*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 [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

**Figure 1.**

*Common acyclic monoterpenes of algae.*

**Figure 2.** *Most representative monocyclic and bicyclic monoterpenes of algae.*

semiochemicals, chemical defense agents, and/or pheromones. They may be acyclic, cyclic, or bicyclic, including several original structures. Among all marine macroalgae, the genus *Laurencia* (red algae) is the most potent source of sesquiterpenes.

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 (Phaeophyta)*

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 [94]: (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 *Phaeophyceae*, are not specific to an order or a family. They have been isolated from diverse groups of brown algae (e.g., the *Zonaria*, *Desmarestia*, *Dictyota*, *Ectocarpus*, *Laminaria*, and *Fucus*); it appears to be most abundant in brown algae of the genus *Dictyopteris* [95].

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 [96]: (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

**9**

**Table 1.**

*Most common sesquiterpenes of macroalgae [88–93].*

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

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, 96–99].

*Abbreviation: D. m., Dictyopteris membranacea; D. d., Dictyota dichotoma; B. f., Bangia fuscopurpurea; C. m., Cystoseira mediterranea; C. g., Callithamnion granulatum; C. e., Cystoseira elegans; P. d., Polysiphonia denudata; L. p., Laurencia papillosa; L. c., Laurencia coronopus; C. v., Cladophora vagabunda; D. div., Dictyota divaricata; Z. m., Zostera marina; P. t., Pyropia tenera; U. p., Ulva pertusa; D. p., Dictyota prolifera; H. p., Halopteris filicina.*

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

#### *Algae Essential Oils: Chemistry, Ecology, and Biological Activities DOI: http://dx.doi.org/10.5772/intechopen.91672*

*Essential Oils - Bioactive Compounds, New Perspectives and Applications*

semiochemicals, chemical defense agents, and/or pheromones. They may be acyclic, cyclic, or bicyclic, including several original structures. Among all marine macroalgae, the genus *Laurencia* (red algae) is the most potent source of sesquiterpenes. 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

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 [94]: (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 *Phaeophyceae*, are not specific to an order or a family. They have been isolated from diverse groups of brown algae (e.g., the *Zonaria*, *Desmarestia*, *Dictyota*, *Ectocarpus*, *Laminaria*, and *Fucus*); it appears to be most abundant in brown algae of the genus

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 [96]: (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

epoxy squalene, the biosynthetic precursors of triterpenes and steroids.

*4.2.1 Odoriferous C11 hydrocarbons from brown algae (Phaeophyta)*

**4.2 Specific volatile compounds of macroalgae**

*Most representative monocyclic and bicyclic monoterpenes of algae.*

**8**

*Dictyopteris* [95].

**Figure 1.**

**Figure 2.**

*Common acyclic monoterpenes of algae.*


*Abbreviation: D. m., Dictyopteris membranacea; D. d., Dictyota dichotoma; B. f., Bangia fuscopurpurea; C. m., Cystoseira mediterranea; C. g., Callithamnion granulatum; C. e., Cystoseira elegans; P. d., Polysiphonia denudata; L. p., Laurencia papillosa; L. c., Laurencia coronopus; C. v., Cladophora vagabunda; D. div., Dictyota divaricata; Z. m., Zostera marina; P. t., Pyropia tenera; U. p., Ulva pertusa; D. p., Dictyota prolifera; H. p., Halopteris filicina.*

#### **Table 1.**

*Most common sesquiterpenes of macroalgae [88–93].*

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, 96–99].

**Figure 3.** *Common sesquiterpenes described in volatile oil of marine algae.*

Moreover, the presence of C11 hydrocarbons is not only limited to marine brown algae. The same compounds have been reported in cultures of diatoms [100], the volatile fraction released during blooms of microalgae in freshwater lakes [101] and,

**11**

**Table 2.**

*C11 and C8 pheromone activities from marine brown algae.*

**Figure 4.**

*Pheromones of brown algae.*

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

mones are either optically pure or enantiomeric mixtures.

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 phero-

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

*Algae Essential Oils: Chemistry, Ecology, and Biological Activities DOI: http://dx.doi.org/10.5772/intechopen.91672*

**Figure 4.** *Pheromones of brown algae.*

*Essential Oils - Bioactive Compounds, New Perspectives and Applications*

Moreover, the presence of C11 hydrocarbons is not only limited to marine brown algae. The same compounds have been reported in cultures of diatoms [100], the volatile fraction released during blooms of microalgae in freshwater lakes [101] and,

*Common sesquiterpenes described in volatile oil of marine algae.*

**10**

**Figure 3.**

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.


**Table 2.** *C11 and C8 pheromone activities from marine brown algae.*

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 [94].
