Effect of pH on growth, cell volume, and production of freshwater ciliates, and implications for their distribution

We investigated the effect of pH on growth, cell volume, and production of the freshwater ciliates Urotricha farcta, U. furcata, and U. castalia in laboratory cultures with Cryptomonas sp. as food. Overall, pH had a significant, species-specific effect on all parameters investigated. The food alga, Cryptomonas sp., showed a wide pH tolerance, with positive growth rates between pH 4.4 and pH 9.65. Among the ciliates, U. farcta was the most pHtolerant and U. castalia was the most pH-sensitive species, with positive growth being confined to pH 6.58.2. The pH optimum was derived from cellular production rates. The pH optima of the three ciliate species were shifted; their production rates peaked at pH 4.45.3 (U. farcta), pH 5.97.3 (U. furcata), and pH 6.87.9 (U. castalia). The pH effect on growth and survival of the ciliates was minor at circumneutral and moderately alkaline pH values, relative to the effect of temperature and food measured in earlier experiments. The widths of the pH tolerances of the ciliates were positively related to the widths of their temperature niches and to their natural distributions. U. farcta and U. furcata were characterized as euryoecious species, with broad pH and temperature tolerances and ubiquitous distribution; U. castalia is a rare, stenoecious species, requiring specific pH and temperature conditions.