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

Abstract

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 ϭ 0.16 m Ϫ1 ). 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). 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 PARϩUV-A and PARϩUV-AϩUV-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 mol photons m Ϫ2 s Ϫ1 , which was the value at which S. araucanus reached a saturation level in the laboratory. In contrast, the incubations at 20 m differed significantly from those in the epilimnion, exhibiting lower values, except when PAR irradiance was higher than 100 mol photons m Ϫ2 s Ϫ1 . 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 mol photons m Ϫ2 s Ϫ1

Bacterial diversity and morphology in deep ultraoligotrophic Andean lakes: The role of UVR on vertical distribution

“…In these lakes, mixotrophic flagellates and ciliates dominate the bacterivorous fraction though in low abundance (Modenutti and Balseiro 2002;Modenutti et al 2005Modenutti et al , 2008. Furthermore, all these lakes exhibit high C : P ratios in accordance with the light-nutrient hypothesis (Sterner et al 1997).…”

“…Another possibility would be that the ciliate O. naumanni, which increases its abundance toward the 1% PAR depth, decreases filament length at deeper layers of the euphotic zone by preying upon them. However, previous work showed that O. naumanni preys on particles ,1 mm (Modenutti and Balseiro 2002) and through catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) dye analysis no filaments were observed in food vacuoles of this ciliate species . Therefore, the inverse relation between filament proportion and O. naumanni abundance was not the consequence of a top-down effect.…”

“…1). The two arms of Lake Mascardi (Catedral and Tronador) were considered two lakes since the two arms have a different hydrology due to the three headwater glaciers (Manso, Overo, and Alerce glaciers) that drain only into Mascardi Tronador (Modenutti et al 2000).…”

“…Cells were counted by epifluorescence microscopy at 12503 magnification, using both UV and blue light, to distinguish heterotrophic nanoflagellates (HNF) from mixotrophic nanoflagellates (MxFl). Enumeration of the ciliate Ophrydium naumanni, a well-known predator of bacteria in these lakes (Modenutti and Balseiro 2002;Modenutti et al 2008), was performed following the Utermö hl technique with an inverted microscope (Olympus DNA extraction and amplification-Water samples (20 mL) from each lake were prefiltered onto 20-mm mesh net and then on NucleoporeH filters, 0.2-mm pore size, 25 mm diameter. Nucleopore filters were then collected and stored at 220uC in phosphate-buffered saline, and transported to the Consiglio Nazionale delle Ricerche-Institute of Ecosystem Study laboratories.…”

“…We studied nine Andean lakes characterized by high transparency and high UVR penetration (Morris et al 1995;Balseiro et al 2007) where planktonic organisms, living in the epilimnetic levels of the euphotic zone, are exposed to damaging ultraviolet wavelengths (UVA and UVB) (Alonso et al 2004;Modenutti et al 2004Modenutti et al , 2005. In these lakes, mixotrophic flagellates and ciliates dominate the bacterivorous fraction though in low abundance (Modenutti and Balseiro 2002;Modenutti et al 2005Modenutti et al , 2008.…”