Endosymbiotic sulphate-reducing and sulphide-oxidizing bacteria in an oligochaete worm

Stable associations of more than one species of symbiont within a single ho st cell or tissue are assumed to be rare in metazoans because competition f or space and resources between symbionts can be detrimental to the host(1). In animals with multiple endosymbionts, such as mussels from deep-sea hydr othermal vents(2) and reef-building corals(3), the costs of competition bet ween the symbionts are outweighed by the ecological and physiological flexi bility gained by the hosts. A further option for the coexistence of multipl e symbionts within a host is if these benefit directly from one another, bu t such symbioses have not been previously described. Here we show that in t he gutless marine oligochaete Olavius algarvensis, endosymbiotic sulphate-r educing bacteria produce sulphide that can serve as an energy source for su lphide-oxidizing symbionts of the host. Thus, these symbionts do not compet e for resources but rather share a mutalistic relationship with each other in an endosymbiotic sulphur cycle, in addition to their symbiotic relations hip with the oligochaete host.