Phaeocystis globosa is a globally important species of marine phytoplankton that forms massive blooms which export significant amounts of carbon to the deep sea and, in the process, remove a significant amount of carbon dioxide from the atmosphere. Phaeocystis is unusual in that it exhibits life cycle changes between solitary cells and large gelatinous colonies. These different life habits can have major impacts on entire food webs. For example, typical finfish fisheries rely on zooplankton for food.  A Phaeocystis bloom can disrupt this fishery because the gelatinous colonies can not be eaten by the zooplankton or the finfish. Instead, the Phaeocystis become food for less desirable members of the marine food web, like jellyfish, which then dominate this ecosystem. Phaeocystis blooms often dominate North Atlantic waters and impact fisheries, fish farming, and coastal tourism by producing odorous foams that accumulate on beaches during the collapse of a bloom. Dense blooms are detrimental to shell fish growth and reproduction, and are responsible for net-clogging, fish mortality, and alteration of fish taste. The success of Phaeocystis is a combination of life cycle stages, a range of colony sizes, and predator-activated defenses that reduces its mortality compared to competing phytoplankton. For these reasons Phaeocystis is an important organism to study the complexity of marine pelagic ecosystems. Molecular-level (e.g. gene expression) responses of P. globosa to ecological stresses and stimuli will be evaluated. Comparative analysis of expressed genes will begin to yield novel information about the biochemistry, physiology and ecology of this important species. Further, the genomic and gene-expression databases from this project will be a community resource and will support many projects on the ecology and evolution of this pivotal organism in marine ecosystems. Further, the PIs will create internships for students in underrepresented groups to train in genomics and bioinformatics.