Experiments were conducted during October 1998 in Lake Titicaca, Bolivia (16° S, 68° W, 3810 m a.s.l), to determine the effects of solar ultraviolet radiation (UVR) on phytoplankton photosynthetic rates and DNA damage. Water samples were taken daily and incubated in situ or in simulated in situ conditions using sharp cut-off filters to eliminate various portions of the UVR spectrum. The total inhibition of photosynthesis due to UVR in surface waters was 85%; the greatest part of this inhibition (65%) was due to UVAR (315-400 nm), the rest (20%) being due to UVBR (280-315 nm). The inhibition of photosynthesis decreased with depth so that there were no significant differences among treatments at 1.3 optical depths (KPAR). The loss of carbon assimilation in the integrated production over the euphotic zone (4.6 optical depths) was 17.4%, with 14% due to UVAR and an additional 3.4% due to UVBR. Lake Titicaca phytoplankton had a threshold for inhibition of photosynthesis at about 0.3 W m-2 for UVBR and 5 W m-2 for UVAR, below which no inhibition was detected. Above this threshold, photosynthetic inhibition increased steadily, with UVAR having the greatest effect. Analysis of biological weighting functions (BWFs) indicated that phytoplankton from Lake Titicaca was less sensitive to UVR than phytoplankton from other regions. DNA damage (evaluated through the formation of cyclobutane pyrimidine dimers, CPDs) was observed for a simulated worst-case situation (i.e. samples incubated in full sunlight) and significant CPD accumulation was found as a result of UVBR exposure, but not with UVAR. However, absolute levels of damage were relatively low when compared with results obtained at other locations, also suggesting the low sensitivity of Lake Titicaca phytoplankton to UVR. It seems that UVBR stress in these organisms acts via at least two cellular targets: the photosynthetic apparatus and nuclear DNA. Our results suggest that an eventual enhancement of UVBR, due to ozone depletion, would have little impact on the phytoplankton of Lake Titicaca.
Bibliographical noteFunding Information:
We especially thank S. Morales and the personnel at the Centro de Investigacio! n y Desarrollo Piscı!cola del Altiplano (CIDPA) who kindly provided support and materials ; C. Mendoza helped during field work and carried out the taxonomic analysis of phytoplankton samples, L. Taibo helped with computer work and L. Sala performed "%C analysis. We also thank the personnel at the Ministry of Foreign Relations (International Cooperation) in Buenos Aires, the Argentinian Embassy in Bolivia, and Fundacio! n Playa Unio! n for logistic support. We thank P. Neale for helpful suggestions on the BWF calculations and for providing BWF data. The comments and suggestions of two anonymous referees and the journal editor are greatly appreciated. This work was partially supported by Fondo Argentino de Cooperacio! n Horizontal - FO-AR (Project ARG}92}004) and Agencia Nacional de Promocio! n Cientı!fica y Tecnolo! gica (BID 802}OC-AR-PICT 97 no. 01-00046-00955). This is Contribution no. 14 of Estacio! n de Fotobiologı!a Playa Unio! n.
- Cyclobutane pyrimidine dimers
- DNA damage
- Lake Titicaca
- Ultraviolet radiation