The first ultrastructural studies of oligochaete symbionts described two bacterial morphotypes (Giere 1981, 1985): large oval bacteria 2-7 |m in length filled with sulfur and PHB vesicles and smaller cocci- to rod-shaped bacteria 0.7-1.5 |m in length without any conspicuous inclusions or structures in their cytoplasm (Fig. 1) (also see Bright and Giere 2005 for a more detailed review of the ultrastructure of the symbiosis). The large oval bacteria are now known to be sulfur-oxidizing gamma Proteobacteria (Gamma 1 symbionts in Fig. 2). In contrast, the small bacteria can not be identified based on their ultrastructure alone, and depending on the host species they occur in, belong to the alpha, gamma, or delta Proteobacteria (see Sect. 5.2). A third very thin and long morphotype (0.3*10 |m), was first described in O. crassitunicatus from the Peru margin (Giere and Krieger 2001), and has now been identified as a spirochete (Fig. 1 and Sect. 5.2.4). The identity of a fourth bacterial morphotype of intermediate size between the large and small bacteria and described in O. loisae (Dubilier et al. 1999) and O. algarvensis (Giere and Erseus 2002) remains unclear. Overall, the ultrastructure of the bacterial symbionts can only be used for identification of the sulfur-oxidizing Gamma 1 symbionts with their obvious sulfur and PHB vesicles and in some cases for the long and thin spirochaete symbionts. The morphology of the other bacterial morphotypes, in particular the small rod- and cocci-shaped bacteria, does not yield enough characters to allow a clear taxonomic identification based on ultrastructure alone.
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