Physical contact and carbon transfer between a lichen-forming Trebouxia alga and a novel Alphaproteobacterium

Recent progress in molecular techniques has begun to alter traditional recognition of lichens as symbiotic organisms comprised of a fungus and photosynthetic partners (green algae and/or cyanobacteria). Diverse organisms, especially various non-photosynthetic bacteria, are now indicated to be integral components of lichen symbiosis. Although lichen-associated bacteria are inferred to have functions that could support the symbiosis, little is known about their physical and nutritional interaction with fungi and algae. In the present study, we identified specific interaction between a lichen-forming alga and a novel bacterium. Trebouxia alga was isolated from a lichen, Usnea hakonensis, and kept as a strain for 8 years. Although no visible bacterial colonies were observed in this culture, high-throughput sequencing of DNA isolated from the culture revealed that the strain is composed of a Trebouxia alga and an Alphaproteobacterium species. In situ hybridization showed that bacterial cells were localized on the surface of the algal cells. Physiological assays revealed that the bacterium was able to use ribitol, glucose and mannitol, all of which are known to exist abundantly in lichens. It was resistant to three antibiotics. Bacteria closely related to this species were also identified in lichen specimens, indicating that U. hakonensis may commonly associate with this group of bacteria. These features of the novel bacterium suggest that it may be involved in carbon cycling of U. hakonensis as a member of lichen symbiosis and less likely to have become associated with the alga after isolation from a lichen.