Diurnal lipid and mucus production in the staghorn coral Acropora acuminata

Net ¹⁴C-accumulation into lipids of Acropora acuminata was rapid and increased with light intensity. Dark ¹⁴C-incorporation was less than 1% noon maximum. Structural lipids were the first radioactively labelled lipid types showing linear ¹⁴C-uptake kinetics. Storage lipids showed non-linear, power-curve kinetics for ¹⁴C-uptake. The rate of ¹⁴C-incorporation into triglycerides and wax esters was maximal during early afternoon and at midday, respectively. Electron microscopic evidence is given for zooxanthellae being primary sites for synthesis of lipids which are exuded from chloroplasts and transferred to animal tissues. Free lipid droplets and crystalline inclusions (wax ester) were common in animal tissues, the inclusions being often associated with mucus-producing cells. The diurnal rate of mucus production was constant. However, ¹⁴C-mucus-lipid production showed a light-dependent diurnal pattern and accounted for 60 to 90% total ¹⁴C of mucus during periods of photosynthetically-saturating light. Here, ¹⁴C was primarily associated with wax esters which were always present in the mucus-lipid. ¹⁴C-triglycerides occur in mucus released only during the day. Lipid and mucus synthesis is discussed in relation to the carbon budget of A. acuminata, in which mucus represented a loss of 40% net C fixation.