**4. Chemistry of marine algae volatile compounds**

The fragrances of terrestrial plants have aroused human interest since antiquity; they were related to spiritual and civilizational aspects. It is not surprising that the first research work on odorous volatile products was carried out on aromatic plants. Phytochemists have quickly associated the odors emanating from trees and shrubs to terpenes (notably monoterpenes), spices to phenols and derivatives, and fruits and flowers to aldehydes, esters, and ketones. The smell connected with marine flora are much less familiar. Unlike the wide number of terrestrial odoriferous plants, relatively few marine seaweeds possess an attractive odor. Although the natural products chemistry of terrestrial organisms was known before the nineteenth century, the one of the marine derived is more recent, and it has only emerged over the past 75 years. This is due to the complexity to access the marine environment. The marine natural products had become an important subdiscipline of natural products chemistry, which has experienced a particular craze which has led to the isolation and characterization of thousands of secondary metabolites belonging to original chemical skeletons without equivalent in the terrestrial environment.

Historically, volatile oils of terrestrial plants were used in Chinese [26] and Egyptian civilizations [27–29] few centuries ago, whereas the first works on the isolation of volatile products of marine algae were carried out, on the brown alga *Fucus* [30] and the red algae *P. fastigiata* and *P. nigrescens* [31] when the seaweeds are exposed to air, at the beginning of the 1930s, followed later by the Katayama

researches in 1951–1961 [32] and Moore prior to 1966 [33]. The volatile organic compounds in marine algae, as in plants and fungi, released into the seawater, are involved in the chemical communications process; these compounds play an important role as either pheromones or allelochemicals for communication and interaction with the surrounding environment [34, 35]. The species produce the volatile organic compounds in closed relation to their physiology; the algae must adapt to abiotic stresses of their ecosystem [36]. The volatile components of marine algae contain a mixture of chemical classes such as terpenes, hydrocarbons, fatty acids, esters, alcohols, aldehydes, ketones [37–41], C11-hydrocarbons [33, 42], polyphenols and derivatives [43, 44], and halogenated [45] and sulfur compounds [46, 47]. The distinctive ocean smell is due to the presence of terpenes, but particularly, to a fraction of acyclic and cyclic non-isoprenoid C11-hydrocarbons acting as pheromones and playing an important role in the chemical communication [48], it seems to be most abundant in brown algae of the genus *Dictyopteris* [33]. As for terrestrial plants, the monoterpenes identified in algae such as linalool, citral, geraniol, and terpinolene, 1,8-cineole, α-pinene and β-pinene, and eugenol and isoeugenol could be valued in perfumery. While the disagreeable odor is related to amines and halogenated, sulfurous, and other specific compounds [49], the dimethyl sulfide, mainly distributed in Chlorophyta and in some Rhodophyta [50], has a very unpleasant odor molecule. It results from the enzymatic cleavage of dimethyl-2-carboxyethylsulfonium hydroxide, from the green algae species (*E. intestinalis* and *A. centralis*) [51].

## **4.1 Common volatile organic compounds of macroalgae**

### *4.1.1 Hydrocarbons and oxygenated hydrocarbons*

The alkanes and alkenes are common compounds in the majority of volatile fraction and essential oils of marine macroalgae. The chemical composition reveals the presence of the linear and branched saturated hydrocarbons from C7 to C36 [37, 52–54], the unsaturated hydrocarbons from C8 to C19 with the presence of 1 [37, 52–54] to 4 degrees of unsaturation [55] in the volatile fraction obtained by several extraction techniques. We also noted the presence of mono- and di-alcohol of C4–C18 [37, 52–54, 56, 57]. Some short-chain (C6, C9) and middle-chain (C10) aliphatic aldehydes are formed in marine algae from fatty acids (C20), whereas they are formed from C18 in higher plants [58–60]. Also, it has been reported that long-chain aldehydes (C14, C17) of the green alga *U. pertusa* [61, 62] are formed by decomposition of fatty acids through the corresponding 2-hydroperoxy acid; this later are encountered in a variety of marine algae [63, 64].

In addition to aldehydes, the ketone compounds were commonly reported in the aroma composition of algae [65]; the presence of β-ionone and 6-methyl-5 hepten-2-one which are formed via the oxidative cleavage of carotenoids such as lycopene and phyotene was mentioned [66]. β-ionone, present in several essential earth oils, is a powerful odorant for the perfume industry. 6-methyl-5-hepten-2-one, in addition to its pleasant fragrant note, is often used as an intermediary in the synthesis of several monoterpenes highly valorized in perfumery. In addition, other simple ketone (C6▬C19) compounds such as maltol [53], octan-3-one [57], nonacosan-2-one [67], and undeca-1,4-dien-3-one [42] are identified in the volatile fractions of algae. Saturated fatty acids from C3 to C18 and their ester derivatives have also been identified in the chemical composition of volatile algae fractions [37]. Unsaturated fatty acids and their corresponding esters, in particular Eicosa-5,8,11,14-methyltetraenoate and Eicosa-5,8,11,14,17-methyl-pentaenoate [42], are usually found; this is probably related to their implications in biosynthetic processes of other metabolites.

**7**

*Algae Essential Oils: Chemistry, Ecology, and Biological Activities*

algae, the amine compounds were especially found in red algae.

The amine compounds have been described several times in marine algae [68–70]; the small amine molecules such as methyl amine, dimethylamine, ethylamine, and propylamine were found in algae [71]. The volatile amines in algae result from decarboxylation of amino acids [71]. Although present in brown and green

The volatile halogen compounds are rare in terrestrial plants, but quite habitual in marine algae because of the presence of chlorine and bromine ions at a high concentration in seawater. The red algae possess the highest abundance of halogenated organic compounds, which are found as terpenoid, phenols, carbonyl compounds, and fatty acid-derived metabolites [45]. They were produced in marine algae and emitted into the atmosphere; the highest amounts of brominated compounds released were done by *L. saccharina* [72]. Chemical investigations of marine algae have shown the presence of 2-bromophenol, 2,4-dibromophenol, and 2,4,6 tribromophenol in numerous red, green, and brown algae. It has been reported the biosynthesis of bromophenols in *U. lactuca* via the bromoperoxidases in the presence of precursors such as phenol, 4-hydroxybenzoic acid, and 4-hydroxybenzyl alcohol [73]. The bromoperoxidases are involved in the biosynthesis of brominated alkanes, such as CHBr3, CH2Br2, CHClBr2, and others in several marine organisms, among them, the red alga *Asparagopsis* sp. [74]. The biosynthesis of organohalogens has known enormous interest as reported in several literature review [75–77]. As indicated for bromocompounds, the iodoperoxidases are responsible of the production of iodinated compounds in marine algae [78–80]. The chemical investigation of 29 macroalgae species reveals their release of volatile iodocompounds iodoethane, 1-iodopropane, 2-iodopropane, 1-iodo-2-methylpropane, 1-iodobutane, 2-iodobutane, diiodomethane, and chloroiodomethane [81]; it has reported that diiodomethane was the main iodinated compound released by brown macroalgae [82].

Terpenes, or terpenoids, are a large and diverse class of plant secondary metabolites, produced by numerous varieties of plants and algae from isoprene building blocks; they play a major ecological role, most notably in defense against plant-feeding insects and herbivores [82]. However, some terpenoids are involved in primary metabolism, such as stability of cell membranes and photosynthesis. The terpenes display enormous structural diversity, are the main constituents of essential oils of terrestrial plants and seaweeds [83], and are characterized by their pleasant strong odor. The terpenoids are biosynthesized mainly via two pathways, the mevalonate pathway and the MEP pathway. The chemical screening of volatile fraction and/or essential oils of algae reveals the presence of high content of monoterpenes and sesquiterpenes and rarely diterpenes [42]. The most significant acyclic monoterpenes found in algae are myrcene (1), ocimene (2), geranial (3), neral (4), citronellol (5), and geraniol (6) (**Figure 1**). Moreover, the most odoriferous compounds identified

in algae are included in the acyclic group of monoterpenes [84].

Likewise, the most common monocyclic algae volatile oil is 1,8-cineole (8) [84], while α-pinene (9) and β-pinene (10) are the most commonly reported of bicyclic monoterpenes (**Figure 2**) [84, 85]. Sesquiterpenes from marine macroalgae constitute a large group, compared to monoterpenes, of secondary metabolites [86]; some of them are halogenated [87]. Some of the algae sesquiterpenes act as

*DOI: http://dx.doi.org/10.5772/intechopen.91672*

*4.1.2 Amine compounds*

*4.1.3 Halogen compounds*

*4.1.4 Terpenoid compounds*
