REGULATION AND CONTROL OF INTRACELLULAR ALGAE (EQUALS ZOOXANTHELLAE) IN HARD CORALS

To examine alg al (= zooxanthellae) regulation and control, and the fa ctors determining algal densities in hard corals, the zooxanthellae mi totic index and release rates were regularly determined in branch lips from a colony of a staghorn coral, Acropora formosa recovering from a coral 'bleaching' event (the stress-related dissociation of the coral -algal symbiosis). Mathematical models based upon density-dependent de creases in the algal division frequency and increases in algal release rates during the post-bleaching recovery period accurately predict th e observed recovery period (approximate to 20 weeks). The models sugge st that (i) the colony recovered its algal population from the divisio n of the remaining zooxanthellae, and (ii) the continual loss of zooxa nthellae significantly slowed the recovery of the coral. Possible reas ons for the 'paradoxical' loss of healthy zooxanthellae from the bleac hed coral are discussed in terms of endodermal processes occurring in the recovering coral and the redistribution of newly formed zooxanthel lae to aposymbiotic host cells. At a steady-state algal density of 2.1 x 10(6) zooxanthellae cm(-2) at the end of the recovery period, the z ooxanthellae would have to form a double layer of cells in the coral t issues, consistent with microscopic observations. Neighbouring colonie s of A. formosa with inherently higher algal densities possess proport ionately smaller zooxanthellae. Results suggest that space availabilit y and the size of the algal symbionts determines the algal densities i n the coral colonies. The large increases in the algal densities repor ted in corals exposed to elevated nutrient concentrations (i.e. betwee n a two- and five-fold increase in the algal standing stock) are not c onsistent with this theory. We suggest that increases of this magnitud e are a product of the experimental conditions: reasons for this state ment are discussed. We propose that the stability of the coral-algal s ymbiosis under non-stress conditions, and the constancy of zooxanthell ae densities in corals reported across growth form, depth and geograph ic range, are related to space availability limiting algal densities. However, at these densities, zooxanthellae have attributes consistent with nutrient limitation.