Dinoroseobacter and dinoflagellate – from killer plasmid to killer gene

Hui Wang, Diana Patzelt, Jürgen Tomasch and Irene Wagner-Döbler

Helmholtz-Center for Infection Research, Inhoffenstr. 7, 38124 Braunschweig, Germany

Phone +49-531-61813080; Email [email protected]

Marine micro-algae influence the diversity and abundance of bacteria living in their phycosphere and thus shape bacterioplankton biogeography on a global scale. During algae blooms, populations of Roseobacter bacteria become abundant. Their interaction with the algae host can be mutualistic or pathogenic depending on conditions and growth stage of the algae. Although many such systems have been described, few are mechanistically understood.

We focus on two model organisms from the Roseobacter group, Dinoroseobacter shibae and Phaeobacter inhibens, and study their interaction with the dinoflagellate Prorocentrum minimum. Under controlled conditions in the laboratory, the co-culture reproducibly shifts from a mutualistic to a pathogenic phase. A large 191 kb plasmid of D. shibae is required for the pathogenic phase. In the absence of this plasmid, the alga is not killed by the bacterium. We have obtained transposon mutants for most of the genes on the “killer-plasmid” and are studying their role for the interaction with the dinoflagellate in co-culture. The data so far show that genes on the killer-plasmid can be grouped into three types: (1) Their deletion has no effect on the co-culture; (2) deletion abolishes the killer-phenotype of the plasmid; (3) deletion abolishes the establishment of the symbiosis. The data suggest that the 191 kb plasmid mediates both symbiosis and killing.