## 4 Bioactivity of Monoterpenes in Macroalgae  
Scientists have shown interest for the bioactivity of monoterenes isolated exclusively from red algae, with genus _Ploaminum_ and _Porieria_ as dominating. El Gamal [7] represented red algae as the main source of monoterenes with cytotoxic activity. As demonstrated in Table 2, anticancer activity of monoterenes against different tumor cell lines, including esophageal [74], brain, renal, colon [75], and cervical cancer [76], was the main bioactivity of novel monoterenes as well as those that were known before. Among all of isolated monoterenes, halomon is the best-studied halogenated monoterene isolated from _Porieria hornemannii_ whose structure is presented in Figure 8. It was firstly isolated and elucidated by [PERSON] et al. [75]. Halomon (**102**) was tested for cytotoxicity against human brain, colon, and renal tumor cell lines and the results indicated high cytotoxicity against mentioned cell lines, while lower activity was observed against leukemia and melanoma cell lines. Interestingly, _P. hornemannii_ collected from different locations showed the absence of halomon, but new monocyclic monoterene (**103**) with lower cytotoxic activity was observed. _Ploaminum hamatum_ collected from six different locations showed significant difference within the content of secondary metabolites which led to different cytotoxic results [16]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Macrolague Species** & **Isolated Monoterpenes** & **References** \\ \hline  & **Anticancer activity** & \\ \hline _Porieria hornemannii_ & **102, 103** & [75] \\ \hline _Ploamin corallorhiza,_ & **54, 55, 58, 60, 104–109** & [77] \\ _Ploaminum cornutum_ & **54, 55, 58, 60, 104–109** & [77] \\ \hline _Porieria hornemannii_ & **110–112** & [78] \\ \hline _Ploamin subrii,_ & **5, 113–118** & [74] \\ _Ploaminum cornutum_ & **52–57** & [17] \\ \hline _Ploaminum carallorhiza_ & **119** & [79] \\ \hline _Porieria hornemannii_ & **102, 120–124** & [80] \\ \hline _Ploaminum carallagineum_ & **30, 125, 126, 127–132** & [76] \\ \hline _Ploamin maxillosum_ & **62, 63** & [64] \\ \hline \multicolumn{3}{c}{**Antiplasmodial (antimalarial) activity**} \\ \hline _Ploamin cornutum_ & 53, 58, 133–139 & [81] \\ \hline \multicolumn{3}{c}{**Insecticidal activity**} \\ \hline _Ploamin curallagineum,_ & **26, 27, 30, 31, 140, 141** & [82] \\ _Pantineuroa picoamoides_ & **142, 143** & [83] \\ \hline _Ploamin tamelfairiae_ & **18–25** & [16] \\ \hline _Ploamin curallagineum_ & **144, 145** & [84] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Isolated monoterpenes from different macroalgal species and their bioactivity. Figure 8: Chemical structure of halomon. The structural diversity among halogenated monoterepenes (Figure 9) has been shown as the important factor for bioactivity intensity. [PERSON] et al. (2017) isolated monoterepene compounds (**54**, **55**, **58**, **60**, **104-109**) and observed that all of them contained from two to five halogen moieties, specifically Cl and/or Br. They selected the molecules whose terpene backbone was identical, but the halogen substitution varied between the compounds. The results indicated that a higher number of halogen atoms resulted in higher anticancer activity against breast cancer cells. Five halogen atoms were found among the most toxic compounds, while non-toxic compounds contained three and four halogens. Hence, the most active against breast cancer cells was the compound which contained only Cl atoms, without Br. When halomon-related halogenated monoterepenes (**110-112**) were isolated and tested for cytotoxicity, the results showed that the halogen at position C-6 was essential for the certain activity ([PERSON] _et al._, 2017). Due to relatively simple structures of the algal compounds, they can be used for novel chemotherapeutics. Even though their activity is lower than commercially used chemotherapeutics, their structures, along with their differential toxicity, promise novel mechanisms of action against cancer cell lines.