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.

The marine, free-living Stilbonematinae (Nematoda: Desmodorida) inhabit the oxygen sulfide chemocline in marine sands. They are characterized by an association with ectosymbiotic bacteria. According to their ultrastructure the bacteria are Gram-negative and form morphologically uniform coats that cover the entire body surface of the worms. They are arranged in host-genus or host-species specific patterns: cocci form multilayered sheaths, rods, and crescent- or filament-shaped bacteria form monolayers. The detection of enzymes associated with sulfur metabolism and of ribulose-1,5 bisphosphate carboxylase oxygenase, as well as elemental sulfur in the bacteria indicate a chemolithoautotrophic nature of the symbionts. Their reproductive patterns appear to optimize space utilization on the host surface: vertically standing rods divide by longitudinal fission, whereas other bacteria form non-septate filaments of up to 100 μm length.

Abstract. Microbial colonization on the leaves of a shoot of the mediterranean seagrass Posidonia oceanica (L.) DELILE was studied using Scanning Electron Microscopy. Methods of field ecology such as transect, random plot and stratified sampling survey were applied to the microbial niveau to gain both qualitative and quantitative information on the microbial assemblage. While macro‐epiphytic growth was significantly greater on the outer leaf sides, microbial colonization density was significantly higher on the inner leaf sides, both on leaf surface and epiphyte surface. Diatoms colonized the surface of incrusting algae and epiphytic animals in significantly lower numbers than the Posidonia leaf surface and were absent on erect epiphytic algae. Bacterial densities on epiphyte surfaces even exceeded values of the corresponding leaf surfaces on algal thalli near the leaf tips and on old leaves. Diatoms reach highest mean density on mature leaves and close to the leaf tips, while bacteria reach their greatest density on the oldest leaf and closer to the leaf base. Diatom density in general increases with exposure time of plant surface, while greatest bacterial density was observed at 7–10 weeks exposure. Basal leaf parts on younger leaves were dominated by rod‐shaped bacteria, while distal leaf parts and old leaves were dominated by small coccoid bacteria. Surfaces of epiphytic algae were always distinctly dominated by small coccoid bacteria, and edges of thalli attracted high microbial densities. Microbial biomass (calculated from cell volumes using standard conversion factors) amounts to 2.3 g dry weight m‐2 in the Posidonia stand where the shoot was sampled. The observed patterns of epiphytic colonization are interpreted as the result of a complex, dynamically changing system of interactions both within the epiphytic community and between the epiphytic community, the host plant, and it's environment. A model of the organization of the epiphytic community on Posidonia leaves is presented. “Ultra‐ecology” is a term introduced to denote a type of SEM research in the micro‐environment which is analogous to in situ investigation in “macroscopic” ecological work.

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