We investigated the morphology, phylogeny of the 18S rDNA, and pH response of Oxytricha acidotolerans sp. nov. and Urosomoida sp. (Ciliophora, Hypotricha) isolated from two chemically similar acid mining lakes (pH ∼ 2.6) located at Langau, Austria, and in Lusatia, Germany. Oxytricha acidotolerans sp. nov. from Langau has 18 frontal-ventral-transverse cirri but a very indistinct kinety 3 fragmentation so that the assignment to Oxytricha is uncertain. The somewhat smaller species from Lusatia has a highly variable cirral pattern and the dorsal kineties arranged in the Urosomoida pattern and is, therefore, preliminary designated as Urosomoida sp. The pH response was measured as ciliate growth rates in laboratory experiments at pH ranging from 2.5 to 7.0. Our hypothesis was that the shape of the pH reaction norm would not differ between these closely related (3% difference in their SSU rDNA) species. Results revealed a broad pH niche for O. acidotolerans, with growth rates peaking at moderately acidic conditions (pH 5.2). Cyst formation was positively and linearly related to pH. Urosomoida sp. was more sensitive to pH and did not survive at circumneutral pH. Accordingly, we reject our hypothesis that similar habitats would harbour ciliate species with virtually identical pH reaction norm.

We investigated the recently described colpodid ciliate Bromeliothrix metopoides in a series of laboratory experiments to reveal the environmental factors that constrain this species to its peculiar habitat, i.e. the tanks of bromeliads. Our results demonstrated that the various life stages of this ciliate (bacterivorous theronts and microstome trophonts, flagellate-feeding macrostomes) have specific demands in terms of food quality and quantity. Bromeliothrix required a high food threshold (>1.4 mg C L−1) in order to thrive. Food quality also affected resting cyst formation of B. metopoides when the experimental containers dried out. Its maximum growth rates (μmax = 4.71 d−1, i.e. 6.8 doublings d−1) belong to the highest ones recorded thus far for free-living ciliates. The pH niche of B. metopoides was relatively wide (pH ∼4 to >9) under optimal food conditions. However, its high sensitivity to unfavourable environmental conditions let the population collapse within several hours. We conclude that B. metopoides is a boom and bust ciliate that is specifically adapted to its peculiar habitat but virtually unviable in other environments.

We investigated the pH response of 5 clones of the oligotrichine ciliate Meseres corlissi originating from 2 temperate localities in Austria, from a subtropical habitat in China, and from a warm-temperate habitat in Australia. The pH reaction norm was investigated under standard laboratory conditions, with the small cryptophyte Cryptomonas sp. provided as food at saturating food levels over the pH range 4.0 to 9.5. Experiments were conducted at 22.5°C and lasted for 24 h. We measured growth rate, cell length and volume, and cellular production at each pH. We found significant clone-specific differences in each measured parameter, which increased with geographical distance of the isolates. Overall, the pH reaction norms of the Austrian isolates were significantly different from those of the Australian and Chinese clones. The tolerance to low pH differed by up to 1.5 pH units between the clones, i.e. intraspecific differences were comparable with interspecific differences measured earlier under similar experimental conditions. Our results suggest that the pH reaction norm is not homogenous for the species, but that genetically and phenotypically different ecotypes may exist among free-living, cosmopolitan ciliates that are adapted to a particular habitat.

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