Yost, Denise MarieDimethylsulfoniopropionate (DMSP) is a multifaceted sulfur compound produced by several groups of marine phytoplankton, macroalgae and higher plants. Additionally, DMSP cleaving enzymes (most of which are thought to be DMSP-lyases) are known to exist in many species of marine phytoplankton, macroalgae and bacteria. Endosymbiotic dinoflagellate microalgae (genus <italic>Symbiodinium</italic>) of reef-building corals produce high intracellular levels of DMSP. The existence of DMSP-lyase(s) in <italic>Symbiodinium</italic> was, until recently, unknown. The function(s) of the DMSP/DMSP-lyase system in cnidarian-algal symbioses is poorly understood. Chapter one introduces coral symbioses, DMSP and the potential roles of the DMSP/DMSP-lyase system in cnidarian-algal symbioses. Chapter two describes the first evidence for <italic>in vivo</italic> DMSP-lyase activity in several isolated <italic>Symbiodinium</italic> strains, revealing varying levels of DMSP and DMSP-lyase activity. These results prompted further characterization of <italic>Symbiodinium</italic> DMSP-lyases. Enzyme assay optimization and substrate kinetics experiments found the measured activity of DMSP-lyase enzymes to be affected by permeabilization buffers, pH, temperature and potential oxidative stress effects (chapter three). Prior to investigations of field-collected intact corals, methods for DMSP analyses were optimized in the laboratory to address the inherent complexities of the coral holobiont. This work compared several preparation techniques for the analysis of particulate (algae only, DMSP_p) and total (coral tissue and algae, DMSP_t) DMSP in several species of stony corals (chapter four). Field-collected corals in chapter five showed DMSP_p and DMSP_t responses when exposed to the oxidative stressor, copper. The second field study (chapter six) describes how DMSP_p and DMSP_t concentrations within five prominent Bermudian corals changed with water depth. Finally, chapter seven presents a synthesis examination of the potential functional attributes and significance of the DMSP/DMSP-lyase system in cnidarian-algal symbioses. The factors influencing variable DMSP production and accumulation as well as differences in DMSP-lyase activity are discussed in light of methodological limitations, the biology and physiology of symbiont and coral, <italic>Symbiodinium</italic> phylotype and environmental variables. The results of this research highlight the existence of DMSP-lyases in <italic>Symbiodinium</italic> and provide insight into the partitioning of DMSP in cnidarian-algal symbioses, furthering our understanding of the production and potential turnover of DMSP while recognizing the limitations inherent in such investigations.DissertationBiology, GeneralcnidariancoraldimethylsulfoniopropionatelyaseSymbiodiniumsymbiosis