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PROGRESS SERIES Prog Ser AND METHODS the large cyanobacteria in quantitative samples Specimens representing could often Fenchel (1968). sory behaviour; published elsewhere small pieces with a scalpel fixed in sucrose in 0.5 h and in point dried transmission electron acetate in 70% ethanol, dehy- drated, embedded minute mineral grains ultramicrotome. Observations microbial mats side through transparent sam- pling cores were made used a halogen lamp fitted with provide focused, cold light a light other 2 sites sulphur bacteria. These were predomi- nantly Beggiatoa; but they mat was Beggiatoa filaments 70 m constituted a biovolume to 6000 dense when fact only thus had a high porosity. This through the volume fraction) filaments constituted 9.0%). The calculated projected 5.0) cm2 Scale bars occurring largely Pleuronema coronatum the mat) occasionally Condylostoma most common the mean and ranae samples) and Heterotrophic flagellates Non-sulphur bacteria Euplotes elegans Unidentified ciliates Unidentified ciliates in Fig. tatively poor in this locality. One sam- deconincki, Geomonh ystera disjuncta Sabatiena pulchra), the first 2 cies occurring within third immediately below it. Quantify- surface layer complete core further revealed in this relative paucity was probably the harbour) many re- smaller (often only from across) reflecting local, visible 128: 171-179, (da Cunha) Plagiopogon loricatus bacteria. Amoebae be reported separately Smirnov unpubl.) Cafeteria roenbergensis Ciliophrys marina Percolomonas cuspidata Pteridomonas danica Strombidium cinctum Trochiloides recta Several species filamentous forms. this in core was atmospheric air. d (Fig. free-swimming sulphur eventually displaced the appearance sulphur bacteria increased eventually the resembled that the harbour. mutually exclusive) free-swimming bacteria and are suitable sites. that during organic material) relatively quickly over time chemocline rapidly; the sediment sur- tolerates anoxia 1989, Fenchel that the eventually monopolise once the beneath the sediment surface. also possible that selective sulphur bacteria. protozoa which sulphur bacteria; when these appear, bacterial populations within contrast few species feed filamentous forms and we Ecol Prog Density (cells smaller, free-swimming shallow site the harbour, decimating the bacterial populations. also likely that populations sulphur bacteria fewer species (dominated by some heterotrophic atmospheric saturation). demonstrate this. The which occurred sulphur bacteria. Beggiatoa filaments; the nematode Scale bars: filamentous forms. marina (which the nematode was observed sulphur bac- site (Fig. observe any feeds on Beggiatoa filaments diameter exceed- species, some the anaerobic occurring immediately beneath the mats consume sulphur bacteria. while filamentous forms spp. (see systematically; very rarely sulphur granules in their vacuoles their small size they would be capable morphologically identifiable forms sulphur bactena. harbour site meiofauna species pm Beggiatoa filaments probably explains the mats not only a variety their role mats remains to Natural Science Research Council to Mr the nematode species Claus Lundsgaard Thrane Haukrogh Fenchel T (1968) marine benthic benthic ecosystem, and the Fenchel T chemosensory behaviour Microbiology 140: Bernard C Bernard C Major microbial bacteria (Beggiatoa (1977) Distribution colorless sulfur (1985) Oxygen mat formation ron Microbiol bacteria. Springer-Verlag, Nelson DC, a chemoautotrophic microgradients. Appl environ anoxic niche microelectrode survey freshwater strains. Microbiol 52: sulfur oxidizing coastal hydrothermal vents. rotifers from waters. Ophelia hydrothermal vents: Oceanogr mar