Anaerobic predatory ciliates track seasonal migrations of planktonic photosynthetic bacteria

Abstract

Caenomorpha medusula was the dominant anaerobic ciliated protozoon in the anoxic water of a stratified freshwater lake. The population distribution was non-random, often with a peak in the middle of the hypolimnion. Abundance was closely correlated with numbers of the photosynthetic bacterium Thiopedia sp., and the latter was selectively eaten. There was no apparent correlation with the distribution of the total bacterial count, nor with dissolved sulphide. These observations strongly suggest that planktonic anaerobic ciliates seek out specific food sources.