Effect of UV-B and different PAR intensities on the primary production of the mixotrophic planktonic ciliate Stentor araucanus

Stentor araucanus is a mixotrophic ciliate that, in Andean lakes, inhabits the upper epilimnetic levels, which are commonly avoided by other planktonic organisms. This freshwater heterotrich has dark pigmented cortical granules and lives autotrophically with endosymbiotic algae. The effect of photosynthetically active radiation (PAR) and ultraviolet (UV)-B radiation on primary production was analyzed during summer 2003-2004 in Lake Moreno Oeste, a highly transparent ultraoligotrophic lake (mean summer K-d = 0.16 m(-1)). Primary production (PP) was measured in the field in the euphotic zone during both static and variable-depth incubations. Static exposure of the organisms was examined at different depths (0.30, 10, and 20 m), and the variable depth exposure involved experimental containers moved continuously up and down the epilimnion (0-15 m). In the static exposure closest to the surface and in the mobile incubation, quartz tubes were incubated with and without a UV-B screen (Mylar (TM)). Additionally, PP was measured in the laboratory with and without previous exposure to a UV-B lamp (290-315 nm). S. araucanus was present throughout the summer with highest abundances at or above 15 m in depth. A high proportion of the ciliate population (80%) was, therefore, exposed to UV radiation, and between 30% and 60% of the population occupied depths at which UV-B (305 nm) exceeded 1% of surface incidence. PP values were higher in the epilimnion than below it and were not reduced by exposure to high irradiances of PAR+UV-A and PAR+UV-A+UV-B. The laboratory experiments showed no difference between UV-B and PAR preexposure treatments. The variable-depth epilimnetic incubations gave similar PP values and did not differ from the static incubations. The average PAR irradiance of the epilimnion was high, around 600 mu mol photons m(-2) s(-1), which was the value at which S. araucanus reached a saturation level in the laboratory. In contrast, the incubations at 20 In differed significantly from those in the epilimnion, exhibiting lower values, except when PAR irradiance was higher than 100 mu mol photons m(-2) s-1. These results indicate that pigmented mixotrophs like S. araucanus achieve high population densities in the epilimnion because they receive sufficient irradiance (PAR between 100 and 1,600 mu mol photons m(-2) s(-1)) to allow endosymbiotic algae to produce.