Scientists at the Max Planck Institute for Marine Microbiology in Bremen/Germany, and their colleagues have discovered a unique symbiosis between bacteria and a marine worm, described in Nature, issue May 17th 2001.
The term symbiosis typically evokes an image of beneficial interactions between two individuals, the symbiont and the host. Associations with multiple endosymbionts are assumed to be rare because competition between symbionts could be harmful to the host, suggesting that in most symbioses three is a crowd. Dr. Nicole Dubilier and her colleagues at the Max Planck Institute for Marine Micorbiology in Bremen have discovered a unique type of symbiosis that could be described as a "mnage trois" in a gutless marine worm from the animal group Oligochaeta. This marine relative of earthworms does not have a mouth or gut, and harbors sulfide-oxidizing bacteria as its primary symbiotic partner. These symbionts use sulfide as an energy source for the fixation of carbon dioxide into organic compounds, which are passed on to the host. Sulfide concentrations in the worm's environment are unusually low, so low that the scientists were at first puzzled how the worm and its primary symbiont could survive. This question was answered when the researchers discovered that the worms harbor sulfate-reducing bacteria as secondary symbiotic partners. The sulfate-reducing symbionts produce sulfide as a metabolic endproduct that can serve as an energy source for the sulfide-oxidizing symbionts. In this endosymbiotic sulfur cycle, the two symbionts not only share a mutualistic relationship with their host but also with each other.
The full article is available at http://www.mpi-bremen.de/deutsch/moloek/dubeng3.html at the Max Planck Institute for Marine Microbiology.