Phylogenetic and functional characterization of symbiotic bacteria in gutless marine worms (Annelida, Oligochaeta)

Symbioses between chemoautotrophic bacteria and eukaryotic hosts are widespread in marine environments. In most chemosynthetic endosymbioses, only a single, or at most two bacterial phylotypes co-occur within a host species.In this study the phylogenetic and metabolic diversity of bacterial endosymbionts in gutless marine worms (Annelida, Oligochaeta) from different environments was investigated. Almost all host species harbor a gammaproteobacterial sulfur oxidizer indicating the importance of these Gamma 1 symbionts for the nutrition of the gutless oligochaetes. A second gammaproteobacterial symbiont and deltaproteobacterial symbionts were detected in hosts from coastal silicate sediments, while in hosts from calcareous sands alphaproteobacterial symbionts were identified. Spirochetes were found in hosts from both types of sediments. The phylogenetic diversity of the bacterial symbionts mirrors their different metabolic capabilities. The Deltaproteobacteria have been identified as sulfate reducers and the secondary gammaproteobacterial symbionts are hypothesized to be sulfur oxidizers. Key genes involved in oxidative and reductive sulfur metabolism, CO2 fixation via the Calvin-Benson-Bassham (CBB) cycle, and nitrogen metabolism were successfully detected. Based on phylogenetic analyses it was possible to make potential assignments of genes to a respective symbiont.The use of comparative metagenomics gave first insights into the genome of a gutless oligochaete symbiont. A contiguous sequence of 51 kb from a bacterial artificial chromosome insert contained genes involved in significant metabolic pathways for these symbioses such as sulfur oxidation and CO2 fixation via the CBB cycle indicating that this sequence originated from a thioautotrophic symbiont. This study showed that the symbiotic community in marine gutless oligochaetes with at least three and as many as six different symbiotic phylotypes is much more complex than previously assumed. Despite the high phylogenetic diversity, these associations are clearly specific and stable for most phylotypes within a given host species.