Conjugational transfer of a Dinoflagellate-killing Plasmid between Marine Rhodobacteraceae

The roseobacter group of marine bacteria is characterized by a mosaic distribution of ecologically important phenotypes. These are often encoded on mobile extrachromosomal elements. So far, conjugation had only been experimentally proven between the two model organisms Phaeobacter inhibens and Dinoroseobacter shibae. Here we show that two large natural RepABC-type plasmids from D. shibae can be transferred into representatives of all major Rhodobacteraceae lineages. Complete genome sequencing of the newly established P. inhibens transconjugants confirmed their genomic integrity. The conjugated plasmids were stably maintained as single copy number replicons in the genuine as well as the new host. Co-cultivation of P. inhibens and the transconjugants with the dinoflagellate Prorocentrum minimum demonstrated that P. inhibens is a probiotic strain that improves the yield and stability of the dinoflagellate culture. The transconjugant carrying the 191 kb plasmid, but not the 126 kb sister plasmid, killed the dinoflagellate in co-culture.