Anaerobiosis and Symbiosis with Bacteria in Free‐living Ciliates

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Abstract
Marine, sediment-dwelling ciliates were examined for cytochrome oxidase activity by a cytochemical method and for fine structural details. Species of Plagiopylidae (Trichostomatida), Plagiopyla frontata, Sonderia vorax and Sonderia sp., and of Heterotrichida, i.e., Parablepharisma pellitum, Parablepharisma sp., Metopus contortus, M. vestitus and Caenomorpha capucina; previously considered to be olligate anaerobes because of their sulfide-containing habitat, do not have cytochrome oxidase activity or mitochondria with cristae or tubuli. The evolutionary origin and significance of anaerobic ciliates is discussed. Most of the anaerobic ciliates harbor a flora of ecto- and endosymbiotic bacteria as demonstrated by transmission and scanning electron micrographs. The bacteria may utilize the metabolic end products of the protozoa for growth and energy yielding processes. These associations are also compared with other, previously described cases of symbiosis involving prokaryotes and protozoa.