Chronic Effect of Low Urea Concentration on the Content of Carotenoid Pigments in Lymnaea stagnalis

Urea is one of the most common nitrogen-containing pollutants in freshwater ecosystems and, if it enters water bodies for a long time, can disrupt the course of metabolic processes in aquatic organisms. Since carotenoids serve as non-enzymatic antioxidants, changes in their content reflect the development of oxidative stress and serve as sensitive biochemical markers of toxic effects. The purpose of the study was to find out the features of the effect of low urea concentration on the content of \(\beta\)-carotene and xanthophylls in the haemolymph, hepatopancreas, mantle, and foot of Lymnaea stagnalis. The study was conducted on sexually mature uninfected mollusks that were kept in an environment with urea at a concentration equivalent to twice the maximum permissible concentration for water bodies used for fisheries purposes (\(2\text{ MPC}_{\text{fisheries}}\)), for 2, 7, 14, and 21 days. The carotenoid content was determined spectrophotometrically, and the results were processed using two-factor analysis of variance and correlation. It was found that with short-term urea exposure (2 days), the content of \(\beta\)-carotene in the mantle and foot of L. stagnalis increased by 1.93-2.88 times (\(p < 0.01\text{-}0.05\)). The content of xanthophylls in these organs significantly decreased by 50.73-56.68%, while no significant differences were found in hepatopancreas. An increase in the duration of exposure to 14-21 days was accompanied by a systemic decrease in the content of \(\beta\)-carotene (by 25.94-69.24%; \(p < 0.001\text{-}0.05\)) and xanthophylls (by 38.89-88.41%) in the hepatopancreas, mantle and foot of L. stagnalis, which is probably conditioned by increased pro-oxidant processes and the use of carotenoids in free radical neutralisation reactions. Correlation analysis revealed a statistically significant negative association between urea exposure and \(\beta\)-carotene and xanthophyll content, which indicates depletion of the carotenoid reserve under toxic load. Consequently, chronic exposure to urea in the studied concentration causes tissue-specific changes in the carotenoid profile of L. stagnalis, which reflect the transition from compensatory-adaptive reactions to depletion of the non-enzymatic link of the antioxidant system. The results obtained can be used by environmental and fisheries organisations for ecotoxicological assessment of the impact of nitrogen-containing pollutants on freshwater ecosystems, and the content of \(\beta\)-carotene and xanthophylls – as an informative biochemical marker of such effects.