**Lipid Composition Modifications in the Blue Mussels (***Mytilus edulis* **L.) from the White Sea**

Natalia N. Fokina, Tatiana R. Ruokolainen and Nina N. Nemova

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67811

#### **Abstract**

Studying biochemical indicators in response to various environmental factors allows revealing the metabolic adaptive strategy of the organism's tolerance and survival under a variety of environmental impacts. This review analyses both the authors' own data and the available literature on the problem of biochemical adaptations of the lipid composition in marine bivalves, particularly blue mussels, *Mytilus edulis* L., to various environmental impacts. Modifications in the composition of lipids and their fatty acids in blue mussels caused by short-term (under laboratory conditions) and chronic (field monitoring) exposure to natural and human factors indicate that homeostasis is maintained in cell membranes and the organism's energy requirements and facilitate the adaptation and tolerance of the mussels to environmental disturbances. The lipid and fatty acid composition indices in White Sea intertidal mussels which reflect their chronic exposure to a wide variety of environmental factors are discussed and compared to data on changes in the lipid composition of blue mussels exposed to some environmental factors (salinity, anoxia, metals) in aquarium experiments. The lipid profile plays an important role in the adaptation of blue mussels to new conditions in the habitat, and it can be used as a biochemical marker for indicating the organism's physiological state.

**Keywords:** lipids, fatty acids, biochemical adaptation, environmental factors, *Mytilus edulis*

### **1. Introduction**

Biochemical processes underlie the development of cell metabolic responses to environmental impacts and allow an organism to adapt and survive in a changing environment [1]. Metabolic modifications up to the level of physiological and morphological disorders are reflected in the

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changes of various biochemical indicators, which allow determining the adaptive strategy of an organism's tolerance and survival under both natural and human impacts. Lipid molecules, which are involved in all the essential physiological-biochemical processes [2], play a major role in the organism's adaptive responses to various factors in the environment [1]. The primary response to stress is modification of the physical state of cell membranes (mainly fluidity), which triggers a change of their lipid and fatty acid composition [3, 4]. The main lipid components of biological membranes are phospholipids and cholesterol. The ratio of phospholipids and cholesterol is considered as an indicator of membrane fluidity. Cholesterol is known to increase the order of the phospholipid fatty acid chains in membranes [5]. Membrane phospholipids in different molecular species and molecular shapes as well as their interaction with cholesterol and membrane proteins determine membrane fluidity and subsequently regulate the activity of membrane-bound enzymes and the functioning of ion channels, pumps and receptors [2, 6, 7]. Besides their effects on membrane fluidity, membrane phospholipids are also a source of bioactive compounds and messengers [8]. In particular, eicosapentaenoic acid (EPA, 20:5n-3) and arachidonic acid (AA, 20:4n-6) are released from phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylinositol (PI) by phospholipase A2 and serve as precursors of short-lived hormone-like substances called eicosanoids (prostaglandins, thromboxanes, leukotrienes, etc.). The bioactive molecules have a wide range of physiological actions, including immune response, inflammatory response, neural function, reproduction and enhancement of an organism's adaptation to environmental stress [2, 8]. PI is also a source of such messengers as diacylglycerols and inositol phosphates (namely, inositol trisphosphate and others). These messengers, as well as the phospholipid phosphatidylserine (PS), are involved in regulating the activity of protein kinase C, which controls many cell functions, such as differentiation, proliferation, metabolism and apoptosis [2, 8, 9]. Moreover, fatty acids are the most labile components of lipid molecules, quickly and accurately reflecting environmental impacts and activating an organism's adaptive abilities. For example, a well-known biochemical response of poikilothermic organisms to low temperature is increased fatty acid unsaturation of both membrane and storage lipids [7, 10, 11]. In addition to membrane lipids, an important role in the adaptive response of organisms to various environmental factors belongs to high-energy storage lipids, chiefly triacylglycerols and their fatty acids [12–15], which cover the energy costs needed for maintaining homeostasis under the new environmental conditions. Since long-chain polyunsaturated fatty acids, particularly such essential fatty acids as EPA and docosahexaenoic acid (DHA, 22:6n-3), cannot be de novo synthesized in marine mussels [16, 17], their incorporation and elimination in membrane and storage lipids are strongly regulated [18, 19]. Thus, lipid and fatty acid composition as a key component of various metabolic pathways that are linked to processes important for survival and tolerance reflects the adaptive response of an organism to environmental effects. It is assumed that lipid composition may be used as a biochemical marker for indicating the organism's physiological state in environmental assessments and biomonitoring.

The blue mussels, *Mytilus edulis* L., are used worldwide as marine sentinel organisms in biomonitoring programmes due to their longevity, sessile nature, global distribution and ability to bioaccumulate high concentrations of pollutants [20–22]. In the White Sea, *M. edulis* L. is the dominant species of coastal (intertidal) ecosystems. Numerous studies on White Sea mussels' response to various environmental effects have identified adaptive mechanisms on molecular, biochemical, cellular, physiological and behavioural levels of biological organization [23–38].

This paper summarizes the results of research on lipid composition effects in White Sea blue mussels, *M. edulis* L. (1758), in response to environmental factors such as temperature, salinity, short-term anoxia, change of nutrition source, metals and oil pollution. Phenotype-specific features of the lipid composition in White Sea blue mussels from different habitat conditions (intertidal zone and aquaculture), as well as compensatory modifications of the lipid composition in intertidal mussels under chronic stress in the natural habitat and under short-term exposure to stress in laboratory experiments, are discussed.
