No 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.

Abstract." The ciliated protozoan communities in the hypolimnion of a highly productive pond were investigated over two years. Three physiological groups could be distinguished: (a) microaerobic ciliates which tracked the oxic-anoxic boundary within the stratified water column; (b) anaerobic ciliates with endosymbiotic methanogens; and (c) anaerobes without endosymbiotic methanogens. Both groups of anaerobes were confined to the anoxic zone of the hypolimnion. Community biomass was dominated by microaerobic ciliates which had on average 20 times larger cells than anaerobic ciliates. Abundance and biomass of microaerobic ciliates decreased over the summer, while anaerobic ciliates increased. This reflected a spatial shift in the availability of inorganic nutrients and, as a result, of ciliate food from the epi-and metalimnion to the hypolimnion. The low biomass production of anaerobic ciliates was consistent with the low theoretical growth efficiency of anaerobic metabolism. Ciliate species displayed characteristic spatial and seasonal distribution patterns within the water column which were similar in both years investigated. Spatial and temporal distribution was mainly governed by two factors: (1) the distribution of dissolved oxygen; and (2) the availability of food. Distribution patterns were not related to chemical gradients other than the oxygen gradient, but they were correlated with the distribution of major food sources.