Microbial Communities Associated with the Intestinal Tract of Grey Mullets from a Mediterranean Aquatic Environment

Introduction: Grey mullets comprise different species that represent the most ubiquitous teleost families in the planet’s coastal waters. They are an important proportion of the Mediterranean lagoon’s production and have been recently considered cultivated marine fish. This study aimed to explore the intestinal microbial communities of grey mullets to understand their possible ecological role for fish and the aquatic environment. Methods: Thirty-four wild-caught mullets were sampled from a Mediterranean lagoon during four seasons and the V3-V4 hypervariable regions of 16S rRNA (Illumina MiSeq) of the fish gut were sequenced. Parameters of the aquatic environment were detected: temperature, salinity, DO, PO4, NH4, NO3, NO2, SiO, DIN, and Chla. Results: The results indicated a high bacterial diversity (mean Shannon index: 4.74 ± 1.12; Simpson index: 0.93 ± 0.08) and variations among seasons. Sixty prokaryotic phyla were identified and the most abundant ones were: Proteobacteria (mean relative abundance 35.4% ± 17.9), Actinobacteriota (mean relative abundance 16.4% ± 9.9), and Firmicutes (mean relative abundance 10.1% ± 10.9). Bacteria belonging to the phylum Chloroflexi were relevant in autumn, Spirochaetota, Verrucomicrobiota, Fusobacteriota, and Cyanobacteria were particularly abundant in winter while Bacteroidota characterized summer fish. A total of 332 prokaryotic families were identified with 26 most abundant ones; Rhodociclaceae (Proteobacteria) were dominant in autumn, Brevinemataceae (Spirocheaetota) and Fusobacteriaceae (Fusobacteriota) were especially present in winter and the Staphylococcaceae (Firmicutes) prevailed in spring. Conclusion: This study sheds light on the variation in the complex gut microbial community structure of Mediterranean grey mullets and their potential ecological role in protecting fish and preserving the aquatic environment.