Symbiotic association of the ciliate Ophrydium naumanni with Chlorella causing a deep chlorophyll a maximum in an oligotrophic South Andes lake

Abstract

Vertical profiles of temperature, light and chlorophyll<it>a</it> concentration were examined in Lake Moreno Oeste, an oligotrophic South Andean lake (Argentina), during the warmest period of the year (November-April), when thermal stratification is characteristic. Concurrent samples for the enumeration of phytoplankton and green ciliates were taken, and the different contribution of these fractions to total chlorophyll <it>a</it> concentration was analysed. The development of a distinctive deep chlorophyll maximum was observed during summer months. The deep chlorophyll maximum was situated near the limit of the euphotic zone and just below the upper limit of the metalimnion. The results showed that the green ciliate <it>Ophrydium naumanni</it> with endosymbiotic <it>Chlorella</it> dominated the metalimnion causing the deep chlorophyll maximum. Additional laboratory experiments revealed a strong dependence of<it>O.naumanni</it> on light. Therefore, the symbiotic association appears to be an effective exploitation of the water column in poor-nutrient-high-light ecosystems like large Andean lakes.