Bridging the gap between theoretical and empirical research and establishing tight and mutually inspiring cooperation between these two approaches is a major challenge for further progress in understanding the evolution of cooperation.’

Doebeli & Hauert 2005, 8: 761

The systems I focus on are the colonial ciliate Zoothamnium niveumCand. Thiobios zoothamnicoli, an ectosymbiont covering the host in a monolayer from shallow-water wood falls and the giant tubeworm Riftia pachyptila and its endosymbiont Cand. Endoriftia persephone from deep-sea hydrothermal vents. In both systems, the host provides chemicals for the symbiont to oxidize sulfide and fix inorganic carbon. In both systems the symbiont provides fixed carbon to the host through immediate release and uptake into the host. While symbiont transmission is vertical in the colonial ciliate, it is horizontal in the giant tubeworm. Mutualism is facultative for the colonial ciliate. Induced by the presence or absence of the symbiont the host exhibits polyphenism. In contrast, mutualism is obligate for the giant tubeworm, but facultative for the symbiont with loner populations in the vent and surrounding deep-sea environment. Hence, both systems differ in symbiont location (ecto- versus endosymbiont), dependency (facultative versus obligate host, facultative symbiont versus obligate symbiont), and transmission (vertical versus horizontal) and therefore are suitable to investigate the impact of these traits on the establishment, maintenance and persistence of symbiotic mutualism.

• Microbial symbiotic mutualism, with emphasis on dual partnership between sulfur-oxidizing bacteria and eukaryotes

• Ecology and evolution of symbiotic mutualism

• The environment as third player

• Maintenance through partner fidelity feedback

• Impact of the loner strategy in the persistence of mutualism

• Host life cycle and symbiont transmission

Approaches

in vivo experiments in high-pressure and ambient pressure flow-through systems to follow host and symbiont fitness under varying conditions

in situ experiments at deep-sea hydrothermal vents and shallow-water wood falls to investigate recruitment, growth and decay of populations

Goals

• We know that the thiotrophic endosymbiont of the giant tubeworm escapes upon host death. We are investigating now how this microbe manages to leave a dead host and how they colonize and proliferate in the environment.

• Currently we can cultivate the giant ciliate with and without its thiotrophic ectosymbiont in natural seawater. We are working hard to develop axenic cultures with and without the microbial partner.