Are there any effects of enhanced UV - B radiation on Chara polyacantha and Nitella hyalina under laboratory conditions ?

Attenuation of UV-B radiation in the water column occurs due to absorption and scatter but in shallow clear freshwater systems enhanced UV-B radiation might be of importance in affecting aquatic organisms. Two laboratory experiments were performed to determine whether two species of charophytes typical for shallow waterbodies, such as Chara polyacantha and Nitella hyalina, are negatively affected by UV-B radiation. Each experiment had two treatments: (i) the photosynthetic active radiation (PAR) treatment and (ii) the enhanced UV-B+PAR treatment. We focussed on the influence on growth rate, photosynthetic pigment concentration and the amount of methanol soluble UV absorbing compounds which can act as a screen preventing radiation damage. UV-B radiation increase in the Extremely Enhanced UV-B Radiation Experiment (> 15 kJ·m·day UV-B dose) reduced average charophyte length in both species after three weeks of incubation and caused depigmentation and cracking of the charophytes. However, in the Enhanced UV-B Radiation Experiment (3.6 kJ·m·day UV-B dose) we found a large variability in the length of charophyte individuals in both treatments after 22 and 49 days what caused that there were not statistically significant differences in the growth rates of both charophyte species. However, significant differences in C. polyacantha growth rate were found after four months of exposure. There were not either significant differences in chlorophyll a concentration in C. polyacantha charophyte tissues among the treatments. On the other hand, N. hyalina increased its chlorophyll concentration in the PAR+UV-B treatment. In both experiments, we did not find statistically significant increased levels of methanol soluble UV absorbing compounds under enhanced UV-B conditions. The lack of induction of this kind of compounds in C. polyacantha and N. hyalina under augmented UV-B doses during our experiments is in agreement with other Authors results for Chara aspera, and make us to consider charophytes as potentially sensitive organisms to UV-B radiation.