Seaweed is considered a good source of hydrosoluble vitamins, lipophilic vitamins and long-chain polyunsaturated fatty acids (LCPUFAs) such as omega-3 fatty acids (n-3), especially eicosa pentaenoic acid (EPA;C20:5 n-3) (Khotimchenko et al., 2002; Adharini et al., 2019). Therefore, traditionally seaweeds have been long used as a food source in many regions of the world and have potential as a functional food (Durucan and Turna, 2014).

Marine plants, expecially seaweed have antioxidant properties. Interest in these properties has increased among consumers and researchers (Farasat et al., 2013; Caf et al., 2015). Antioxidants can be of both synthetic and natural origin. Natural antioxidants contain mainly phenolic compounds. The utilization of natural antioxidants from plants generally does not produce negative effects, although some synthetic antioxidants have been found to induce genotoxic effects (Barlow, 1990). The biological effects of highly reactive oxygen species are controlled by a wide spectrum of antioxidant mechanisms (Yu, 1994). Flavonoids are an important group of plant metabolites and also represent the most important group of polyphenolic compounds. They have high chemical and biological activities, including antioxidant and free radical scavenging properties (Kahkonen et al., 1999). In addition, α-tocopherol (vitamin E) is one of the most important lipophilic vitamins because of its high antioxidant effect. In particular, it is effective for preventing lipid peroxidation (LPO) (Houston, 2005).

Furthermore, fatty acids are one of the most important molecules in aquatic ecosystems because of transference among different trophic levels (Filimonova et al., 2016). The main dietary sources of n-3 PUFAs for humans are fish. However, the sustainability of fish as a PUFA source is not certain due to decreasing fish stocks and increasing global demands. Worm et al., (2009) indicated that some fish populations are rapidly being depleted worldwide. Hence, new alternative sources of PUFAs must be found for food and feed applications (Vizetto-Duarte et al., 2015). The interest in the lipid composition of seaweeds has gradually increased because seaweeds contain a high level of PUFAs, especially alpha-linolenic (ALA, 18:3 n-3) arachidonic acid (ARA, 20:4 n-6) and EPA (Dawczynski et al., 2007). In fish, n-3 LCPUFA supplies mainly depend on the primary producers in the marine seafood chain and their supply to fish’s diets (Ivanova et al., 2013a). Producers synthesize LC PUFA from ALA and linoleic acid through desaturation and elongation of the contained fatty acids (Strobel et al., 2012).

Therefore, seaweeds may be a new alternative source of PUFAs for the food industry. Turkey is one of the countries with the highest potential for seaweed production. However, information about the biochemical contents of Turkish seaweeds is limited. Therefore, the aim of this study was to determine the lipophilic vitamin and fatty acid contents in several edible seaweeds in Turkey.

Four seaweeds (C. elongata, J. rubens, L. obtusa and S. vulgare) were collected in April, 2014 from Lara (Antalya, Turkey), and C. barbata was collected in May, 2012 from Kepez (Çanakkale, Turkey). The epiphytes, epizoons, sediment particles and additional organic matter in the seaweeds were removed using sea water. Cleaned seaweed samples were kept in the freezer (–80 °C) until analysis.

The predominant fatty acids in five seaweed species (C. barbata, C. elongata, J. rubens, L. obtusa and S. vulgare) were studied. The 16:0 fatty acid was detected in C. barbata at 40.55%, in C. elongata at 36.5%, in J. rubens at 34.22%, in L. obtusa at 34.57% and in S. vulgare at 37.11%. 18:1n-9 and 16:1n-7 were the most abundant MUFAs in the analyzed species. A study by Gressler et al., (2010), showed that 16:0 was the most predominant fatty acid in seaweed. Our findings showed that total PUFA contents were higher than total MUFA contents in the seaweed species, except for S. vulgare. We found that PUFAs were the highest in L. obtusa from Rhodophyta whereas it was the lowest in S. vulgare from Phaeophyta. PUFA content was higher in C. barbata (26.71%) taken from Çanakkale than S. vulgare (18.13%) taken from Antalya station. This difference can be related to location, seasonal differences etc. Frikha et al., (2011) and Vizetto-Duarte et al., (2015) reported that 16:1n-7 and 18:1n-9 were the main MUFAs in C. barbata. Our results are in accordance with the literature. Moreover, diets rich in MUFAs were found to decrease the total and low-density lipoprotein (LDL) cholesterol levels in plasma (Degirolamo and Lawrence, 2011).

All seaweeds also contained 18:2n-6. LA was most abundant in C. barbata compared to the other seaweeds. Regional differences may cause high 18:2n-6 in C. barbata, which was collected from Çanakkale, whereas the other seaweeds were collected from Antalya. Multivariate analysis showed differences among the locations (p = 0.003) (p < 0.01). The biochemical composition of seaweeds is directly related to season and location (Renaud and Luong-Van, 2006). Also, fatty acid content can depend on environmental and genetic differences (Nelson et al., 2002). Notably, 18:3n-3 was found in all brown seaweeds; in red seaweeds, 18:3n-3 was found in C. elongata and J. rubens. The eicosanoid precursors ARA and EPA were also detected in all species. EPA was found in significant levels in the red seaweeds, and ARA was found at similar levels in the red and brown seaweeds. In addition, HUFA was higher in red seaweeds than in brown seaweeds (Table1). Simiarly, we encountered significant differences between Sargassaceae (brown seaweed) and Corallinaceae (red seaweed) (p = 0.006) (p < 0.01) and partial differences between Sargassaceae (brown seaweed) and Rhomelaceae (red seaweed) (p = 0.013) (p < 0.01) according to multivariate analysis. However, no differences among the seaweed species as a whole were found (p = 0.09–0.11) (p < 0.01).

Several studies indicated the presence of a reverse correlation between the PUFA:SFA ratios and cardiovascular diseases. Greater concentrations of PUFA relative to SFA in the human diet were found to decrease problems associated with cardiovascular diseases (Simopolous, 2000; Erkkila et al., 2008). Therefore, the PUFA:SFA ratio was an important parameter in our study. Also, the British Department of Health suggested that the average ratio of PUFA:SFA should be 0.45 or higher. In this study, the PUFA:SFA ratio of the seaweeds was determined to range from 0.31–0.60 (extreme values corresponding with S. vulgare and L. obtusa, respectively), and all analyzed species demonstrated a favorable PUFA: SFA ratio except for S. vulgare (Table 1).

Dawczynski et al., (2007) reported that HUFA synthesis pathways have not been studied in seaweeds to date. Seaweeds can produce high levels of ARA and EPA but generally have low levels of or completely lack DHA (C22:6 n-3, Docosahexaenoic acid). EPA and DHA are major components of membranes. EPA and DHA are basically derived from ALA by elongation and desaturation reactions (Narayan et al., 2006). They are also necessary precursors to bioactive compounds such as eicosanoids (Lee and Hirota, 1973). In the present study, DHA was not present in the analyzed species; in different phaeophytes, DHA is generally absent or exists at low levels (Li et al., 2002). Meanwhile, red and brown seaweeds usually have a high content of fatty acids with 20 carbons such as EPA and ARA (Banerjee et al., 2009). We observed that PUFAs had the highest relative concentration in the red seaweeds, and EPA ranged from 9–15.09% of the total determined fatty acid content.

We also evaluated the lipophilic vitamin content of the five seaweed species. The results clearly indicated that all tested seaweeds possessed vitamins. Specifically, α-tocopherol, δ-tocopherol, vitamin K1, vitamin K2, stigmasterol, ß-sitosterol and ergosterol were detected. Seaweeds were previously found to be a significant source of vitamins (Sanchez-Machado et al., 2004). In this study, α-tocopherol was the most important lipophilic vitamin, ranging from 1.65 to 20.46 μg/g. d.w. in L. obtusa, which contained the highest content. We determined that α-tocopherol was generally low in C. barbata (4.40 μg/g). Similarly, Panayotova et al., (2013a) and Panayotova et al., (2013b) reported that C. barbata had high amounts of α-tocopherol (15.77 ± 0.21 mg/g, 29.13 mg/g; respectively). In addition, Durmaz et al., (2008) found that Cystoseira spp. had an α-tocopherol content of 17.10 µg/g in the Black Sea of Turkey. The vitamin composition of seaweed is affected by species, algal growth level, geographical region, salinity, season and temperature (Škrovánková, 2011); yet, overall, α-tocopherol is the most abundant lipophilic vitamin in seaweeds (Škrovánková, 2011). In the present study, α-tocopherol content was found to be higher than δ-tocopherol content. The α-form has a higher antioxidant effect than other forms (Yamamoto et al., 2001). Also, α-tocopherol is associated with decreased blood pressure and the prevention of lipid peroxidation in biological systems, which is its main function (Houston, 2005).

Finally, we examined sterols, which are one of the main components of lipid classes phytosterols (C₂₈ and C₂₉ sterols) are essential for the synthesis of some vitamins. For instance, ergosterol is necessary for the synthesis of vitamin D₂ and cortisone (Ibañez et al., 2011). We observed that ergosterol was the main sterol in L. obtusa (137±1.41 μg/g). β-sitosterol was the main sterol in S. vulgare, and stigmasterol was the main sterol in C. elongata (43.23 μg/g) (see Table 3).

Important bioactive molecules from marine resources can be used in many fields, including the drug, cosmetic and food industries, as they have been found to have positive effects on human health (Hamed et al., 2015). Our findings showed that seaweeds can be used in different industries because of their biochemical contents. In particular, the high PUFA content of seaweeds has increased their importance in the food industry. PUFA concentration was high, especially C₂₀, ARA and EPA in the studied species. However, they did not contain DHA. C. barbata, S. vulgare from Phaeophyta presented the lowest contents of EPA (> 2%) whereas J. rubens, L. obtusa and C. elongata from Rhodophyta presented the highest contents of EPA (> 9%). These contents vary according to different factors, such as species, location and family. In addition, α-tocopherol had the highest amount within lipophilic vitamins. These species can use instead of synthetic antioxidants because of their high antioxidant effect.

Our results showed that species, family and location had an important effect on the biochemical contents of the studied seaweeds. Further research on different marine seaweed species is necessary, including the applications of seaweed species in the health and food industries in Turkey. The findings will be useful as a guide to further studies.