Leaf litter is one of the allochthonous sources of dissolved organic carbon (DOC) in freshwater ecosystems. Throughout the degradation of leaves, a lot of chemical compounds are set free into water, which may promote oxidative stress via generation of reactive oxygen species and / or by disturbance of the intracellular oxidative balance in aquatic organisms. We investigated concentration- and time-dependent relationships for the activities of oxidative stress enzymes in macrophytic algae cells of Nitellopsis obtusa, induced by beech Fagus sylvatica leaf extracts, respectively, 0.11–55.0 mg L^–1 DOC (aft er 24 h) and 1.1 mg L^–1 DOC after (15 min – 24 h). The oxidative stress enzymes included catalase (CAT – E. C. 1.11.1.6), glutathione reductase (GR – E. C. 1.6.4.2), cytosolic and microsomal glutathione S-transferase (GST – E. C. 2.5.1.18). A significant increase in sGST and mGST activities after a 24-h exposure to 1.1–11.0 mg L^–1 and to 5.5 mg L^–1 DOC of beech leaf extracts was detected. sGST activity significantly increased in the whole period of exposure, while mGST activity decreased below control levels when exposed for 1–4 h. Glutathione reductase activity increased significantly at all DOC concentrations of beech leaf extracts after 24 h of exposure. The highest elevation of GR activity was observed in extracts containing 5.5 mg L^–1 DOC and the lowest activity increment at 0.11 mg L^–1 DOC. A statistically significant increase in GR activity started after 1 h (at 1.1 mg L^–1 DOC) and was evident until the end of the 24-h exposure. The activity of catalase increased significantly only at the lowest (1.1 mg L^–1) DOC concentration after 24 h of exposure. Beech leaf extracts induced a statistically significant time-dependent elevation of CAT activity during the whole period of exposure. In general, an increase of antioxidative enzyme activities in charophyte Nitellopsis obtusa indicated that the defence mechanisms against oxidative stress were induced when cells had been exposed to beech leaf extracts.

Keywords: Nitellopsis obtusa, Fagus sylvatica, oxidative stress, catalase, glutathione S-transferase, glutathione reductase, dissolved organic carbon, leaf litter