FATTY-ACIDS OF THE SCLERACTINIAN CORAL GALAXEA-FASCICULARIS - EFFECT OF LIGHT AND FEEDING

In order to investigate nutritional interactions in the symbiotic scle ractinian coral-zooxanthella association, fatty acids of the coral Gal axea fascicularis were analysed in two groups of cultured microcolonie s. The first group was fed with Artemia sp. while the second group was starved. After an initial 1-month period during which both groups wer e subjected to the same ''normal'' light conditions (constant irradian ce of 125 mu E . cm-2 . s(-1) and 14:10 h light:dark), a light cap was used to cover the aquarium and keep all the microcolonies in permanen t darkness for 20 days. During the light phase of the experiment it wa s shown that the nutritional status lead to large variations in the pe rcentage of saturated, mono-unsaturated and polyunsaturated fatty acid s. Palmitic acid (C16:0) was the most abundant fatty acid in both grou ps. Important differences between fed and starved microcolonies occurr ed during the dark phase of the experiment. In the fed group the dark phase was characterized by a significant increase in polyunsaturated f atty acids. Particularly arachidonic acid (C20:1 n-6) became the most important fatty acid followed by docosatrienoic acid (C22:3 n-3). A sl ight increase in these two fatty acids was also found in the starved g roup but the bulk of polyunsaturated fatty acids was significantly dec reased. In this group, palmitic acid remained the most important fatty acid while an increased concentration of cis-vaccenic acid (C18:1 n-7 ) was found at the end of the experiment. The increased concentration of cis-vaccenic acid might indicate that bacteria serve as a source of energy. While the number of zooxanthellae per milligram of protein an d the chlorophyll a to protein ratio strongly decreased in the starved microcolonies immediately after the beginning of the dark period, the decrease in fed microcolonies was delayed for about 10 days. Furtherm ore, after 20 days of dark incubation the chlorophyll a to protein rat io was the same as measured at the beginning of the dark period. This suggests that in the dark the metabolic requirements of the zooxanthel lae are in part met from the animal host through a heterotrophic mode of nutrition.