Sulfur-oxidizing prokaryotes (SOP) catalyse a central step in the global S-cycle and are of major functional importance for a variety of natural and engineered systems, but our knowledge on their actual diversity and environmental distribution patterns is still rather limited. In this study we developed a specific PCR assay for the detection of dsrAB that encode the reversely operating sirohaem dissimilatory sulfite reductase (rDSR) and are present in many but not all published genomes of SOP. The PCR assay was used to screen 42 strains of SOP (most without published genome sequence) representing the recognized diversity of this guild. For 13 of these strains dsrAB was detected and the respective PCR product was sequenced. Interestingly, most dsrAB-encoding SOP are capable of forming sulfur storage compounds. Phylogenetic analysis demonstrated largely congruent rDSR and 16S rRNA consensus tree topologies, indicating that lateral transfer events did not play an important role in the evolutionary history of known rDSR. Thus, this enzyme represents a suitable phylogenetic marker for diversity analyses of sulfur storage compound-exploiting SOP in the environment. The potential of this new functional gene approach was demonstrated by comparative sequence analyses of all dsrAB present in published metagenomes and by applying it for a SOP census in selected marine worms and an alkaline lake sediment.

Abstract. The Stilbonemutinue (marine free‐living nematodes) arc remarkable for cctosymbiotic bacteria, which cover the greatest part of their body in a highly ordered and species specific pattern. Using SEM we describe the main types of symbiotic cover and give evidence for the role of the bacteria in the nutrition of their host on the basis of stable carbon isotope ratios. In experimental systems the worms migrated repeatedly across a sulfide gradient during 12 h when sulfide concentrations were low, but stayed above the sulfide maximum at high concentrations. The migration across the chemocline exposes the symbionts alternately to reduced sulfur compounds and oxygen; this constitutes an alternative strategy to the ventilation/circulation systems in symbiotic macrofauna from sulfidic habitats.

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Zoothamnium niveum is a giant, colonial marine ciliate from sulfide-rich habitats obligatorily covered with chemoautotrophic, sulfide-oxidizing bacteria which appear as coccoid rods and rods with a series of intermediate shapes. Comparative 16S rRNA gene sequence analysis and fluorescence in situ hybridization showed that the ectosymbiont of Z. niveum belongs to only one pleomorphic phylotype. The Z. niveum ectosymbiont is only moderately related to previously identified groups of thiotrophic symbionts within the Gammaproteobacteria, and shows highest 16S rRNA sequence similarity with the free-living sulfur-oxidizing bacterial strain ODIII6 from shallowwater hydrothermal vents of the Mediterranean Sea (94.5%) and an endosymbiont from a deep-sea hydrothermal vent gastropod of the Indian Ocean Ridge (93.1%). A replacement of this specific ectosymbiont by a variety of other bacteria was observed only for senescent basal parts of the host colonies. The taxonomic status "Candidatus Thiobios zoothamnicoli" is proposed for the ectosymbiont of Z. niveum based on its ultrastructure, its 16S rRNA gene, the intergenic spacer region, and its partial 23S rRNA gene sequence.

Although thiotrophic symbioses have been intensively studied for the last three decades, nothing is known about the molecular mechanisms of symbiont acquisition. We used the symbiosis between the marine nematode Laxus oneistus and sulfur-oxidizing bacteria to study this process. In this association a monolayer of symbionts covers the whole cuticle of the nematode, except its anterior-most region. Here, we identify a novel Ca 2؉ -dependent mannose-specific lectin that was exclusively secreted onto the posterior, bacterium-associated region of L. oneistus cuticle. A recombinant form of this lectin induced symbiont aggregation in seawater and was able to compete with the native lectin for symbiont binding in vivo. Surprisingly, the carbohydrate recognition domain of this mannose-binding protein was similar both structurally and functionally to a human dendritic cell-specific immunoreceptor. Our results provide a molecular link between bacterial symbionts and host-secreted mucus in a marine symbiosis and suggest conservation in the mechanisms of host-microbe interactions throughout the animal kingdom.Stilbonematinae (Desmodoridae, Chromadoria) (27, 28) are especially abundant in tropical calcareous sands, where an oxidized surface layer overlies a reduced one. In the Belize Barrier Reef, two species, Laxus oneistus and Stilbonema majum, even dominate the nematode fauna of shallow sands. Stilbonematids repeatedly cross the boundary between oxidized and reduced sediment layers and thus represent an ideal substrate for bacteria that require both oxygen and sulfide. The worms, in turn, appear to obtain most of their nutrition by grazing on their symbionts (29).Another peculiar feature of symbiotic marine nematodes is a conspicuous system of glandular sensory organs (GSOs) underlying their cuticle (see Fig. 2D). The GSOs produce a mucus envelope in which symbionts may be embedded. In each GSO the secretory granules accumulate in the canal and are released onto the cuticle surface through a hollow seta (18,19).There is no evidence of vertical transmission of the symbionts, but even very small juveniles carry a complete microbial coat. Colonization of recently hatched or molted stilbonematids must be a rapid process because field collections rarely yielded nonsymbiotic stilbonematids.In L. oneistus, whose bacterial coat is composed of a single phylotype of rod-shaped ␥-Proteobacteria (26, 31, 30), symbiont recruitment must be highly selective. The microbial coat starts with a sharp onset some distance behind the anterior end, and the bacterial rods are aligned perpendicularly to the worm's surface. The absence of symbionts on the anterior region does not correlate with fewer or smaller GSOs or with reduced mucus production.Incubation in D-mannose specifically led to symbiont detachment from nematodes belonging to the genus Laxus but not from S. majum. Furthermore, this monosaccharide was found on the surface of the symbionts, but not on L. oneistus cuticle (22). These data led to the hypothesis that L. oneistus binds its s...