Host-Zooxanthella Interactions in Four Temperate Marine Invertebrate Symbioses: Assessment of Effect of Host Extracts on Symbionts.

Photosynthesis and translocation of photosynthetic products from symbiotic zooxanthellae in four species of temperate-latitude invertebrates were investigated in vivo and in vitro. In vivo, zooxanthellae fixed 14C and translocated a substantial proportion of fixed products to host tissues. In vitro, the effect of host tissue extracts on isolated zooxanthellae varied. Extracts of the soft coral Capnella gaboensis, lysed zooxanthellae after a relatively short exposure. Those of the zoanthid Zoanthus robustus and the nudibranch Pteraeolidia ianthina had little effect on translocation of organic carbon from zooxanthellae. In contrast, host extract of the scleractinian coral Plesiastrea versipora stimulated the release of up to 42% of the total 14C fixed, and the magnitude of release was positively correlated with the protein concentration of the extract. Host extracts had no effect on photosynthetic rates in algal symbionts. The effect of P. versipora extract on isolated zooxanthellae was studied. This extract caused zooxanthellae to divert photosynthetic products from lipid synthesis to the production of neutral compounds, principally glycerol, and these compounds were the predominant form of carbon detected extracellularly after incubating zooxanthellae in this extract. Only organic compounds made during the period of exposure of zooxanthellae to host extract, and not pre-formed photosynthetic products, were translocated. The translocation-inducing activity of host extract was almost completely destroyed by heating (100{deg}C), and a preliminary attempt to fractionate the tissue extract revealed that the active constituent did not pass through dialysis tubing of nominal pore size 10,000 D. These results are discussed in relation to host control of symbiotic partners, and to previous reports of "host-release factors" in other invertebrate symbioses.