Phylogenetic diversity and metabolic versatility of the bacterial endosymbionts in marine gutless oligochaete worms

Abstract

Marin gutless oligochaete worms (Annelida, Phallodrilinae) live in an obligate association with bacterial endosymbionts. Each host, belonging to one of the two genera Olavius or Inanidrilus, harbours a specific, but morphologically, phylogenetically and metabolically diverse symbiont community. The primary symbionts of gutless oligochaetes, called Gamma 1, are large chemoautotrophic sulfur-storing bacteria that form a monophyletic clade within the Gammaproteobacteria and had been found in all host species studied so far. Secondary symbionts of gutless oligochaetes belong to the Alpha-, Gamma- and Deltaproteobacteria and to the Spirochaetes. In this PhD thesis the diversity and function of gutless oligochaete symbiont communities was investigated.In a first part, the phylogenetic and metabolic diversity of I. exumae was studied. The symbiont community of this host differed markedly from that of other gutless oligochaetes. Sulfate-reducing deltaproteobacterial symbionts co-occurred with alpha-proteobacterial symbionts in this host, showing that these do not mutually exclude each other as previously assumed. Furthermore, a large novel gammaproteobacterial symbiont only distantly related to the Gamma 1 symbionts, but morphologically similar, dominated the symbiont community, while no indication was found for a Gamma 1 symbiont. The presence of sulfur and genes diagnostic for autotrophy and sulfur oxidation indicate that this new symbiont is a sulfur-storing chemoautotroph. Thus, the novel symbiont seems to share its morphology and its function with the Gamma 1 symbionts and may have replaced the Gamma 1 symbiont in I. exumae.To learn more about the ecophysiology of gutless oligochaete symbioses, the autotrophic activity was investigated in a second project with tracer incubation experiments. Analyses of radiolabelled inorganic carbon uptake and sulfur content of individual Olavius algarvensis worms showed that in the presence of oxygen, internally stored sulfur was used as an energy source for the incorporation of inorganic carbon into biomass. In the absence of oxygen, inorganic carbon was taken up at lower rates. The electron donors and electron acceptors used under anoxic conditions could not be unambiguously identified. However, increased carbon fixation occurred in the presence of nitrate, sulfide and thiosulfate in a few worms.Identification of the autotrophic symbionts in the O. algarvensis symbiont community was achieved in a third project by applying in situ hybridization combined with microautoradiography (MARFISH) or high resolution mass spectrometry (nanoSIMS-HISH). The Gamma 1 symbionts immediately incorporated inorganic carbon into biomass under oxic conditions in the absence of external energy sources suggesting the usage of internally stored sulfur as electron donor. Uptake rates of individual cells varied, but were on average in the range of those found for free-living sulfur bacteria and chemoautotrophic symbionts. For the first time, the autotrophic symbiont could be directly identified and the inorganic carbon uptake analyzed for individual symbionts within the gutless oligochaete symbiosis.