Enzymatic response of Moina macrocopa to different sized zinc oxide particles: An aquatic metal toxicology study

Zinc oxide particles (ZnOPs) of both nanometer and sub-micron sizes are important components of high demand consumer products such as sunscreen, paint, textile, food packaging, and agriculture. Their ultimate discharge in the aquatic ecosystem is nearly unavoidable. For sustainable use of ZnOPs, there is an urgent need to assess its ecotoxicity using ecological indicator organisms. Moina macrocopa, an important component of the aquatic ecosystem is one such less explored indicator organism. In the present investigation, ZnOPs of two different sizes (250 +/- 20 and 500 +/- 50 nm) were selected for risk assessment as most of the previous reports were based on the use of 10-100 nm ZnOPs. ZnOPs of 500 nm were more lethal than that of 250 nm size, with respective LC50 of 0.0092 +/- 0.0012 and 0.0337 +/- 0.0133 mg/L against M. macrocopa after 48 h of exposure. We further used a sublethal concentration of 500 nm (0.00336 mg/L) and 250 nm (0.00092 mg/L) ZnOPs followed by measurement of enzymatic biomarkers of toxicity (acetylcholinesterase, digestive enzymes, antioxidant enzymes). A sizedependent variation in enzymatic response to 250 and 500 nm ZnOPs was seen. Exposure to ZnOPs inhibited acetylcholinesterase and digestive enzymes (trypsin, amylase), and elevated antioxidant enzymes (catalase, glutathione S-transferase) levels. The exposure also decreased the superoxide dismutase activity and increased that of beta-galactosidase. Microscopic investigation revealed the accumulation of ZnOPs in the digestive tract of M. macrocopa that possibly disrupts enzyme activities. The present study will contribute to establishing regulatory policy on the maximum permissible limit of ZnOPs in different water bodies.