THIS INVESTIGATION ADDRESSES THE PROGRAM AREA PRIORITY TO SOLVE CRITICAL DISEASE ISSUES IMPACTING COMMERCIAL AQUACULTURE SPECIES. HATCHERY PRODUCTION OF SHELLFISH SEED IS REQUIRED TO SUPPLEMENT NATURAL RECRUITMENT AND MEET RISING INDUSTRY DEMAND FOR OYSTERS, SCALLOPS, AND HARD CLAMS. DESPITE CONSIDERABLE MEASURES AIMED AT THE ELIMINATION OF BACTERIAL PATHOGENS FROM AQUACULTURE SYSTEMS, DISEASE CONTINUES TO BE A MAJOR IMPEDIMENT TO RELIABLE LARVICULTURE PRODUCTION IN HATCHERIES, WITH PERIODIC BACTERIAL OUTBREAKS AFFECTING THE AVAILABILITY OF SEED AVAILABLE TO FARMERS. THERE IS A CRITICAL, UNMET NEED FOR NOVEL TOOLS TO PREVENT AND TREAT DISEASE OUTBREAKS AT AQUACULTURE FACILITIES. THE LONG-TERM GOAL OF OUR INVESTIGATION IS TO DEVELOP NEW TOOLS FOR BACTERIAL DISEASE MANAGEMENT IN AQUACULTURE (AND ALTERNATIVES TO ANTIBIOTIC TREATMENT) BY EXPLOITING KNOWLEDGE ON MICROBE-MICROBE-HOST-ENVIRONMENT INTERACTIONS. WE HAVE ISOLATED AND CHARACTERIZED THE MECHANISMS OF ACTION OF A PROBIOTIC BACTERIUM, PHAEOBACTER INHIBENS S4, ABLE TO SAFELY AND EFFECTIVELY PREVENT MORTALITY IN BIVALVE LARVAE CAUSED BY BACTERIAL PATHOGENS. HOWEVER, KNOWLEDGE GAPS REMAIN IN UNDERSTANDING THE MOLECULAR MECHANISMS THAT DICTATE FAVORABLE OUTCOMES FOR THESE INTERSPECIES INTERACTIONS, AND HOW THIS PROBIONT MAY AFFECT THE BACTERIAL COMMUNITY IN SHELLFISH HATCHERIES. WE HYPOTHESIZE THAT THE PROBIOTIC ACTIVITY OF S4 IS THE RESULT OF MULTIPLE CONTRIBUTING FACTORS, INCLUDING BENEFICIAL ALTERATION TO THE HOST AND CULTURE SYSTEM MICROBIOMES.THIS PROJECT WILL CHARACTERIZE CHANGES IN THE COMPOSITION (OBJECTIVE 1) AND FUNCTION (OBJECTIVE 2) OF THE MICROBIAL COMMUNITIES (THE MICROBIOMES) IN OYSTER HATCHERIES FOLLOWING EXPOSURE TO THE PROBIONT S4 USING SEQUENCING ANALYSIS OF THOSE COMMUNITIES (16S RRNA GENE LIBRARY AND METATRANSCRIPTOME ANALYSIS). IT WILL ALSO EVALUATE THE IMPACT OF ENVIRONMENTAL FACTORS ON S4 ACTIVITY (OBJECTIVE 3). THIS RESEARCH WILL ILLUMINATE HOW THE LARVAL OYSTER MICROBIOME MAY BE ALTERED TOPROMOTE EITHER DISEASE RESISTANCE OR SENSITIVITY, AND MAY REVEAL ADDITIONAL COMMUNITY MEMBERS OR TRANSCRIPTS ASSOCIATED WITH HOST PROTECTION OR DISEASE. DETAILED KNOWLEDGE ON MECHANISMS OF ACTION OF PROBIONT S4 WILL FACILITATE OPTIMIZATION OF ITS USE AS A DISEASE MANAGEMENT TOOL IN SHELLFISH HATCHERIES.