Summary: Three species of marine, anaerobic free-living ciliates ( and ) were studied with respect to their relation to O. Survival was adversely affected at O tensions exceeding 1–2% of atmospheric (air) saturation (atm. sat.): survived for about 1 h at 100% atm. sat., while the other two species survived the treatment for up to 2 d. Survival in the presence of O depended on the composition of the medium; cells survived longer in clean seawater than in culture medium which, when exposed to O, became toxic. This is probably related to peroxide production mediated by solutes in the culture medium: addition of catalase prolonged survival. The ciliates did not contain catalase. Two of the species harbour endosymbiotic methanogens; at O tensions exceeding 2% atm. sat. the bacteria were inactivated, but they remained viable even after more than 5 h exposure to atmospheric O tension. and respired O at rates similar to those of aerobic species; this O uptake is not coupled to energy conservation since the ciliates do not contain cytochromes, and low O tensions do not stimulate growth. O consumption is probably a detoxification mechanism which can maintain an intracellular anaerobic environment at a low ambient O tension. and showed chemosensory behaviour in response to O; this allowed them to find and remain within anaerobic microhabitats.


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