| Summary: | The digestive glands of marine molluscs are colonized by a large number of microorganisms, and the structure and function of bacterial community could be severely affected by diets. Microalgae is the main food and energy sources for bivalves, while the impact of phytoplankton composition on the bacterial community as well as the health of bivalves are still not well understood. In the present study, the bacterial communities in digestive glands of oyster Crassostrea gigas fed with different diets were compared based on the high-throughput sequencing of partial 16S rRNA gene. There were significant differences of bacterial composition rather than diversity in digestive glands between the oysters fed with diatom dominant diet (Group N, mainly made up of Nitzschia closterium f.minutissima) and dinoflagellate dominant diet (Group P, mainly made up of Prorocentrum micans). The abundances of Prevotella, Vibronaceae, Ruminococcaceae, and Polaribacter were significantly higher in Group N (p < 0.05), and the abundances of Streptophyta and Acidimicrobiales were significantly higher in Group P (p < 0.05). According to the functional prediction results, the bacterial community in Group P displayed weaker capacities of Kdo2-lipid A biosynthesis as well as taurine degradation, and a stronger capacity of glycolysis compared with the bacterial community in Group N. The higher phylogenetic clustering degree of the bacterial community in Group P (p < 0.05) indicated the higher host selectivity on bacteria. These results suggested that the change of phytoplankton composition of diet would have large effects on bacterial communities in oyster digestive glands. The bacterial community in digestive glands of oysters living in dinoflagellate dominant waters would produce harmful impact to hosts. The present study provided a new perspective to explore the potential mechanism for the massive mortalities of oysters.
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