Reference: Biol. Bull. 166: 419-426. (April, 1984) ALANINE UPTAKE BY ISOLATED ZOOXANTHELLAE OF THE MANGROVE JELLYFISH, CASSIOPEA XAMACHANA. II. INHIBITION BY HOST HOMOGENATE FRACTION SUSAN CARROLL* AND RICHARD S. BLANQUET Department of Biology, Georgetown University, Washington, DC 20057 ABSTRACT A low molecular weight fraction (Inhibitory Fraction, IF) was isolated from whole animal homogenates of the symbiotic jellyfish, Cassiopea xamachana. This fraction (2-10 kDa) strongly inhibited the uptake of L-(U-14 C)alanine by isolated zooxanthellae at concentrations as low as 5 ^g protein ml" ' . The magnitude of suppression was unaffected by the length of exposure time to IF prior to monitoring uptake. Uptake rates were linear for control and experimental cell suspensions, suggesting that algal metabolic integrity was maintained in the presence of IF. Zooxanthellae incubated in IF, and then washed free of this fraction, continued to show suppressed alanine uptake with only partial recovery. These cells, when placed in culture media, exhibited normal morphology, continued to grow and divide, and developed flagellated stages. The possible regulation of nutrient exchange between symbionts by symbiosis-specific host factors is discussed. INTRODUCTION Algal-invertebrate associations provide unique opportunities for investigating the regulation of the metabolic interactions necessary for the maintenance of this symbiosis in two such diverse organisms. While it has been recognized that endozoic algae translocate photosynthate to their animal hosts (for reviews, see Muscatine, 1974, 1980), it has only recently been appreciated that they also may compete for the available pool of nutrients obtained via host feeding (Cook, 1971; Thorington and Margulis, 1981). Since the activity of various metabolic pathways is directed by the availability of substrate, the stability and widespread occurrence of algal-invertebrate symbioses may result from an ability of the symbionts to regulate reciprocal transfers. Few studies, however, have focused on this regulation and the mechanisms involved. An intriguing possibility is that this might be accomplished, in part, through the induction of certain regulatory molecules or factors specific to the symbiotic state. Factors that increase the rate of photosynthate liberation by isolated zooxanthellae have been reported in various cnidarians (Trench, 1971; Muscatine et ai, 1972). In addition, these factors appear to lack species specificity (Muscatine, 1967). The present investigation shows that a homogenate fraction of the jellyfish Cassiopea xamachana can significantly inhibit the back-transfer of nutrients such as alanine to zooxanthellae. Should such factors be common to algal-en idarian associations, their differential production could conceivably regulate the levels of organic compounds available to each partner. The regulation of nutrient availability to zooxanthellae, or the production of appropriate host factors, could also serve to regulate algal populations within cells. Received 5 July 1983; accepted 16 December 1983. * Present address: Department of Physiology, Uniformed Services University of the Health Sciences, Bethesda, Maryland 20814. 419
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