THE SYMBIOTIC INTERFACE IN AN ALGA INVERTEBRATE SYMBIOSIS

Both the cell membranes of the symbiotic alga Symbiodinium and the surrounding membranes of the sea anemone host Anemonia viridis have ATPase activity, as determined by enzyme cytochemistry. The interface between Symbiodinium and Anemonia also bears phosphatase activity, with substrate specificity for beta-glycerophosphate and alpha-naphthylphosphate. An immunocytochemical analysis of the distribution of the weak base N-(3-[2,4-dinitrophenyl-amino]propyl)-N-(3-aminopropyl)methylamine dihydrochloride (DAMP) suggests that the membrane-bound compartment surrounding Symbiodinium cells is not markedly acidic. These data suggest that the symbiosome membrane of A. viridis may control the availability of phosphorous compounds and other nutrients to Symbiodinium. They are not readily compatible with the diffusion-depletion hypothesis, by which symbiont demand is hypothesized to be the primary determinant of the nutrient supply to the symbiont cells.