Predicting the performance of aquatic organisms in a future warmer climate depends critically on understanding how current temperature regimes affect the organisms’ growth rates. Using a meta-analysis for published experimental data, we calculated the activation energy (E_a) to parameterize the thermal sensitivity of marine and freshwater ciliates, major players in marine and freshwater food webs. We hypothesized that their growth rates increase with temperature but that ciliates dwelling in the immense, thermally stable ocean are closely adapted to their ambient temperature and have lower E_a than ciliates living in smaller, thermally more variable freshwater environments. The E_a was in the range known from other taxa but significantly lower for marine ciliates (0.390 ± 0.105 eV) than for freshwater ciliates (0.633 ± 0.060 eV), supporting our hypothesis. Accordingly, models aiming to predict the ciliate response to increasing water temperature should apply the environment-specific activation energies provided in this study. 

We used the Web of Science to collect experimentally obtained data for ciliate growth rates which we accessed the last time on 15th Oct 2021. The keywords applied were “ciliate*” and “growth rate*”. From the search results, we first selected references that contained further keywords in the title and/or its abstract such as “plankton”, “aquatic systems”, “experiment”, “free-living” and variations of the terms and immediately discarded titles and abstracts where terms as “in situ”, “benthos”, “terrestrial”, “parasitic” or “rumen” and their synonyms and variations were mentioned. Then, we read the entire references that passed the first selection round to determine which would finally be included in our dataset. We only included papers that focused on planktonic ciliate growth rates under experimental conditions assuming optimal growth (i.e., saturating food concentration) and reported the temperature of the experiments. In those studies that measured growth rates at multiple temperatures, the growth rate at each temperature was included in our data set.

Occasionally, growth rate and volume data had to be extracted from figures, using DataThief III (Tummers 2006), and – when needed and possible – we sought clarification directly from the authors. In cases where the volume was not measured in a study or provided by the authors, estimates were made based on similar work (Supplement 1, Table S1). The analyses covered a temperature range from 5 to 30 °C, for which data from both marine and freshwater ciliate taxa were available. Eventually, these selection criteria yielded 57 studies: 40 marine, 17 freshwater, which provided data on 58 taxa (42 from marine water, 16 from freshwater).