Environmentally co-occuring mercury resistance plasmids are genetically and phenotypically diverse and confer variable context-dependent fitness effects.

Plasmids are important mobile elements that can facilitate genetic exchange and local adaptation within microbial communities. We compared the sequences of four co-occurring pQBR environmental mercury resistance plasmids and measured their effects on host competitive fitness of Pseudomonas fluorescens SBW25, which was isolated at the same site. Fitness effects of carriage differed between plasmids and were strongly context dependent, varying with medium, plasmid status of competitor and levels of environmental = mercury. The plasmids also varied widely in their rates of conjugation and segregational loss. We found that few of the plasmid-borne accessory genes could be ascribed functions, although we identified a putative chemotaxis operon, a type IV pilus-encoding cluster, and a region encoding putative arylsulfatase enzymes that were conserved across geographically distant isolates. One plasmid, pQBR55, conferred the ability to catabolise sucrose. Transposons, particularly the Tn5042 mer transposon, appeared to have been acquired by the different pQBR plasmids by recombination, indicating an important role for horizontal gene transfer in the recent evolution of pQBR plasmids. Our findings demonstrate extensive genetic and phenotypic diversity amongst co-occurring members of a plasmid community and suggest a role for environmental heterogeneity in the maintenance of plasmid diversity.