Abstract:To investigate the microbiota variation of Pacific oyster Crassostrea gigas in the hot and cold seasons and its potential impact on the quality of C.gigas and food safety.The 16S rRNA gene sequencing was used to detect the composition and variation of the microbial community in the soft tissues of C.gigas during the hot and cold seasons，which were sampled at the Aoshan Bay in Qingdao，Shandong Province.The high-throughput sequencing results showed that bacteria colonized in C.gigas sampled in the hot season were dominated by Proteobacteria（40.35%），Firmicutes（19.87%），Actinobacteria（12.23%），and Bacteroidetes（10.39%）.At the genus level，Bifidobacterium Spp.（5.11%），Enhydrobacter（5.03%），Endozoicomonas（4.81%），and Methy－lobacterium（3.04%）were predominant.However，in the cold season，the main changes of microbiota in the soft tissue of C.giga were that Actinobacteria dropped to 4.14%，and Cyanobacteria increased from 2.61%to 9.05%.At the genus level，the dominant bacteria were Sphingomonas（21.13%），Cyanobacteria（7.70%），and Mycoplasma（5.08%），which significantly increased compared with those in the C.gigas sampled in the hot season.In contrast，Bifidobacterium（0.58%）and Enhydrobacter（0.05%）significantly decreased.The Vibrio spp.accounted for the highest proportion（3.03%）in mantle tissue，and the overall abundance reduced from 1.25%in the hot season to 0.10%in the cold season，which was consistent with the results of the culture-based method.From hot season to cold season，the richness of microbial community in C.gigas samples increased slightly，while the diversity decreased.These results provided insights for further assessing the impact of microbiota on the quality and safety of C.gigas collected in different seasons.