## 2 Results and Discussion
### Headspace and Volatile Oil Composition of T. atomaria  
The headspace of _T. atomaria_ is dominated with germacrene D (32.06% (PDMS/DVB fibre); 27.89% (DVB/CAR/PDMS fibre)) and sesquiterpene bicyclic hydrocarbons of cadinane type skeleton epi-bicyclosesquiphellandrene (27.49%; 25.13%) followed by cadina-3,5-diene (2.45%; 3.60%), _trans_-cadina-1(6),4-diene (1.21%; 2.39%) and zonarene (2.25%; 2.53%). An array of other cadinane type structures with minor percentages were identified (Table 1). Another abundant compound with tricyclic sesquiterpene skeleton of cubebane type was \(\beta\)-cubebene (12.80%; 10.74%) with a minor percentage of its isomer \(\alpha\)-cubebene (2.53%; 2.48%). In addition, axane type (bicyclic spiro[4,5]decane) sesquiterpene alcohol gleenol was found among major constituents (9.68%; 11.02%). Several compounds were found as minor constituents exclusively in the headspace (Table 1) such as: dimethyl sulfide, 2-ethylhex-1-yl acetate, bornyl acetate, \(\gamma\)-elemene, cyclosativene, \(\alpha\)-ylangene, and \(\beta\)-patchoulene. The headspace composition of this algae is reported for the first time. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Compound**} & \multirow{2}{*}{**RI**} & \multicolumn{4}{c}{**Area Percentages (\%)**} \\ \cline{3-6}  & & & **HS-SPME** & **HS-SPME** & **HS-SPME** \\  & & & **(PDMS/DVB) \(\pm\) SD** & **(DVB/CAR/PDMS) \(\pm\) SD** & **HD \(\pm\) SD** \\ \hline
1 & Dimethyl sulfide [5] & 523 & 0.06 \(\pm\) 0.01 & 0.02 \(\pm\) 0.01 & - \\
2 & \(p\)-Xylene [5] & 870 & - & - & 0.01 \(\pm\) 0.01 \\
3 & 2-Ethyl-1-hexyl acetate & 1151 & 0.03 \(\pm\) 0.01 & 0.03 \(\pm\) 0.00 & - \\
4 & Albene & 1160 & - & - & 0.01 \(\pm\) 0.00 \\
5 & Bornyl acetate [5] & 1286 & - & 0.01 \(\pm\) 0.00 & - \\
6 & \(\gamma\)-Elemene & 1341 & 0.06 \(\pm\) 0.01 & 0.09 \(\pm\) 0.01 & - \\
7 & \(\alpha\)-Cubebene [5] & 1353 & 2.53 \(\pm\) 0.06 & 2.48 \(\pm\) 0.05 & 0.16 \(\pm\) 0.01 \\
8 & Cyclosativene [5] & 1368 & 0.01 \(\pm\) 0.00 & 0.01 \(\pm\) 0.00 & - \\
9 & \(\alpha\)-Ylangene & 1374 & 0.02 \(\pm\) 0.00 & 0.06 \(\pm\) 0.01 & - \\
10 & \(\alpha\)-Copaene [5] & 1379 & 0.04 \(\pm\) 0.01 & 0.12 \(\pm\) 0.02 & 0.02 \(\pm\) 0.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Headspace and volatile oil composition of _T. atomaria_ (RI from the literature used for identifying the compounds). Germancrene D (22.24%), epi-bicyclosesquiphellandrene (20.83%) and gleenol (15.35%) were dominant compounds in the hydrodistillate as well. Smaller percentages of other cadinane types as well as selinane type sesquiterpenes were found (Table 1). However, in the distillate, several higher molecular compounds were found (that were not present in the headspace) such as diterpenes (pachydictyl A, cembra-4,7,11,15-tetraen-3-ol and isochemrol), higher aliphatic compounds (e.g., tridecanal or pentadecanal), sesquiterpenes (_trans_-\(\alpha\)-bergamotene, \(\alpha\)-amorphene, junenol, di-epi-1,10-cubenol, or \(\delta\)-cadinol), trisnorsesquiterpene albene and \(p\)-xylene. The major VOCs found in the headspace and oil are known to exhibit different biological activities. Germacrene D is involved in plant-insect interaction, acting as a pheromone on receptor neurones [25] and is also an important deterrent and insecticidal agent against different parasites such as mosquitos, aphids and ticks [26, 27]. Germacrene D could be considered responsible for the cytotoxic activity of  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.