THE INFLUENCE OF SITE AND SEASON ON THE GUT AND PALLIAL FLUID MICROBIAL COMMUNITIES OF THE EASTERN OYSTER Crassostrea virginica.  

The microbial communities and overall health of the eastern oyster, Crassostrea virginica, have long been topics of interest due to the fundamental economic and ecological roles this species maintains. A broad scale characterization of the oyster microbiome over spatial and seasonal scales, however, has never been conducted. The goal of this study was to examine factors mediating microbial communities of the gut and pallial fluid of three populations of C. virginica within Long Island Sound (LIS) estuary, with a focus on genetic structure (T-RFLP), composition (fluorescence in situ hybridization, FISH), and physiological profiling (Biolog EcoPlates) of the microbiome. One population inhabited western LIS (Norwalk), whereas the other two populations inhabited eastern LIS (Noank & Avery Point; ca.  130 km away).  Results indicated that the genetic structure of microbial communities of oysters were minimally separated across the three sites, but were strongly influenced by season (Table 1). Although the microbial community structure was similar, the number of carbon sources utilized by these communities (i.e., substrate richness) varied across site, season and anatomical location within the host (Table 2). Composition data from FISH revealed that Bacteroidetes contributed 0.34 to 29.7% of total bacterial biomass in pallial fluid samples and 0.17 to 29.8% in gut samples. Psuedomonas (Groups I) contributed 0 to 45% of total bacterial biomass in gut samples and 0 to 31% in pallial fluid samples.  The contributions of both groups changed throughout the year and from site to site.  Parameters including oyster condition index, Dermo-disease infection status, and ambient water temperature were measured to assess their influence on the oyster microbiome. Only water temperature was found to have a significant relationship with microbial community structure and richness. These results suggest that a core microbiome may exist within the eastern oyster, specifically for those populations that are not genetically distinct.