Microbially produced exopolymeric substances (EPS) are believed to play a critical role in the weathering and degradation of oil in marine environments. However, the mechanisms by which EPS promotes the aggregation and/or dispersion of oil and dispersants are not well understood. Furthermore, current understanding of microbial community dynamics, especially between bacteria, archaea and eukaryotes, after exposure to oil and/or Corexit is poorly resolved. We prepared 79L seawater mesocosms to investigate the responses of natural microbial communities to the water accommodated fraction of oil with and without Corexit (CWAF/WAF, respectively). Prior to experiment initiation, phytoplankton were collected from coastal Galveston, TX waters and added to each tank to stimulate interactions between phytoplankton, oil, and Corexit. The goal of this project is to monitor the composition and structure of microbial communities over time as they respond to the oil and/or Corexit and explore the relationships between hydrocarbon-degrading prokaryotes and phytoplankton. Data collected during these mesocosms will help elucidate the role microorganisms play in the aggregation, dispersion, and degradation of oil in marine environments.