The impact of bleaching on the metabolic contribution of dinoflagellate symbionts to their giant clam host

Bleaching (loss of symbiotic dinoflagellates) is known to significantly decrease the fitness of symbiotic marine invertebrates resulting in reduced growth, fecundity and survival. This report is the first to quantify the effects of bleaching on inorganic carbon (C_i) and ammonium flux, fixation and export of photosynthate to the host, in this case the giant clam Tridacna gigas. The 1998 bleaching event was found to decrease the zooxanthellae population 30‐fold when comparing bleached to non‐bleached clams. This resulted in significant increases in haemolymph C_i and decreases in haemolymph pH and glucose concentration, the predominant photosynthate exported from zooxanthellae in this symbiosis. There was also a decrease in the expression levels of host carbonic anhydrase, an enzyme involved in C_i transport to the zooxanthellae, and although host glutamine synthase levels were unaffected, the clams ability to assimilate ammonium was eliminated in bleached individuals, suggesting that photosynthate from the zooxanthellae is required for ammonium assimilation. In an artificial bleaching experiment haemolymph C_i (r² = 0.97), pH (r² = 0.94) and glucose levels (r² = 0.95) were correlated to zooxanthellae numbers during both bleaching and recovery. Recovery of the zooxanthellae population, was enhanced four‐fold by the addition of organic and inorganic nutrients, as were related haemolymph characteristics. These results highlight the profound physiological changes that occur in symbiotic organisms during and after a bleaching event.