Monika Bright
University of Vienna, Austria
Title: Testing evolutionary theory in thiotrophic mutualism
Biography
Biography: Monika Bright
Abstract
Evolutionary theory depicts inter-species cooperation as mutual beneficial for both partners. However, empirical efforts to investigate underlying, stabilizing mechanisms require systems, which can be experimentally manipulated or cultivated. One of the most diverse microbial mutualisms involves sulfur oxidizing, chemoautotrophic bacteria and protist or animal hosts known from diverse marine ecosystems. The system we study is the tubeworm Riftia pachyptila and its endosymbiont Cand. Endoriftia persephone from deep-sea hydrothermal vents. We also focus on the ciliate Zoothamnium niveum and its ectosymbiont Cand. Thiobios zoothamnicoli from shallow-water wood falls, which we managed to cultivate. Transmission, considered to play a key role in the evolution, is horizontal in Riftia and resembles pathogen infections. Here we show an active escape of Endoriftia upon host death and seeding of the environment. This crucial process enables the host to find the cooperating symbiont over generations thus facilitating persistence. New findings point to a mixed transmission mode in Zoothamnium. In contrast to Endoriftia, Thiobios escapes dead hosts as well as living hosts when sulfide flux ceases. Cultivation experiments point to partner fidelity feedback mechanisms over a wide range of environmental conditions, but also show under which circumstances the ciliate mutualism terminates rendering an aposymbiotic host with reduced fitness. This loner strategy is not possible for the gutless tubeworm entirely nourished by its symbiont. Such studies outline the importance of bridging the gap between theory and empirical tests in various beneficial microbial associations to better understand cooperation.