DNA damage and photosynthetic inhibition induced by solar ultraviolet radiation in tropical phytoplankton (Lake Titicaca, Bolivia)

Experiments were conducted during October 1998 in Lake Titicaca, Bolivia (1 6 degrees S, 68 degrees 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 ir? situ or in simulat ed in situ conditions using sharp cut-off filters to eliminate various port ions of the UVR spectrum. The total inhibition of photosynthesis due to WR in surface waters was 85 %; the greatest part of this inhibition (65 %) was due to WAR (315-400 nm), the rest (20 %) being due to UVBR (280-315 nm). T he inhibition of photosynthesis decreased with depth so that there were no significant differences among treatments at 1.3 optical depths (K-PAR). 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 addition al 3.4% due to UVBR. Lake Titicaca phytoplankton had a threshold for inhibi tion 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, photosynthe tic inhibition increased steadily, with UVAR having the greatest effect. An alysis of biological weighting functions (BWFs) indicated that phytoplankto n from Lake Titicaca was less sensitive to UVR than phytoplankton from othe r regions. DNA damage (evaluated through the formation of cyclobutane pyrim idine dimers, CPDs) was observed for a simulated worst-case situation (i.e, samples incubated in full sunlight) and significant CPD accumulation was f ound as a result of UVBR exposure, but not with UVAR. However, absolute lev els of damage were relatively low when compared with results obtained at ot her locations, also suggesting the low sensitivity of Lake Titicaca phytopl ankton to UVR. It seems that UVBR stress in these organisms acts via at lea st two cellular targets: the photosynthetic apparatus and nuclear DNA. Our results suggest that an eventual enhancement of UVBR, due to ozone depletio n, would have little impact on the phytoplankton of Lake Titicaca.