The content and composition of algal lipids vary with species, geographical location, season, temperature, salinity, light intensity, or combination of these factors. In gen­eral, algae contain up to 1-3% of dry weight of lipids, being glycolipids the major lipid class in all algae, followed by neutral and phospholipids.

The major polar lipids that can be found in microalgae are monogalactosyl dia — cylglycerols (MGDGs), digalactosyl diacylglycerols (DGDGs), and phosphatidylg — lycerol (PG) [2]. Although these compounds, primarily MGDGs and DGDGs, have been known for more than 40 years, their importance has been recently raised by the description of their different, mainly anti-inflammatory, functional activities [15]. For example, glycol analogs of ceramides and of PG with antithrombotic and anti­inflammatory activities have been reported in cyanobacteria [2] . MGDGs and DGDGs contain a galactose linked to the sn-3 position of the glycerol backbone. These polar lipids are found in the thylakoid membrane of the cells. For instance, several polar lipids have been identified in Spirulina platensis, such as, four MGDGs, three PGs and two sulfoquinovosyl diacylglycerol [57], in Croococcidiopsis sp. [2], in Sargassum thunbergii [81], and Phormidium tenue [124] among others.

On the other hand, most of the alga’s lipid content is made of polyunsaturated fatty acids (PUFAs) which accumulation also relies on environmental factors. For example, it is known that algae accumulate PUFAs when there is decrease in the environmental temperature [80]. In this sense, it has been described that tropical species contain less lipid (<1%) than cold water species (1.6%) [125].

PUFAs are essential nutrients for humans, and must be obtained from food. w-3 and w-6 long chain PUFAs are structural and functional components of cell mem­branes. The w-3 to w-6 ratio is closely matched, a factor that has been found to be important in balanced diet [176]. Likewise, these fatty acids are precursors of eico — sanoids, which exert hormonal and immunological activity. This means w-3 and w-6 should be consumed in a balanced proportion, with the ideal ratio w-6: w-3 ranging from 3:1 to 5:1 [184].

The properties of the long-chain w-3 fatty acids eicosapentaenoic acid (EPA) (w-3 C20 5) and docosahexaenoic acid (DHA) (w-3 C22 6) have been followed with considerable interest in the last few years. In particular, the vascular protective effects of long-chain w-3 fatty acids are well documented [17, 170, 207]. Green algae show interesting levels of alpha linolenic acid (w-3 C18:3). The red and brown algae are particularly rich in fatty acids with 20 carbon atoms: EPA and arachidonic acid (w-6 C20:4).

S. platensis is a microalga belonging to the group of cyanobacteria (or blue-green algae) and is a natural source of DHA, which can account for up to 9.1% of the total fatty acids content [199].

Table 1 [133, 161] presents the typical composition of different fatty acids in algae. As can be seen, in all algae studied except Undaria pinnatifida and Ulva lac — tuca the single most abundant fatty acid was palmitic acid (which in Phorphyra sp. accounted for 63.19% of all fatty acids) while in U. pinnatifida the palmitic acid

content (16.51%) was only exceeded by that of octadecatetraenoic acid (w-3 C18.4) (22.6%), and in U. lactuca the C16.0 content (14.0%) was only exceeded by that of oleic acid (w-9 C181) (27.43%). However, all the seaweeds also contained the essen­tial fatty acids linoleic acid (w-6 C18.2) and linolenic acid and the icosanoid precur­sors, arachidonic acid and EPA. Furthermore, the w-6:w-3 ratio, which the WHO currently recommends should be no higher than 10 in the diet as a whole, was at most 1.49 so that these algae may be used for reduction of w-6:w-3 ratio. Saturated fatty acid contents were higher in the red algae (Palmaria sp. and Porphyra sp.) than in the brown and green algae, and vice versa for relative total unsaturated fatty acid contents. Whereas in the red algae, C20 PUFAs were as a class 8-12 times more abundant than C18 PUFAs, in green algae the opposite occur while in brown algae these two classes of fatty acids were more or less equally abundant. Relative essen­tial fatty acid contents were higher in brown and green algae than in red algae.

Several researchers have reported the fatty acid composition of total lipids of different species of Sargassum. Heiba et al. [47] studied the fatty acids present in four different Sargassum species in the Phaeophyta class that contained heptade — canoic acid (C17.0), eicosanoic acid (C20.0), eicosatrienoic acid (w-3 C20.3), and DHA. On the other hand, Khotimchenko [ 80 ] . working with seven Sargassum species from different parts of the world, determined similar fatty acid compositions in all of them. The site of collection only seemed to affect palmitic acid (C16.0) and C.0 PUFA contents and was connected mainly with water temperature.

Aquatic plants possess conjugated fatty acids (CFA) with carbon chain length varying from 16 to 22, as natural constituents in their lipids; both trienes and tet — raenes occur in aquatic plant lipids. There is not much information available on the literature, only a few reports on the occurrence of these conjugated polyenes in Tydemania expeditionis, Hydrolithon reinboldii [69], Ptilota [205], Acanthophora [8], and Anadyomene stellata [6] have been published. Various enzymes in aquatic plants are thought to be responsible for the formation of conjugated trienes/tet — raenes endogenously. The enzymes responsible for the formation of CFA can be grouped into three main categories of conjugases, oxidases, and isomerases. Hideki and Yuto [58] studied the selective cytotoxicity of eight species of marine algae extracts to several human leukemic cell lines. It has been reported recently that conjugated PUFA, such as conjugated EPA, conjugated AA, and conjugated DHA, prepared by alkali isomerization had profound cytotoxic effects against human can­cer cell lines [102].

Besides fatty acids, unsaponifiable fraction of algae contain carotenoids (see Sect. 4.1), tocopherols (see Sect. 4.5), and sterols. The distribution of major sterol composition in macroalgae has been used for chemotaxonomic classification. Recent biological studies have demonstrated that sterols and sterol derivatives pos­sess biological activities. Currently, phytosterols (C28 and C29 sterols) are playing a key role in nutraceutic and pharmaceutical industries because they are precursors of some bioactive molecules (e. g., ergosterol is a precursor of vitamin D2, also used for the production of cortisone and hormone flavone and has some therapeutic applica­tions to treat hypercholesterolemia). Phytosterols have also been shown to lower total and LDL cholesterol levels in human by inhibiting cholesterol absorption from the intestine [37]. High serum concentrations of total or LDL cholesterol are major risk factors for coronary heart disease, a major cause for morbidity and mortality in developed countries. In addition to their cholesterol lowering properties, phytoster­ols possess anti-inflammatory and anti-atherogenicity activity and may possess anticancer and antioxidative activities [37] .

From a chemotaxonomic point of view, literature data show that major sterols in red algae are C27 compounds and cholesterol occur in substantial amount. It is gen­erally the primary sterol. Desmosterol and 22E-dehydrocholesterol are present in high concentrations and may even be the major sterols in any red algae.

Sterol content in green algae is similar to higher plants, and also contains large amounts of cholesterol. But in green algae, the dominant sterol seems to vary within the order and within the family.

In brown algae, the dominant sterol is fucosterol and cholesterol is present only in small amounts.

Fucosterol content in H. elongata and U. pinnatifida was 1,706 mg/g of dry weight and 1,136 mg/g of dry weight, respectively, as demonstrated by Sanchez — Machado et al. [ 162] . Mean desmosterol content in the red algae ranged from 187 mg/g for Palmaria sp. to 337 mg/g for Porphyra sp. Cholesterol, in general, was present at very low quantities, except in Porphyra sp. that can contain up to 8.6% of the total content of sterols as cholesterol [162].

Sterol content determined in red alga Chondrus crispus showed that the main sterol was cholesterol (>94%), containing smaller amounts of 7-dehydrocholes­terol and stigmasterol and minimum amounts of campesterol, sitosterol, and 22-dehydrocholesterol [188].

According to the investigation carried out by Kapetanovic et al. [77], the sterol fractions of the green alga Codium dichotomum and the brown alga Fucus vir — soides contained practically one sterol each, comprising more than 90% of the total sterols (cholesterol in the former and fucosterol in the latter). The main sterols in the green alga U. lactuca were cholesterol and isofucosterol, while in the brown algae Cystoseira adriatica, the principal sterols were cholesterol and stigmast-

5- en-3 beta-ol, while the characteristic sterol of the brown algae, fucosterol, was found only in low concentration [77]. However, fucosterol was the major sterol present in Cystoseira abies-marina (96.9%), containing low concentration of 24-methylenecholesterol (1.1%), brassicasterol (1.2%), and cholesterol (0.7%) [120].