ADAPTATION OF SOLITARY CORALS AND THEIR ZOOXANTHELLAE TO LOW-LIGHT AND UV-RADIATION

Adaptation of solitary corals, Fungia repanda and F. echinata, and the ir zooxanthellae to low light and ultraviolet light B (UV-B) was studi ed with respect to changes in their protein contents, photosynthetic p igment contents and the photosynthesis-irradiance (P-I) curves. The co rals were collected from 1 to 50 m depths in the Republic of Belau (Pa lau) in 1990 and 1991. The chlorophyll a content in a unit surface are a of the coral did not change significantly with the depth of the habi tat, whereas cellular chlorophyll a in the algae increased with the de pth. Zooxanthellae density and protein content in a unit surface area of Fungia spp. decreased with the depth. Photosynthetic parameters nor malized by a unit surface area of the Fungia spp., maximum gross photo synthetic rate (P-gmax area(-1)) and dark respiration rate (R area(-1) ), were negatively correlated with the depth, while initial slope of t he P-I curve (alpha) did not show significant correlation with the dep th. Compensation light intensity (Ic) decreased with the depth. In iso lated zooxanthellae, P-max chl a(-1), and R chl a(-1) decreased with t he depth, while alpha(chl a) was constant. P-gmax cell(-1) and R cell( -1) did not change significantly but alpha(cell) increased with the de pth. Ic decreased with the depth as in the intact corals. Reduction of protein content in a unit area of the coral from deeper habitat impli es decrease of host animal tissues. Reduction of Ic can be explained b y decrease of R area(-1), which may be due to the diminution of animal tissues. The photoadaptational response to low light intensity of int act Fungia spp. was found to be a combination of the photoadaptation o f symbiotic algae and the decrease of host animal tissue. In order to study their adaptation to ultraviolet (UV) radiation, P-I curves of Fu ngia spp. and isolated zooxanthellae were analyzed before and after UV -B irradiation. 1 h UV-B irradiation showed no effect on the photosynt hetic rate of the shallow water (1 m) corals, while it inhibited the p hotosynthesis of the deep water (30 m) corals and zooxanthellae isolat ed from both shallow and deep water corals. These results indicate tha t the host, Fungia spp., in shallow water have protective mechanism fo r intense UV-B in their habitat. These photoadaptational mechanisms se em to allow the Fungia spp. to have wide vertical distribution where l ight intensity spans more than two orders of magnitude.