**4. Conclusions**

of membrane lipid components (cholesterol, phospholipids and their fatty acids) [83]. Thus, mussels from contaminated sites had an elevated level of triacylglycerols and an increased triacylglycerol/phospholipid ratio, which implies a reduced rate of mobilization of triacylglycerols into the phospholipid pool with serious consequences for the structure and function of cell membranes. There was also a substantial decrease in phospholipids, apparently in connection with membrane destruction [84–86]. Some papers have reported the modifications in lipid and fatty acid composition of hydrobionts, including marine mussels, in response to organic and inorganic pollutants' effect [34, 37, 87–93]. Since the lipid metabolism plays an important role in living organism, it is believed that the lipid and fatty acid profile may be used to indicate the organism's health under stress conditions of pollutant effect. Exposure of blue mussels from the White Sea to various concentrations of oil products in an aquarium experiment led to an increase in the level of phospholipids and a reduction of cholesterol concentration in gills and mantle, i.e. the gateway organs for external impacts [25, 34]. These modifications in membrane lipids are believed to make cell membranes more permeable to oil products and create the conditions for their accumulation in these organs for further detoxification. A significant decrease in the level of membrane lipids—phospholipids (mainly at the expense of PC and PE) and cholesterol—simultaneously with an increase in triacylglycerols was observed in gills and digestive glands of mussels exposed to various concentrations of cadmium [37]. These modifications of the lipid profile reflect the destructive effect of cadmium on cell membranes realized through the activation of lipid peroxidation processes. It is worth noting that a significant decrease of the cholesterol concentration under the impact of oil products, mainly their high concentrations, was observed in all the studied organs (gills, mantle, mantle edge and foot) of *M. edulis* [25, 34], as well as under the impact of cobalt on *Mytilus galloprovincialis* [94]. One of the presumed examples of the toxic effect of oil products, as well as some heavy metals on bivalves, is the inhibition of cholesterol synthesis, leading to high membrane permeability. At the same time, the effect from exposure to copper as an essential metal was the opposite (significant increase of cholesterol concentration), probably meant to stabilize the membranes under the metal's oxidative action and to reduce their permeability. It was noted also that when exposed to cadmium and copper [37], as well as to relatively low concentrations of oil products [95], mussels demonstrated an elevated level of arachidonic acid. Apparently, AA involvement in the synthesis of eicosanoids ensures high resistance of the mussels to these xenobiotic impacts. On the other hand, when the concentrations of oil products were high, the level of this acid in the mussels decreased, probably due to inhibition of its biosynthesis, given the observed elevated concentrations of linoleic acid, its metabolic precursor [95]. The results of studies on the lipid composition of gills and digestive glands of intertidal blue mussels, *M. edulis*, collected from different sites in the Gulf of Kandalaksha, White Sea, prove that the composition of lipids and their fatty acids depends not only on the hydrological conditions in the habitat but also on the degree of human impact on it [38]. To wit, the fatty acid profile of the intertidal mussels living in habitats with high human impact is noted for the prevalence of oleic (18:1n-9) acid among total lipids of gills and digestive glands. A similar effect in the fatty acid composition of total lipids was observed in the mussels exposed to various doses of copper in an aquarium experiment [37]. Elevated content of non-essential oleic acid in bivalves may be associated with its additional synthesis under the toxic effect of pollutants and have the goal of binding and detoxifying xenobiotic substances. Unsaturated fatty acids are known

150 Organismal and Molecular Malacology

The lipid profile of White Sea blue mussels, *M. edulis* L., is modified in response to various environmental factors in order to protect cell membranes, maintain or recover their homeostasis, replenish the cell's energy and metabolic resources and thus to secure the mussels' adaptation to the change in environmental conditions. Organ-specific distribution of lipids and fatty acids in White Sea blue mussels, as well as the dependence of the lipid and fatty acid composition response on the effect of various environmental factors on the studied organ, was detected. Modifications in the lipid composition predominantly in gills reflect the acute effect of environmental factors in aquarium experiment conditions, whereas changes in the lipid composition of digestive glands represent an adaptation of the lipid metabolism in response to chronic exposure to ambient factors (field monitoring). The composition of lipids and their fatty acids in intertidal mussels evidences their chronic exposure to abiotic environmental factors and human impact and is in agreement with data on the modifications of the lipid profile in White Sea blue mussels subjected to such environmental factors (namely, salinity, short-term anoxia, heavy metal and oil pollution) in aquarium experiments. The data discussed above prove that the lipid profile plays an important role in the adaptation of blue mussels, *M. edulis*, to new conditions in the habitat. Assessment of the lipid composition in intertidal and cultured mussels helps disclose the metabolic strategy to ensure resistance and adaptation of the organisms to environmental impacts of different nature and can be used as a biochemical marker for indicating the organism's physiological condition. This knowledge is necessary for environmental safety assessment under both natural and human impacts, as well as to predict an organism's and population's status in biomonitoring.
