## 3 Isolation and Biodiversity of Monoterpenes from Macroalgae
### Monoterpenes Isolated from Different Macroalgae Species  
Monoterpenes are divided into cyclic and acyclic groups, with cyclic monoterenes being mono- or bicyclic. Regular acyclic monoterepenes are head-tail linked (the branched end of one isoprene unit binds to the unbranched end of another isoprene unit). Figure 5 shows the most significant representatives of acyclic monoterepenes found in the algae: Myrcene (**9**), ocimene (**10**), gerani (**11**), neral (**12**), citronellol (**13**), and geraniol (**14**). The most pleasant odorous compounds found in the algae [53] are included in acyclic group of monoterepenes. Monocyclic monoterepenes are usually derived from methyl-isopropyl cyclohexane by its dehydrogenation [54]. 1,8-Cineole (**15**) is the most common monocyclic monoterepene found in the algae [53; 55], while \(\alpha\)-pinene (**16**) and \(\beta\)-pinene (**17**) are the most common representatives of the bicyclic monoterepenes (Figure 6) [53; 56]. Even though monoterepenes isolated from macroalgae have been known for centuries, new emerging methods of their isolation and analysis allowed further research of these compounds with the discovery of many new monoterepenes produced by macroalgae (Figure 7, Table 1). Four new monoterepenes based on 1-(2-chlorovinyl)-2,4,5-trichloro-1,5-dimethylcyclohexane skeleton, (**18-21**), and four known cyclic monoterepenes, (**22-25**), were isolated and characterized from _P. cartilagineum_ collected along the central coast of Chile [57]. [PERSON] et al. [58] collected this alga also from Chilean coast and found that isolated tetrahydrofuran monoterepenes, furo  
Figure 4: Monoterpenes isolated with SC-CO\({}_{2}\) extraction from _Plocamium cartilagineum_. Figure 5: Acyclic monoterepenes. Figure 6: Cyclic monoterepenes. chlorobromo vinyl functional group which is unusual among the macroalgae monoterpenes. It is known that monoterpenes isolated from the algae of _Plocamium_ genus possess terminal bromovinylic or chlorovinylic system, but the findings of dihalovinyl system enabled the discovery of regiochemistry of new polyhalogenated monoterpenes. Hence, _P. cartilagineum_ collected at the same location was investigated for its minor constituents to better understand the formation of oxane ring-containing monoterpenes. [PERSON] et al. [59] described two new tetrahydrofuran halogenated monoterpenes (**32**, **33**), as well as new acyclic polyhalogenated monoterpene (**34**). _P. cartilagineum_ collected from the Spanish coast contained two new polyhalogenated acyclic monoterpenes, (**35**) and (**36**) [60]. [PERSON], [PERSON], and [PERSON] [61] confirmed that the location influenced the secondary metabolites of _P. cartilagineum_, while in 1999, [PERSON], [PERSON], and [PERSON] [16] observed this pattern in _Plocamium hamatum_ [PERSON] collected from different regions in Australia and reported the compounds (**37**-**47**). In 1979, [PERSON] and [PERSON] [62] studied _P. cartilagineum_ collected in Antarctica and found several monoterpenes as minor constituents, while [PERSON] et al. [63] found that these monoterpenes were present at higher concentrations in the same alga species collected also in Antarctica, but during different seasons. The compounds (**48**-**51**) were discovered as new, undescribed halogenated monoterpenes from _P. cartilagineum_, named anverenes B-E.  
A recent study by [PERSON] et al. [64] stated that red algae of the families _Plocamiaceae_ and _Rhizophyllidaceae_ developed pathways to oxidize chloride and bromide ions from seawater and incorporate them into monoterpene structures. This resulted in a wide variety of different cyclic and acyclic monoterpenes with multiple halogen atoms, mostly chloride and bromine [27]. Although _P. cartilagineum_ was the most studied algae of the genus _Plocamium_, the authors observed that other algae from the same genus also produce halogenated monoterpenes. [PERSON] et al.