Phototrophic consortia

Phototrophic consortia consist of a fixed number of green sulfur bacteria epibionts surrounding a central colorless rod-shaped betaproteobacterium. Our group established the first laboratory cultures of phototrophic consortia that represent the only laboratory model system currently available for the study of highly specific interactions between nonrelated bacteria. Both partners divide in a highly coordinated manner. The entire consortium swims towards the light (scotophobic response), which the epibionts need for anoxygenic photosynthesis, whereas the central bacterium provides the necessary motility. The cells form specific cell-cell-adhesion structures and exchange multiple signals. Recent ultrastructural analyses have provided evidence for intracellular sorting of antenna structures in the epibiont cells, which occurs exclusively when they are in the symbiotic state. "Symbiosis genes" of the epibiont were identified by genomic approaches. Putative large exoproteins and a putative protein with a RTX toxin-type ß-roll could be identified. So far, such genes have not been found in any of the free living green sulfur bacteria, indicating a specific role in the interaction.

Autotroph-heterotroph interactions

The beneficial effect of heterotrophic bacteria on the growth of algae and cyanobacteria has been observed by phycologists decades ago. One example is the diatom Thalassiosira rotula, which represents one of the most prominent cosmopolitan algae, but grows lousy under axenic conditions. Many phototrophs are auxotrophic for the synthesis of vitamins that are provided by their bacterial partners, which in turn benefit from the released exudates. However, with respect to the microbial communities, there is a conspicuous gap of knowledge between environmental studies and the systematic investigation of cultured phototrophs. We are systematically investigating the community composition of non-axenic microalgae and cyanobacteria with a new 16S-ITS high-throughput sequencing approach and metagenome sequencing in order to identify the key player(s) of the communities. The usage of defined resources from the curatorship „Protists and Cyanobacteria“ allows a subsequent isolation of promising strains, which is an unique feature in comparison with conventional community analyses of environmental samples. Furthermore, the interaction of model organisms is investigated to understand the specific metabolic interplay between the autotrophic and heterotrophic partners. Dinoroseobacter shibae shows in coculture with the dinoflagellate Prorocentrum minimuma „Jeckyll and Hyde“ behavior correlating with a shift from mutualism to pathogenicity in senescent algal cultures. We identified a conjugative ‚killer plasmid‘ that is responsible for morbid life style and currently investigate its mode of action.

Projects

  • ŸProject A5, TRR51-Roseobacter
  • ŸProject A7, TRR51-Roseobacter

Selected References

  1. Segev E, Wyche TP, Kim KH, Petersen J, Ellebrandt C, Vlamakis H, et al. (2016). Dynamic metabolic exchange governs a marine algal-bacterial interaction. eLIFE 5: e17473.
  2. ŸWang H, Tomasch J, Michael V, Bhuju S, Jarek M, Petersen J, et al. (2015). Identification of genetic modules mediating the Jekyll and Hyde interaction of Dinoroseobacter shibaewith the dinoflagellate Prorocentrum minimum. Front. Microbiol. 6: 1262.
  3. ŸFrank O, Michael V, Päuker O, Boedeker C, Jogler C, Rohde M, Petersen J(2015). Plasmid curing and the loss of grip - The 65-kb replicon of Phaeobacter inhibensDSM 17395 is required for biofilm formation, motility and the colonization of marine algae. Syst. Appl. Microbiol. 38: 120–127.
  4. ŸWagner-Döbler I, Baumgart M, Brinkhoff T, … Petersen J, et al. (2010). The complete genome sequence of the algal symbiont Dinoroseobacter shibae– a hitchhiker`s guide to life in the sea. ISME J. 4: 61–77.

Partners in high-throughput enrichments

Often, the growth of one bacterium depends on the presence of another bacterium. Such mutualistic interactions may depend on, e.g., metabolite flux between the two partners. In order to identify novel partners and potential keystone taxa in microbial communities we determine the microbial community composition of thousands of liquid enrichment cultures. These were derived from a series of different growth media that were each inoculated with strong dilutions from natural communities. Using statistical permutation approaches, we will identify those pairs of taxa which co-occur much more frequent than can be expected under random distribution. We expect to disentangle the effect of habitat and incubation medium on identifying novel pairs of potentially interacting bacterial taxa.