Age related acute effects of microcystin-LR on Daphnia magna biotransformation and oxidative stress

As a result of eutrophication among other factors, bloom forming species of cyanobacteria frequently thrive and dominate communities in freshwater ecosystems. Cyanobacteria are considered as a nuisance and hazardous due to the production of toxins. The most potent and studied cyanobacterial toxins are the microcystins, especially the variant microcystin-LR (MC-LR). Daphnia magna plays a central role in freshwater pelagic food webs and its populations are negatively affected by cyanobacteria due to nutritional inadequacy, feeding interference and toxic effects on growth and reproduction. Biotransformation and antioxidant enzymes, amongst others enable D. magna capacity to cope with MC-LR up to a certain concentration. Aim of our study was to investigate the age related acute effects of MC-LR on D. magna biotransformation and antioxidant enzymes (glutathione S-transferase, GST and catalase, CAT), subsequent lipid peroxidation and furthermore to lactate and the enzyme lactate dehydrogenase (LDH). D. magna neonates (<3 days old) and young adults (7 days old), were exposed to increasing concentrations of MC-LR (up to 100 μg/L) for 24 and 48 h. D. magna sGST activities at both ages, but more pronounced in adults, increased at low MC-LR concentrations followed by decreased activities at the higher ones suggesting diminished ability to biotransform the cyanotoxin. Oxidative protection was more efficient in neonates, where CAT was elevated strongly and contributed to buffer oxidative stress, evidenced by the constant TBARS concentrations. LDH was negatively affected by MC-LR exposure leading to constant and low concentrations of lactate. As this enzyme is involved in the production of rapidly required energy, the findings suggest an energetic impairment due to MC-LR.