BackgroundGut microbiota plays important roles in host animal physiology, homeostasis, metabolism, and environmental adaptation while the interplay between the gut microbiome and urochordate ascidian, the most closet relative of vertebrate, remains less explored. In this study, we characterized the gut microbial communities of urochordate ascidian (Halocynthia roretzi) across the changes of season and starvation stress using a comprehensive set of omic approaches including 16S rRNA, metagenomic, metabiotic, and transcriptome sequencing.ResultsThe 16S rRNA amplicon profiling revealed that ascidians harbor indigenous gut microbiota distinctly different to the marine microbial community and significant variations in composition and abundance of gut bacteria, with predominant bacterial orders representing each season. Depressed alpha-diversities of gut microbiota were observed across starvation stress when compared with the communities in aquafarm condition. Synechococcales involving photosynthesis and its related biosynthesis was reduced in abundance while the enrichments of Xanthomonadales and Legionellales may facilitate bile acid biosynthesis in starvation stress condition. Meanwhile, the metabolomics analysis found that the long chain fatty acids, linolenic acid, cyanoamino acid, and pigments derived from gut bacteria were upregulated, suggesting a beneficial contribution of the gut microbiome to ascidian under starvation stress.ConclusionsOur results revealed the seasonal variation of ascidian gut microbiota. Furthermore, we found that the defense and energy-associated metabolites derived from gut microbiome were responded to the starvation stress. The data provide insights into understanding the adaptive interplay between the gut microbiome and ascidian host that maintains a beneficial metabolic system across season and starvation stress. The diversity-generating metabolisms from both microbiota and host might lead to the co-evolution and environmental adaptation.