Mobile genetic elements shape the evolution and adaptation of the marine Sulfitobacter genus

Mobile genetic elements (MGEs) are essential for facilitating horizontal gene transfer and play crucial roles in the evolution and adaptive capabilities of bacterial species. Here, we analyzed closed genomes from the marine Sulfitobacter genus to assess plasmid contributions to ecological adaptability and evolutionary diversification. Our analysis of 153 Sulfitobacter plasmids from 36 strains representing 8 species shows extensive plasmid conservation within species (e.g., >95% nucleotide identity for flagellar plasmids) alongside significant mosaicism across 60% of plasmids. Insertion sequences (IS) elements are nearly ninefold more concentrated on plasmids relative to chromosomes, suggestive of active genetic exchange in this replicon class. Network analysis identified 14 primary plasmid clusters, with species-specific conservation patterns and evidence of inter-species gene transfer. In Sulfitobacter pontiacus strain CB2047, we discovered chromosomal integration of a 280 kb plasmid encoding a toxin-antitoxin system, rrn operon, as well as a chromosomal partitioning system. These findings demonstrate that plasmids function as key drivers of evolution and adaptation in Sulfitobacter, serving as both repositories of conserved adaptive traits and platforms for ongoing genetic innovation.

Plasmids are increasingly recognized as crucial drivers of bacterial evolution and adaptation, yet their roles in shaping marine microbial communities are poorly understood. Here, we provide a comprehensive analysis of plasmid diversity and evolution within Sulfitobacter, a broadly distributed and metabolically versatile marine bacterial genus, in which ~15% of genome content is plasmid-encoded. We propose that Sulfitobacter plasmids serve dual evolutionary roles: maintaining highly conserved species-specific traits essential for survival (such as flagellar motility and biofilm formation), while simultaneously functioning as platforms for genetic innovation through extensive horizontal gene transfer. The discovery of a large plasmid integrated into the chromosome of one strain highlights that episomal elements can transition to stable chromosomal inheritance in this genus. These findings advance our understanding of how marine bacteria balance genomic stability with adaptive flexibility, providing insights applicable to microbial evolution in dynamic ocean environments.