Behavioural responses in oxygen gradients of ciliates from microbial mats

Microaerophilic ciliates which live in the chemocline of marine sediments and of microbial mats orient themselves in O-2-gradients where they show a more or less distinct and species-specific preference for an oxygen tension somewhere within the range of 1 to 20% atmospheric saturation. In sufficiently steep O-2-gradients some of the species maintain their position within +/- 200 mu m relative to their preferred pot-value. Different types of behavioural responses are involved in the orientation in O-2-gradients; in steep gradients the ''fine tuning'' is at least in some cases based on a phobic response which allows the cells to turn 180 degrees when they accidentally leave their optimum zone. This behaviour was studied in detail in the case of the scuticociliate Uronema filificum. In natural sediments ciliates perform diel vertical migrations in response to changes in O-2-gradient profiles caused by photosynthetic activity.