Lucina floridana is an intertidal clam that contains intracellular, sulfur-oxidizing symbiotic bacteria in its gills. These bacteria are chemolithoautotrophic, using the energy from the oxidation of reduced sulfur compounds to power carbon fixation. Although sulfide is an energy-rich compound, it is also toxic to aerobic metabolism and autooxidizes in the presence of oxygen. Flow-through experiments were designed to examine the metabolic response to sulfide of the intact symbiosis. Three questions were addressed: 1) What are the major sulfide oxidation products in the gill and in the hemolymph? 2) How are sulfide and its major oxidation products distributed between the hemolymph and the gill? and 3) What is the metabolic poise (use of aerobic or anaerobic metabolism) in the gills during sulfide oxidation? The production of sulfide in the gills in response to various sulfur starvation regimes was also examined. Sulfide and its major oxidation products were determined using HPLC analysis of incompletely oxidized sulfur compounds. The metabolic poise was assessed by measuring the concentrations of substrates and products of anaerobic metabolism. Sulfide was produced in the gills of Lucina floridana after five days of sulfur starvation. The addition of invertebrate feed, a source of organic carbon for the clams, to the seawater significantly reduced the concentration of sulfide in the gills following sulfur starvation, suggesting that sulfide production was a consequence of symbiont oxidation of reduced sulfur compounds in the gills to provide energy for carbon fixation. Sulfide was oxidized to thiosulfate which was preferentially distributed in the hemolymph relative to the gills, a pattern that is consistent with further oxidation of thiosulfate by the bacteria in the gills for energy generation. This study was the first to investigate the metabolic poise of a lucinid in response to varying oxygen and external sulfide treatments; evidence suggests that anaerobic metabolism may be an important metabolic mode for Lucina floridana. / Graduation date: 1998