Absence of photosynthesis inhibition by UVR in the mixottrophic planktonic ciliate Stentor araucanus

MODENUTTI, B., E. BALSEIRO, C. CALLIERI, R. BERTONI, Y C. QUEIMALIÑOS

Figueira da Foz, Portugal

Congreso; Plankton Symposium III; 2005

Stentor araucanus is a mixotrophic ciliate that inhabits the upper epilimnetic levels of Andean lakes that are commonly avoided by other planktonic organisms. During the summer 2003-2004, S. araucanus population development was studied in Lake Moreno Oeste (Patagonia, Argentina). Light, temperature and plankton vertical profiles were analysed. The effect of different underwater light intensities on net primary production (NPP) was measured in static (0.30, 10 and 20 m depth) and running incubations (0-15 m depth). S. araucanus was recorded all over the season with highest abundances at or above 15 m; therefore a high proportion of the ciliate population (80%) was exposed to ultraviolet radiation (UVR). NPP incubation experiments indicated that the epilimnetic irradiance levels were more suitable for photosynthetic activity giving higher NPP values. In addition, endosymbiotic algae did not experience photoinhibition in the upper layers since we did not find a NPP decrease due to UV-B wavelength at 0.3 m depth. The NPP resulted in similar values in the presence (quartz tubes) or absence (Mylar) of UV-B. Furthermore, both treatments were not significantly different than that incubated at 10 m depth and in the running incubations. At 20 m depth, with PAR irradiances less than 100 µmol photons m-2 s-1, NPP decreased sharply and this would reveal that at this level endosymbiotic algae were light limited.