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Chemosensory Behaviour of Strombidium purpureum, an Anaerobic Oligotrich with Endosymbiotic Purple Non‐Sulphur Bacteria

ABSTRACT. Strombidium purpureum Kahl, is an anaerobic oligotrichous ciliate with endosymbiotic phototrophic bacteria. Like other anaerobic ciliates, S. purpureum reacts to O2‐pressure. In the light, the ciliates avoid even traces of O2 (< 1% atmospheric saturation). In the dark, however, the ciliates accumulate in water with a pO2 of 1–4% atmospheric saturation. Experiments show that ciliates which have accumulated in the dark under microaerophilic conditions react when the light is turned on, and their ability to escape is enhanced by steep O2‐gradients. The ciliates orient themselves in O2‐gradients by a series of phobic responses. They tumble whenever they swim towards a higher O2‐tension and thus they eventually all swim in the direction of lower pO2. The ecological implications of such behaviour are discussed.