This chapter focuses on the essential oils and volatile fractions of seaweed. It includes an introduction to the essentials and volatile fractions and the main chemical classes found. This part is completed by a presentation of the fundamental aspects of biodiversity and the chemodiversity of the marine environment followed by the taxonomy and systematics of marine macroalgae. The heart of this chapter concerns the chemistry of volatile products extracted from marine algae. It reports the specificities of the marine natural products chemistry in comparison to that of terrestrial organisms. The description of volatile compounds in seaweed is divided into two parts, the first reports the common compounds identified in main volatile fractions and the second cover the specific volatile components. These include C11 hydrocarbons, sulfur compounds, and halogenated hydrocarbons. These latter are playing a very important role in communication and chemical defense. The last part includes aspects of chemical ecology and biological activities of volatile products.
- essential oils
- marine algae
- C11 hydrocarbons
- sulfur compounds
- halogenated sesquiterpenes
- chemical ecology
- biological activities
The origin of the distillation methods is an invention attributed to the Arab alchemists and to the Persian scientist Avicenna (980–1037) with the establishment of the steam distillation process. Avicenna invented a setup to prepare essential oils and aromatic waters. Essential oils, sometimes called quintessence, are a very complex mixture of volatile compounds produced by the secondary metabolism in various plant organs (flowers, fruits, seeds, leaves, etc.) and algae. According to ISO and AFNOR standards, essential oils are defined as volatile composition obtained from raw materials by steam distillation and/or by cold expression from citrus peels (known as essences) . The definition of an essential oil excludes other volatile fractions obtained by steam distillation and/or hydrodistillation from the crude extract resulted from solvent extraction, supercritical fluid extraction, solvent- and water-free microwave extraction, ultrasound-accelerated solvent extraction, solid-phase microextraction, and headspace extraction. The chemical composition of essential oils and volatile fraction could be quite similar. Moreover, it should be pointed out the clear difference between the physical and chemical properties of essential oils and fixed or fatty oils. The fixed oils contain mainly triglycerides, esters composed of three saturated fatty acids linked to glycerol, characterized by high boiling and low volatility. The chemical composition of essential oils is principally composed of terpenes derived from the mevalonate and methylerythritol pathways . Monoterpenes and sesquiterpenes are commonly the main contributor group of compounds identified in several essential oils . Moreover, some essential oils contain other chemical classes, such as phenols (derived from shikimic acid pathway); the saturated and unsaturated fatty acids, acting as biosynthetic precursors; alkanes; and, more rarely, nitrogen and sulfur derivatives . The essential oils play an important role in the allelopathic interaction of plants. They are involved in defense and signaling processes  and attraction of pollinating insects . They constitute an important raw material source for the pharmaceutical, food, cosmetics, and perfume industries . The essential oils of different plants exhibit a broad spectrum of biological activities. They show antibacterial activities attributed, in some cases, to the presence of phenolic compounds . The literature reports also the excellent antioxidant , anti-inflammatory , and cancer chemoprotective activities .
2. Marine biodiversity and chemodiversity
More than 70% of the Earth’s surface are oceans and seas. It is not surprising to affirm that the marine environment is characterized by an important biodiversity in comparison to terrestrial organisms. In 2010, 230,000 marine species were listed . Consequently, with the increase of biological space (biodiversity), more novel metabolites (high chemodiversity), involved in ecological interactions, are produced in order to ensure easy adaptation of the species [13, 14]. Furthermore, the chemodiversity of the marine ecosystem has no equivalent in terrestrial environment. The large groups of the sea organisms, such as red algae and soft corals, are known to produce a great variety of quite unique secondary metabolites, such as highly halogenated terpenes, definitely due to the high halogen concentration of the sea water, and acetogenins from
3. Systematics and taxonomy of macroalgae
It was the French botanist Joseph Pitton de Tournefort (1656–1708) who grouped the species into genera and then the Swedish naturalist Carl von Linné (1707–1778), founder of systematics (or taxonomy), who classified the organisms into increasingly large groups: species, genera, families, orders, classes, phylum (or phyla), and kingdoms. Algae, according to Feldmann and Chadefaud [20, 21], are classified into six branches differentiated by the nature of the pigments, the nature and situation of carbohydrate reserves, and the presence or absence, number, and arrangement of flagella:
Pyrrophycophyta: unicellular marine or freshwater algae
Euglenophycophyta: unicellular freshwater algae rich in organic matter
Chrysophycophyta: most are single-celled; freshwater and sea water
Chlorophycophyta: green algae; single or multi-cell; marine, freshwater, and terrestrial environments
Phaeophycophyta: brown algae; always multicellular and almost exclusively marine
Rhodophycophyta: red algae; mainly multicellular and mostly marine
There are about 2000 species (in 265 genera) of brown algae , and less than 1% are known from freshwaters (3–7 genera) . The brown color is due to Fucoxanthin (carotenoid pigment) and in some species to the presence of tannins (phenolic compounds).
There are estimated to be at least 600 genera with 10,000 species within the green algae  recognized inhabiting mostly in the water’s surface of the calmer seas. They are characterized by the presence of chloroplasts with two envelope membranes, stacked thylakoids, and chlorophyll a and b. In their fundamental biochemistry (photosynthetic pigments, storage polysaccharides, etc.), the Chlorophyta resemble the higher plants .
They are primarily marine in distribution sometimes inhabiting the deep water, with less than 3% (150 species from 20 genera) of the over 6500–10,000 species occurring in truly freshwater habitats . The red algae are characterized by eukaryotic cells, with the complete absence of flagellar structures, food reserves of starch, presence of phycobilins, chloroplasts without stacked thylakoids, and no external endoplasmic reticulum.
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  and Egyptian civilizations [27, 28, 29] few centuries ago, whereas the first works on the isolation of volatile products of marine algae were carried out, on the brown alga
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, 53, 54], the unsaturated hydrocarbons from C8 to C19 with the presence of 1 [37, 52, 53, 54] to 4 degrees of unsaturation  in the volatile fraction obtained by several extraction techniques. We also noted the presence of mono- and di-alcohol of C4–C18 [37, 52, 53, 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, 59, 60]. Also, it has been reported that long-chain aldehydes (C14, C17) of the green alga
In addition to aldehydes, the ketone compounds were commonly reported in the aroma composition of algae ; 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 . 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 , octan-3-one , nonacosan-2-one , and undeca-1,4-dien-3-one  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 . Unsaturated fatty acids and their corresponding esters, in particular Eicosa-5,8,11,14-methyltetraenoate and Eicosa-5,8,11,14,17-methyl-pentaenoate , are usually found; this is probably related to their implications in biosynthetic processes of other metabolites.
4.1.2 Amine compounds
The amine compounds have been described several times in marine algae [68, 69, 70]; the small amine molecules such as methyl amine, dimethylamine, ethylamine, and propylamine were found in algae . The volatile amines in algae result from decarboxylation of amino acids . Although present in brown and green algae, the amine compounds were especially found in red algae.
4.1.3 Halogen compounds
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 . They were produced in marine algae and emitted into the atmosphere; the highest amounts of brominated compounds released were done by
4.1.4 Terpenoid compounds
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 . 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 , 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 . 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 .
Likewise, the most common monocyclic algae volatile oil is 1,8-cineole (8) , 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 ; some of them are halogenated . Some of the algae sesquiterpenes act as semiochemicals, chemical defense agents, and/or pheromones. They may be acyclic, cyclic, or bicyclic, including several original structures. Among all marine macroalgae, the genus
The most common sesquiterpenes reported in marine algae (10–53) are grouped in Table 1 and illustrated in Figure 3. The only diterpene and triterpene described as volatile compounds are, respectively, phytol and squalene. Phytol is a degradation product of chlorophyll and the precursor of vitamin E. The squalene is via the epoxy squalene, the biosynthetic precursors of triterpenes and steroids.
4.2 Specific volatile compounds of macroalgae
4.2.1 Odoriferous C11 hydrocarbons from brown algae (
The brown algae produce a variety of volatile derivatives whose chemical nature and biological function are different from those of red algae. They are hydrocarbons with 11 carbon atoms without halogens which can be classified according to their chemical structure into four groups : (a) derivatives of cyclopropane, (b) derivatives of cyclopentene, (c) derivatives of cycloheptadiene, and (d) acyclic olefins. The only volatile hydrocarbon with eight carbon atoms identified in brown algae is fucoserratene. These metabolites, which are known in all the species of
They are involved in the reproduction process of the alga; they are sex pheromones. To date, it has been revealed that these algal pheromones are involved at least in three well-defined ecological interactions : (i) synchronization of the mating of male and female cells by the controlled release of male spermatozoids, (ii) enhancement of the mating efficiency by attraction, and (iii) chemical defense of the plant due to the presence of high amounts of pheromones within and release from the thalli into the environment. Furthermore, the relationship between structures of pheromones and the taxonomic classifications of algae are still not established. Until now, a series of 12 (54–65) hydrocarbons and epoxides (Figure 4) have been characterized, and more than 50 stereoisomers are known within the pheromone bouquets of more than 100 different species of brown algae [48, 96, 97, 98, 99].
Moreover, the presence of C11 hydrocarbons is not only limited to marine brown algae. The same compounds have been reported in cultures of diatoms , the volatile fraction released during blooms of microalgae in freshwater lakes  and, inquisitively, in higher plants [102, 103]. Table 2 reports the pheromones described in Figure 4, the algae from which they are derived, as well as their attraction or release activities. In comparison to the number of brown algae species, the chemodiversity of pheromones is relatively limited, so, the semiochemical activity of the same molecule is noted in more than one species. Female gametes secrete a mixture of products, not just one pheromone and depending on species; released pheromones are either optically pure or enantiomeric mixtures.
However, it has been verified that the biological activity is associated with a single constituent which may not be the major product. These by-products sometimes play a role of modulator of response of the gametes, and in general, they do not have a determined biological function .
4.2.2 Sulfur compounds in the genus Dictyopteris
The organic sulfur compounds are widespread in terrestrial and marine plants . Due to the relatively high sulfate concentration in seawater, and the particularly high sulfide concentration in anoxic environments, it was expected that many sulfides would occur in the marine environment . They are reported in few taxa and act as chemical defenses against herbivores . As part of this single group, some
4.2.3 Halogenated terpenes from red algae (Rhodophyta)
As noted previously, the halogenated compounds are common in the marine environment. They are formed among diverse species such as bacteria, sponges, molluscs, algae, and several marine worms. Among all marine algae, the Rhodophyta class possesses a privileged biosynthetic pathway for organohalogen compounds. A huge number of organohalogens have been isolated from most genera of Rhodophyta [108, 109]. The genus
The same species collected along the central coast of Chile  conduct to the isolation of eight monoterpenes (88–95), four of which are based on the 1-(2-chlororovinyl)-2,4,5-trichloro-1,5-dimethylcyclohexane skeleton (Figure 7). As in the genus
The first brominated sesquiterpene (Figure 8) ketone spirolaurenone (96), chamigrane skeleton, was described in the essential oil of
The volatile compounds play an important role in the inter- and intraspecies chemical communication in marine algae. They act as pheromones  or allelochemicals, chemical defenses against herbivores [132, 133], and inhibition of bacterial and fungal biofilms . The genus
In green algae, the volatile compounds, such as (Z)-8- heptadecane, act also as allochemicals . In the genus
4.2.5 Biological activities
There are several reports of secondary metabolites, among them are numerous volatile compounds, derived from macroalgae which exhibit a broad range of biological activities such as antibiotics [40, 143].
The essential oil of
The cytotoxicity is the most common activity observed for halogenated organic compounds isolated from the family Rhodomelaceae. A large number of these compounds were shown to be cytotoxic to a wide range of cancer cell lines .
Among many of the halogenated sesquiterpenes evaluated for their in vitro cytotoxic effects against HeLa and HEP-2 cancer cell lines, and against nontumoral VERO cells, during both lag- and log-phase cell growth , elatol (103) turned out the most active compound with IC50 values of 4.1 and 1.3 μM to HeLa, 2.4 and 2.0 μM to HEP-2, and 2.3 and 25.0 μM to VERO cells, in lag- and log-phase, respectively . Further studies were carried on the evaluation of the cytotoxicity against several tumor cell lines of chamigrane  and Laurane- and Cuparane-type sesquiterpenes and were found to display a wide range of potency levels [151, 152]. Other activities of halosesquiterpenes such as antibacterial activity , antifungal activity , and antiviral activity  were investigated and conducted to promising results.
Essential oils from terrestrial plants have been known for a very long time. They have been applied in different domain, particularly in aromatherapy. Essential oils from seaweed are much more recent. The fragrant note of marine origin is becoming more and more interesting among perfumers, the species of the genus
This work is supported by the General Directorate of Scientific Research and Technological Development (DGRSDT)—Algeria.