In order to meet the increased demand for seafood, aquaculture has experienced significant progress over recent decades. According to the latest FAO report, the world aquaculture production reached 114.5 million tonnes and was valued at 263.6 billion USD in 2018 (FAO, 2020). According to the same statistics, fish farming accounted for almost half (47%) of the global aquaculture production. Mullets are members of the Mugilidae family and are among the most ubiquitous fish resources worldwide. Their farming has been practiced for centuries (FAO,2015a). In the Mediterranean basin, the thick-lipped grey mullet Chelon labrosus is cultured in natural earthen ponds under extensive or semi-intensive regimes (De las Heras et al., 2015). It has been described as an easily cultivable species and constitutes a promising species for aquaculture diversification (Zouiten et al., 2008; Ben Khemis et al., 2013). Indeed, as other mullet species, C. labrosus is a low trophic level feeder, obtaining its energy directly from the first trophic level (Brusle, 1981). In addition, the eurytherm and euryhaline species, C. labrosus is able to tolerate wide ranges of temperature and salinity (Cardona, 2006; Rabeh et al., 2013). Furthermore, several authors have reported that C. labrosus’ osmoregulation abilities appear early during in their development, allowing them to maintain elevated growth rates even under hyposaline conditions (Nordlie et al., 1982; Cardona, 2006). In most North African regions, the production of C. labrosus may be carried out in a variety of ecosystems, such as coastal lagoons with brackish to hyper saline waters (Crosetti, 2016) or reservoirs and ponds with fresh waters (Losse et al., 1991). For many years, Tunisia has opted to breed fish in reservoirs and artificial lakes as a strategy for suppling aquatic products to the interior regions and for providing work opportunities for local communities (Besbes et al., 2020). For instance, natural water flow from artesian sources is used directly as a culture medium in the south of Tunisia (Béchima locality) to facilitate in the raising of some freshwater and marine fish species. In this respect, recent investigation has shown the aptitude of Oreochromis niloticus and C. labrosus to live in geothermal waters (26 to 30°C) and to display good growth rates (Azaza et al., 2008a; Azaza et al., 2008b).

It is well established that the capacity of an organism to efficiently adapt environmental changes is predominantly dependent on its metabolic flexibility (Smith et al., 2018). This phenomenan includes a series of metabolic reorganizations and biochemical adjustments allowing the animal to meet increases in energy requirements and to maintain homeostasis under new environmental condition (Soengas et al., 2007). As the densest form of energy in marine ecosystems and fundamental components of the cell membrane, lipids and their key components, fatty acids (FA), play key roles in the adaptation of aquatic organisms to new environmental conditions (Fokina et al., 2017). In this regard, several studies have reported high variability in the FA composition of fish depending on different abiotic and biotic factors such as the type and amount of food available, water temperature, pH, salinity, and reproduction cycle (Shirai et al., 2002; Kaushik et al., 2006). Changes in FA composition are likely to induce conformational remodeling of membrane proteins, including receptors and channels, and ultimately affect cell responses to extracellular challenges (Brown, 1994). FA are distributed into two major sub-classes which represent an essential and integral part of these compounds. Lipid classes can be broadly divided into neutral, mainly triacylglycerol (TAGs), and polar lipids (such as phospholipids). Phospholipids (PLs), are the major structural constituents of biological membranes. They are involved in the maintenance of membrane integrity and permeability and provide the matrix for the function of a large variety of catalytic processes (Dowhan et al., 2008). The most biologically important phospholipids of organisms are phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI), and phosphatidylserine (PS). These lipid classes are particularly known for their role in the maintaining of membrane integrity and fluidity and in the ensuring of the fish acclimatization process to changes in environmental conditions (Murzina et al., 2020). Neutral lipids which includeTAG, serve mainly as a depot for lipids and provide most of the energy consumed (Sargent et al., 1976). These molecules, which mainly come from food sources, tend to directly accumulate in fish muscle ( Sushchik et al., 2020). The accumulation of these lipids can be differentiated by either external factors, such as fluctuations in environmental conditions, temperature, and food availability, or by internal factors, such as metabolic and physiological activities.

Despite their commercial importance, there are few studies on the biochemical composition of grey mullets (Rabeh et al., 2015; Ben Khemis et al., 2019). According to our hypothesis, lipids could indicate the ability of C. labrosus to adapt to abiotic factors, including water temperature and salinity. Hence, the objective of the present study was to identify and compare the FA composition of individual PL (PC, PE, PI and PS) and NL (mainly TAG) classes in C. labrosus reared in geothermal and seawater conditions in order to gain further insight into the physiological fitness of mullets.

Four major classes of phospholipids: phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI) and phosphatidylserine (PS) in the muscle of C. labrosus with a dominant neutral lipid (TAG) in different rearing conditions were identified. Indeed, C. labrosus from geothermal water was richer in PLs and TAG than those from seawater. A detailed distribution of the fatty acids within the classes is given in Tables 2 and 3. The results show that the amount of neutral and polar lipids of C. labrosus from geothermal water was more important than those from seawater. The predominant FAs in the two groups (seawater and geothermal) were C16:0, C18:0, C18:1, C18:2n-6, C20:4n-6, C20:5n-3 and C22:6n-3 in PL and TAG. Among saturates, palmitic acid (C16:0) was the major fatty acid in all lipid classes and exhibited the highest levels, mainly in the PC, PS, and TAG of the two studied fish groups (Tables 2 and 3). In addition, stearic acid (C18:0) appeared to be particularly abundant in both polar and neutral lipid fractions. The main monounsaturated fatty acid (MUFA) was oleic acid (C18:1) and was found in high amounts in PC as well as in TAG (in both seawater and freshwater groups). Regarding polyunsaturated fatty acids (PUFA), we noted a significant increase in C18:2n-6 in the major polar lipid classes of the geothermal group. C22:6n-3 (DHA) and C20:5n-3 (EPA) were particularly abundant in PI. Indeed, 23 to 25% of PI were made of DHA for both fish groups.

To better visualize the differences in FA in the lipid classes, five-line charts referring to the major fatty acid groups, namely satured fatty acids (SFA), MUFAs and PUFAs, are presented in Figure 1. The results showed that in almost all lipid fractions of the seawater fishes, PUFA (representing up to 50% of the total FA of PS species) was by far the major FA group followed by SFA with corresponding increases in UI and U/S. By contrast, in geothermal fish the distribution proportions of the PUFA series substantially differed between polar lipid fractions. Indeed, PUFA n-3 was particularly abundant in PS and PI fractions while the n-6 series dominated the PC and PE subclass. Additionally, it was found that the level of n-3/n-6 ratio was markedly increased in the PE fraction of the geothermal group. It is also important to note that, SFA stood out as the most abundant fatty acid group in TAG mainly in the geothermal group.

The principal component analysis (PCA) was performed to gain better insight into the effects of rearing conditions on the fatty acid composition of lipid classes in the flesh of the thick-lipped grey mullet, C. labrosus. Figure 2 depicts the twoprincipalcomponents that described 57.72% of the total data variability (PC1 34.22% and PC2 23.5%). The PCA biplot of the overall data described a clear separation between samples from the seawater and the geothermal water. The projection of individuals (each fatty acid from each lipid class of the different fish groups) on the same factorial plan (1:2) showed that samples could be clustered into two groups (I and II). Group I, which consisted of individuals sampled from geothermal water, showed a positive contribution to the first component (PC1), which was characterized by the substantial PUFA n-6 in the PE class. Group II was, however, characterized by high PUFA, particularly n-3 PUFA, DHA and EPA of the phospholipid class representing individuals sampled from seawater (Figure 2).

Teleosteen fish are exposed to varying and occasionally extreme environmental conditions that can produce potent effects on their physiology (Somero, 2004). The Thick-lipped grey mullet C. labrosus, is an euryhaline and eurytherm teleost that presents the ability to live under different environmental conditions of salinity and temperature (Azaza et al., 2008; Cardona, 2006; Rabeh et al., 2013). It is known that temperature and salinity are the key factors in teleost fish that cause fluctuations in the fluidity of cell membranes (Soengas et al., 2007) which largely depends on their lipid components. In this context, changes in membrane lipid composition is a key molecular mechanism of adaptation that is commonly called homeoviscous adaptation (Hazel,1995).

In the current study, lipid class composition and the fatty acid profiles of the two fish groups analyzed were shown to respond selectively and significantly to environmental conditions. Our data revealed that the lipids of the examined fractions from the seawater fishes were characterized by higher values for PUFA n-3, particularly in the PI fraction. However, a high content of PUFA n-6 was recorded in the PI fraction of the geothermal group. In addition, a substantial amount of SFA was observed in the TAG for the two groups. Our findings corroborate with those recorded by some authors who have reported that long-chain fatty acids were probably important for PI to successfully conduct particular cellular functions, including the role in cell growth, signal transduction processes and the membrane anchoring of proteins in plants (Riekhof and Benning, 2009). Consequently, we hypothesize that an increase in PUFA n-6 in the PE of geothermal water fish might compensate for the effects of warm and lower salinity water, which tends to increase lipid rigidity.

The detailed investigation of C. labrosus groups considered in this study indicated that among the major fatty acids of the different lipid classes are C16:0, C18:1, C22:5n3, C18:2n6, C20:4n6, EPA, and DHA. First, with respect to saturates, the highest levels are usally contained in PE and TAG in the two C. labrosus groups. Among SFA, the major components were C16:0, followed by C18:0. Previous studies have reported that SFA, mainly palmitic acid, which is the main fatty acid synthesized de novo in fish, is classically associated with HUFAs in the diacyl phospholipids of fish (Steffens, 1997; Li et al., 2011). In fact, the important levels of SFA may result from a lipogenic activity (Dias et al., 1998) that can be biosynthesized by fish through a conventional pathway catalyzed by cytosolic fatty acid synthetase (Sargent et al., 2002). In addition, the TAG fraction in fish can also contain high levels of monoenoic C16-C18 fatty acids that are intensively synthesized in so-called “fatty” fish species to provide energy reserves. Likewise, the findings of Arts et al. (2009) revealed that triacylglycerol synthesis notably begins with a polar lipid that mostly has one of five common FA (e.g., myristic, C14:0; palmitic, C16:0; palmitoleic, C16:1n-7; stearic, C18:0; or oleic, C18:1n-9) in the sn-1 position.

The highest MUFA detected in the lipid class of both fish groups was C18:1 with markedly impotant levels in the PC of the two studied groups. This is presumably due to its dominance in the commercial feed used in this survey. Another aspect to be taken into account is that C18:1 is a typical MUFA in fish which is most often considered from the standpoint of its energy importance (Sargent et al.,1989). It is also interesting to point out that PC in fish tissues is commonly rich in C18:1 and appears to be more easily influenced by dietary fatty acids than other phosphoglycerides (Tocher, 2003) and it should be the precise reason for the preponderance of these fatty acids.

It is remarkable that the phospholipids of the geothermal groups were found to contain significantly higher contents of C18:2n-6 compared to the seawater groups. The same conclusion was reached in a experiment in which M. cephalus fry were acclimatized to freshwater (El Cafsi et al., 2003). The drop in salinity in the geothermal habit may have led to an increase in the percentages of PUFAs and particularly the n-6 series in the PE fraction. On the other hand, it is also possible that the mechanism involved in the catabolism of C18:2n-6 was more active in the fresh water fish than those from seawater (Sargent et al., 1989; Kheriji et al., 2003). It is also noteworthy that C18:2n-6, which is a well-known key dietary compoment constitutes a good energy source from fish (Castell et al., 1972; Glencross, 2014). Herein, the dominant FA in the food is C18:2n-6 (29%), which allow us to explain the high levels of PUFA n-6 in the fillet of C. labrosus. Indeed, dietary lipids are a source of fatty acidswhich are required for the synthesis of new cellular lipids and for the turnover of existing lipids.

In the present study, an increased proportion of physiologically significant FAs (C22:6n-3 and C20:5n-3) in the phospholipid class was observed for both groups. It has been well established that fish phospholipids are usually considered as a physiologically crucial lipid classes since they are rich in PUFA, predominately DHA and EPA (Sargent et al., 1993). Typically, EPA occurs in lower proportions than DHA due to the availability of these FA as dietary sources. Interestingly, C22:6n-3 is abundant in PI and PS particularly in geothermal specimens. The importance of DHA in PS was previously attributed to the ability of C. labrosus to assimilate this FA. The abundance of DHA observed in the polar lipids started with recognition that this FA has a unique conformation dictacted by helical or an angle iron shape with an overall length similar to that of C16:0 (Applegate et al., 1986). This structure favors the formation of the hexagonal phase in phosphoglycerides, above all in C22:6n-3 phosphoglycerides containing small head groups such as phosphatidylserine and this will facilate very fast conformational changes in membranes (Brown,1994). In a general way, such essential FA are, for example, involved in the modulation of the properties of the lipid phase of the cell membrane, membrane bound enzymes as well as the precursor of functionally important lipooxygenase products (Koven et al., 2011). Previous investigations have shown that variations in polar lipid contents and their individual fractions particularly in muscle are key compensatory mechanisms in organisms that guarantee optimum performance of several membrane-bound enzymes under different environmental conditions (Los, 2001; Cengiz et al., 2012).

This study provides initial insight into the lipid class composition of C. labrosus reared under different conditions. From the obtained results we can conclude that variations in the salinity and temperature chiefly affected the PUFA group. Indeed, we noticed that the juvenile C. labrosus reared in seawater conditions were characterized by the predominance of the n-3 series in phospholipid fractions. As for specimens from the geothermal system, high levels of n-3 PUFA were recorded in the PS and PI fraction, while PC and PE were dominated by the n-6 series. This suggests the aibility of C. labrosus to remodel its lipid composition to adapt to extreme environmental conditions. The obtained results can provide useful basic information that can help in the management of inland aquaculture practices. However, futher investigations are needed to better understand the impact of abiotic parameters on the lipid metabolism of of this promising species.