Characterization of bacterial endo- and ectosymbionts of oligochaete worms from marine sediments: Phylogeny and metabolic potential

Two obligate bacterial endosymbioses and a facultative ectosymbiosis of gutless (1, 2) and gut-bearing (3) marine oligochaetes from coastal sediments were characterized. (1, 2) The gutless oligochaetes Olavius algarvensis and O. ilvae live in coastal sediments with very low sulfide concentrations. Similar bacterial consortia were found in both hosts with two sulfide-oxidizing Gammaproteobacteria and two sulfate-reducing Deltaproteobacteria. The presence of sulfate-reducers providing the sulfide-oxidiziers with an internal source of sulfide could explain how the worms could colonize a sulfide-poor environment. (3) Tubificoides benedii lives in Wadden Sea sediments and is adapted to extreme fluctuations of oxygen and sulfide. Its posterior end is facultatively colonized by filamentous bacteria. This community was dominated by two morphologically distinct phylotypes: A thicker Gammaproteobacterium attached to the exterior of the cuticle and a thinner Epsilonproteobacterium penetrated it. Both ectosymbionts belonged to clades that consisted nearly exclusively of bacteria associated with deep-sea hydrothermal vent invertebrates.