Mixotrophic ciliates in an Andean lake: dependence on light and prey of an Ophrydium naumanni population

Abstract

1. Planktonic ciliates were examined during a spring–summer period (November 1998–April 1999) in the ultraoligotrophic Lake Moreno Oeste (41°5′ S and 71°33′ W, 758 m a.s.l), which belongs to the Nahuel Huapi System (Patagonia, Argentina). The lake is deep (Zmax=90 m) and warm monomictic.2. Sampling was performed at a mid‐lake station, where vertical profiles of temperature and light were measured in situ, and samples for bacteria and ciliates enumeration were taken throughout the water column.3. The peritrich Ophrydium naumanni, a freshwater pelagic ciliate with endosymbiotic Chlorella, was the dominant ciliate in the lake.4. Ophrydium naumanni and autotrophic picoplankton exhibit a clear coincidence in their vertical distribution (P < 0.05), preferring levels at or near the 1% of surface photosynthetically active radiation (PAR) irradiance. Both may have the same light requirements, or the coincidence may reflect a trophic relationship.5. Dependence on light and prey by O. naumanni were studied using field experiments, in which we analysed ciliate grazing on bacteria, and in laboratory experiments, in which we compared particle uptake under dark and light conditions.6. Ophrydium naumanni was able to ingest particles [latex microspheres and fluorescently labelled bacteria (FLB)] in field and laboratory experiment, indicating that it has the potential to affect bacteria population of Lake Moreno Oeste.7. Ciliate particle ingestion was observed to be dependent on light availability because under dark conditions, the ingestion was lowered (P < 0.05).