Abstract: Spectacular blooms of Phaeocystis antarctica in the Ross Sea, Antarctica are the source of some of the world's highest concentrations of dimethylsulfoniopropionate (DMSP) and its volatile degradation product, dimethylsulfide (DMS). The flux of DMS from the oceans to the atmosphere in this region and its subsequent gas phase oxidation generates aerosols that have a strong influence on cloud properties and possibly climate. In the oceans, DMS and DMSP are quantitatively significant components of the carbon, sulfur, and energy flows in marine food webs, especially in the Ross Sea. Despite its central role in carbon and sulfur biogeochemistry in the Ross Sea, surprisingly little is known about the physiological functions of DMSP in P. antarctica. The research will isolate and characterize DMSP lyases from P. antarctica, with the goal of obtaining amino acid and gene sequence information on these important enzymes. The physiological studies will focus on the effects of varying intensities of photosynthetically active radiation, with and without ultraviolet radiation as these are factors that we have found to be important controls on DMSP and DMS dynamics. The research also will examine the effects of prolonged darkness on the dynamics of DMSP and related compounds in P. antarctica, as survival of this species during the dark Antarctic winter and at sub-euphotic depths appears to be an important part of the Phaeocystis' ecology. A unique aspect of this work is the focus on measurements of intracellular MSA, which if detected, would provide strong evidence for in vivo radical scavenging functions for methyl sulfur compounds. The study will advance understanding of what controls DMSP cycling and ultimately DMS emissions from the Ross Sea and also provide information on what makes P. antarctica so successful in this extreme environment. The research will directly benefit and build on several interrelated ocean-atmosphere programs including the International Surface Ocean Lower Atmosphere Study (SOLAS) program. The PIs will participate in several activities involving K-12 education, High School teacher training, public education and podcasting through the auspices of the Dauphin Island Sea Lab Discovery Hall program and SUNY ESF. Two graduate students will be employed full time, and six undergraduates (2 each summer) will be trained as part of this project.