Local adaptation among geographically distant clones of the cosmopolitan freshwater ciliate Meseres corlissi. II. Response to pH

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 labora- tory conditions, with the small cryptophyte Cryptomonas sp. provided as food at saturating food lev- els over the pH range 4.0 to 9.5. Experiments were conducted at 22.5°C and lasted for 24 h. We mea- sured 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.