Some microaerobic ciliates are facultative anaerobes

Microaerobic ciliates prefer low oxygen tensions somewhere between 1 and 20% atmospheric saturation, but their tolerance to high values of P-O2 and to anoxia is variable. Some microaerobic ciliates (Uronema marinum, U. filificum) were shown to survive complete anoxia for only a limited time and they were incapable of growth, while other species (Cyclidium cf. flagellatum, Euplotes sp., E. aberrans, Strombidium sp. and Paranophrys sp.) survived and grew under anoxic and sulphidic conditions at rates which were lower or almost similar to those attained under oxic conditions. These species are thus facultative anaerobes. They all had relatively low aerobic growth rates compared to those reported in the literature for other similarly sized aerobic ciliates. The facultative anaerobes were tolerant to high concentrations of CN- ( > 0.5 mM) and S2- (in two cases up to 5 mM). When grown under anoxic conditions mitochondrial volume as well as volume fraction increased, suggesting that mitochondria play a role in anaerobic energy metabolism.