Elevated Seawater Temperatures Decrease Microbial Diversity in the Gut of

The gut microbial community is critical for the host immune system, and in recent years, it has been extensively studied in vertebrates using 'omic' technologies. In contrast, knowledge about how the interactions between water temperature and diet affect the gut microbiota of marine invertebrates that do not thermoregulate is much less studied. In the present study, the effect of elevated seawater temperature and diet ( and var. ) on the gut microbial community of the commercial mussel, , was investigated. The 16S rRNA gene sequencing was used to characterize the microbial community in gut. The mortality of exposed to a high water temperature (31°C) increased after 3 days and the diversity of the bacterial community in the gut of live was significantly reduced. For example, the abundance of (Bacteroidetes) and norank_ (Bacteroidetes) increased in the gut of fed . In fed , the abundance of (Proteobacteria) and norank_ increased and the abundance of unclassified_ (Bacteroidetes) decreased. The results obtained in the present study suggest that high temperatures favored the proliferation of opportunistic bacteria, including and , which may increase host susceptibility to disease. Microbial community composition of the gut in live was not impacted by the microalgal diet but it was modified in the group of mussels that died. The present study provides insight into the potential effects on the gut microbiome and mussel-bacteria interactions of rising seawater temperatures.