Diversity and evolution of endosymbiotic bacteria in the siphonous green alga <i>Bryopsis</i> (Bryopsidales, Chlorophyta)

Many algae maintain close associations with bacteria that are linked with various metabolic functions and influence the shape and life cycle of the host. Siphonous green algae frequently contain bacteria within their giant cells, which form interesting biotic environments for bacterial communities. We examined the diversity and evolution of endosymbiotic bacteria in the genus Bryopsis. Using algal cultures and molecular methods (fluorescence in situ hybridization, denaturing gradient gel electrophoresis and 16S rRNA gene clone libraries), we show that Bryopsis species harbor well-defined and rather stable bacterial communities composed of Labrenzia, Mycoplasma, Rickettsia, Rhizobiaceae, Phyllobacteriaceae, Bacteroidetes and Flavobacteriaceae. These mixed communities of generalists and specialists are differently influenced by host phylogenetic relationships, geographic and environmental factors. The presence of Flavobacteriaceae is strictly determined by host phylogeny, indicating an obligate symbiotic association and vertical transmission of these bacteria. Comparative phylogenetic analyses of symbiont and host indicate a complex pattern of coevolution that is obscured by factors such as host-switching and incomplete sorting of symbionts within host lineages