Cobalt (Co
2+) is present in many nanoscaled materials created for various applications. The key goal of our study was to develop sensitive approaches for assessing the bio-risks associated with using novel Co
2+-containing nanoscaled polymeric complex (Co-NC). Freshwater bivalve mollusk
Anodonta cygnea (Unionidae) was subjected to 14 d action of the developed Co-NC, as well as of Co
2+ applied in the corresponding concentration (50 μg L
−1) or polymeric substance (PS). All experimental groups under study have demonstrated signs of toxic targeting, notably changes in DNA characteristics, oxidative stress (with particularities in each exposed group) and activation of anaerobiosis (Co
2+ and Co-NC). However, the group exposed to Co-NC showed some advantages that can be related to the activation of metallothionein (MT) function (increase in the level of MT-related SH-groups (MT-SH)): low level of oxyradical formation, no increase in protein carbonylation and vitellogenin-like proteins concentration unlike in Co
2+ and PS exposed groups. On the other hand, Co
2+ increased metal (Co, Cu, Zn and Cd) binding to MT (MT-Me) without changes in MT-SH level jointly with activation of oxyradical formation and apoptosis and decreasing of lysosomal membrane stability. PS
per se initiated unbalanced changes in activities of the biotransformation enzymes ethoxyresorufin-
O-deethylase and glutathione-S-transferase. Thus, Co
2+ complexing with the developed PS prevented bio-toxic effects of free Co
2+ ions and PS
per se, at least in the studied hydrobiont. The MT-SH was the main distinguishing index of Co-NC group selected by classification and regression tree analysis.
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