Alpha Proteobacterial Symbionts

To date, alpha proteobacterial symbionts have been found in three gutless oligochaete species that are all from similar environments, shallow water coral reef sediments, but from different geographic locations (I. leukodermatus from Bermuda, I. makropetalos from the Bahamas, and

0. loisae from the Australian Great Barrier Reef). All three host species harbor symbionts belonging to a clade of relatively closely related (92.6% sequence similarity) bacteria called Alpha 1 symbionts (Fig. 2). Within this clade, different phylotypes can co-occur in the same host species (Fig. 2), e.g., the Alpha 1a and b symbionts of I. leukodermatus (Blazejak et al., submitted) and the Alpha 1a-1 and 1a-2 phylotypes in O. loisae (Dubilier et al. 1999). In addition to the Alpha 1 clade of symbionts, I. makropetalos and

1. leukodermatus harbor additional lineages of alpha proteobacterial symbionts (Alpha 2 symbionts in Fig. 2) (Blazejak et al., submitted). All alpha proteobacterial symbionts have been identified as the small, rod- to cocci-shaped morphotype described in (12.) 4, and they appear to be evenly distributed throughout the symbiotic region (Blazejak et al., submitted).

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Fig. 4. a Live gutless oligochaetes. Note the characteristic white color of the worms, due to the bacterial symbionts just below the cuticle. The extended front ends of the worms have less symbionts, and are therefore more transparent. b - d Fluorescence in situ hybridization (FISH) identification of bacterial symbionts in O. crassitunicatus (b) and O. algarvensis (c, d). Dual hybridization with GAM42a and DSS658/DSR651 probes, showing gamma proteobacterial symbionts in green, and delta proteobacterial symbionts in red. Note the different distribution of the symbionts in the two host species, with the delta proteobacterial symbionts of O. crassitunicatus directly below the cuticle, and those of O. algarvensis more evenly distributed throughout the symbiotic region. Bars b and d 10 |m, c 20 |m. e Model of the syntrophic sulfur cycle between the sulfur-oxidizing and sulfate-reducing symbionts. The bacteria exchange oxidized (Sox) and reduced (Sred) sulfur compounds such as sulfate and sulfide. The sulfur oxidizer fixes CO2 into organic compounds which are passed on to the host. In environments with low sulfide concentrations, the dominant flow of electrons is through the sulfate reducer in the form of organic carbon or H2 if the sulfate reducer is autotrophic. Anaerobic metabolites such as succinate and the fatty acids proprionate and acetate that the worm produces under oxygen-limiting conditions and normally excretes, can be recycled by the sulfate reducer

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