Estes, A.M.*; Kempf, S.C.; Henry, R.P.: Carbonic anhydrase induction by zooxanthellae in Cassiopea xamachana

Endosymbiotic dinoflagellates of cnidarians are encased in multiple membranes, causing difficulty in acquiring inorganic carbon for photosynthesis. Host carbonic anhydrase(CA), which catalyses the reversible dehydration of HCO₃-, can supply endosymbionts with unlimited CO₂ by accessing the large HCO₃– pool in seawater. Bell tissue from Cassiopea xamachana, was examined to determine if the localization and induction of CA activity were correlated with the density and location of zooxanthellae. Symbiotic and regenerated recolonized C.xamachana bell tissue had 5 times and 1.7 times, respectively, greater CA activity than aposymbiotic native and regenerated tissue. Symbiotic native and recolonized tissue contained respectively, 3 and 6 fold greater algal populations than aposymbiotic tissues. The oral epithelial layer, which receives more intense sunlight, had 1.2 greater CA activity and 5 times the algae than aboral. CA localized using the fluorescent inhibitor 5-dimethylaminonapthalene-1-sulfonamide (DNSA), revealed symbiotic and oral epithelial tissues 1.5 times brighter than aposymbiotic and aboral tissues. CA activity and algal density in symbiotic and aposymbiotic animals were highest at the bell edge and lowest near the manubrium. Thus, the postion and density of algal populations are positively correlated with CA concentration and location. Symbiotic C.xamachana incubated in the Photosystem I inhibitor, 3-(3,4 dichlorophenyl)-1,1-dimethylurea (DCMU)showed a 1.7 fold decrease in CA activity after five days. Thus, not only algal density, but also photosynthetic rate determines CA concentration.