Comparative Genomics of the First Plasmid Identified in the Genus Halovibrio Revealing a Mosaic Structure Shaped by Horizontal Gene Transfer

Exchange of genetic information within extreme environments is mediated by mobile genetic elements, most notably plasmids, which play a crucial role in microbial adaptation and survival in harsh environmental conditions. In this study, we characterized a novel plasmid pHv20-59a (194 kb) identified in Halovibrio sp. isolated from a hypersaline brine in Solivar near Prešov (Slovakia). Although the absence of canonical relaxase suggests that autonomous conjugative transfer is unlikely, bioinformatic approaches revealed that the plasmid possesses highly conserved regions shared with other members of the Halomonadaceae family. Plasmid annotation revealed a functional potential that may contribute to a selective advantage under stress conditions, particularly through a broad repertoire of heavy metal tolerance genes ( mer and cop operons) and increased heavy metal tolerance correlating with the identified heavy metal content. Since initial experiments to remove the plasmid from the cells were unsuccessful and other curing strategies still require testing, these findings suggest that the putative toxin-antitoxin systems may contribute to the maintenance of this replicon in the host cell, highlighting its potential biological significance in halophilic bacteria. Our finding of a conserved plasmid backbone shared across phylogenetically distinct halophilic hosts provides evidence for its dissemination by horizontal gene transfer in hypersaline environments. This work represents the first characterization of a plasmid from the genus Halovibrio and expands current knowledge of plasmid diversity and evolution in extreme environments.