosynthesis in zooxanthellae per unit of chlorophyll is
somewhat similar to that in phytoplamkton. The assimilation number of
chlorophyll varies in them ca. 1-5mgCh- 1 1 mg- I of chlorophyll a (Wethey
and Porter 1976; Falkovsky and Dubinsky 1981). Average photosynthesis
rates in zooxanthellae are 10-20 Ilg 0 2 10- 6 cells (Zvalinski et al. 1980;
Muscatine 1980; Titlyanov et al. 1988a). The respiration of zooxanthellae
estimated by different methods ranges from 1 % of their photosynthesis