Culturable Gut-Associated Bacteria of Red Mullet (Mullus barbatus) from the Black Sea Show Broad Heavy-Metal Tolerance and High Phylogenetic Diversity

The present study sought to determine the phylogenetic diversity and heavy metal resistance profile of culturable gut-associated bacteria isolated from Black Sea red mullet (Mullus barbatus) specimens. Twelve bacteria were isolated using culture-dependent methods and characterized by 16S rRNA gene sequencing. The three dominant bacterial phyla represented were Actinomycetota, Bacillota, and Pseudomonadota. One of the isolates, Z1T07, was less than 98.65% identical at the sequence level to the closest type strain, which suggests the possibility of a new species within the genus Luteococcus. The heavy metal resistance of the bacteria was tested against cadmium, chromium, copper, mercury, nickel, and zinc. The great majority of the strains were tolerant of the highest level of chromium and nickel tested and had slightly inhibitory effects of mercury and cadmium. Of the strains examined, the widest range of resistance was shown by the Acinetobacter sp. Z2T15. The results of this work demonstrate that the marine sedentary fish microbiota of a species such as M. barbatus serves as a significant ecological indicator. In addition, a recommendation of M. barbatus as a good sentinel for the monitoring of heavy metal contamination within the Black Sea is presented. Future genomic work should aim at elucidating the resistance mechanisms as a preface to the full determination of the biotechnological potential of these heavy metal-tolerant bacteria.