High temperature aggravates mortalities of the Pacific oyster (Crassostrea gigas) infected with Vibrio: A perspective from homeostasis of digestive microbiota and immune response

Long-lasting high temperature and bacterial exposure are two critical environmental factors contributing to the mass mortality of oysters in summer. In this study, we investigated the effects of high-temperature exposure and Vibrio alginolyticus infection on the mortality of the Pacific oysters, Crassostrea gigas. The results of survival experiment demonstrated that the overall survival of Vibrio-infected oysters was significantly reduced at high-temperature stress. The 16S rRNA amplicon sequencing showed that high temperature significantly disrupted the structure of the infected oyster's digestive gland microbiota and increased the abundance of potentially pathogenic bacteria. Transcriptome profiling showed that expression of critical genes associated with inflam-mation was suppressed in the early phase of infection but was induced in the later phase. In contrast, genes related to apoptosis and oxidative stress were continuously upregulated, indicating that homeostasis of in-flammatory response was maintained in the early phase but was eventually destroyed upon continuous high temperature exposure. This work is of practical significance for deciphering the cause of oyster mortality in summer and monitoring disease outbreaks in the oyster culture.