# Characteristic Volatile Composition of Seven Seaweeds from the Yellow Sea of China
## 2 Results and Discussion
### Characteristic VOC Molecules and Potential Application in Chemotaxonomy  
#### 2.2.1 PCA on Total VOCs Variables  
Principal component analysis (PCA) on data from fiber DVB/CAR/PDMS exhibited that, except for _D. dichotomous_ and _E. prolifera_, the other five seaweeds were clustered together (Figure 2a). The PCA results for fiber PDMS show that _D. dichotomous_ was distinct from the other six seaweeds (Figure 2b).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**NO.**} & \multirow{2}{*}{**Compound**} & **Molecular** & **Compound** & \multirow{2}{*}{**RI \({}^{*}\)**} & \multicolumn{2}{c}{**Area Percentage (\%)**} & \multirow{2}{*}{**Identification**} \\ \cline{4-7}  & & **Formula** & & & & **EP \({}^{b}\)** & **UL \({}^{b}\)** \\ \hline
**33** & Dihydroactinidiolide & C\({}_{11}\)H\({}_{8}\)O\({}_{2}\) & C\({}_{11}\)- & & & \\  & & Norisoprenoid & 1525 & 0.00 & 0.29 \({}^{*}\) & MS, RI \\
**34** & Tetradecanal \({}^{5}\) & C\({}_{14}\)H\({}_{23}\)O & Aldehyde & 1608 & 3.34 \({}^{*}\), 6.32 \({}^{*}\) & 0.79 \({}^{*}\) & MS, RI \\
**35** & _E_-Heptadecene & C\({}_{2}\)H\({}_{4}\) & Allee & 1681 & 2.72 \({}^{*}\), 5.32 \({}^{*}\), 19.70 \({}^{*}\) & MS, RI \\
**36** & _(Z_)-11-Pentadecan \({}^{\dagger}\) & C\({}_{5}\)H\({}_{23}\)O & Aldehyde & 1689 & 0.40 \({}^{*}\), 0.81 \({}^{*}\) & 0.77 \({}^{*}\) & MS \\
**37** & Pentadecanal & C\({}_{5}\)H\({}_{23}\)O & Aldehyde & 1711 & 8.49 \({}^{*}\), 24.40 \({}^{*}\) & 3.83 \({}^{*}\), 7.36 \({}^{*}\) & MS, RI \\
**38** & _(Z_)-11-Hexadecan & C\({}_{56}\)H\({}_{23}\)O & Aldehyde & 1793 & 0.59 \({}^{*}\), 1.18 \({}^{*}\) & 0.00 & MS, RI \\
**39** & _(Z_,_Z_)-6,9-Pentadecan-1-ol \({}^{\dagger}\) & C\({}_{56}\)H\({}_{23}\)O & Unsaturated & 1889 & 0.83 \({}^{*}\) & 0.54 \({}^{*}\), 0.93 \({}^{*}\) & MS \\
**40** & _(Z_,_Z_)-7,10,13- & Hexadecririental & C\({}_{56}\)H\({}_{23}\)O & Unsaturated & 1890 & 0.46 \({}^{*}\), 6.13 \({}^{*}\) & 2.83 \({}^{*}\), 4.96 \({}^{*}\) & MS \\  & & & aldehyde & & & & \\  & & Total Identified (\%) & & & & 65.07 \({}^{*}\), 56.43 \({}^{*}\) & 69.79 \({}^{*}\), 69.10 \({}^{*}\) & \\ \hline \hline \end{tabular} \({}^{\dagger}\) tentatively identified. \({}^{5}\) identified by mass spectra (MS) and retention index (RI) compared with authentic standard. \({}^{a}\) RI: retention index relative to C7–C30 alkanes. \({}^{b}\) EP—Enteromorpha prolifera; UL—Ulva lactuca. \({}^{*}\) data from fiber DVB/CAR/PDMS \({}^{a}\) data from fiber PDMS.  
\end{table}
Table 3: _Cont.__D. dichotomous_ is an outlier in PCA, being very different from the other six seaweeds. The molecules that contributed most to this difference were revealed in the loading plot and are reported in the Supplementary Materials (Figure S1 and Table S1). As expected, the main differences were regarding sesquiterpenes and diterpenes.  
Based on the revised biogenetic scheme that is widely cited, _Dictyota_ diterpenes can be divided into three groups (I-III), resulting from the first formal cyclization of the geranylgeraniol precursor, which makes them potentially useful as chemotaxonomic and phylogenetic markers [28]. Unfortunately, the number of diterpenes available in _Dictyota_ VOCs is very limited, making it difficult for them to be used in chemotaxonomy.  
On the other hand, _Dictyota_ contains large numbers of sesquiterpenes, distinguishing them from other seaweeds in this study. It is still not clear whether volatile sesquiterpenes can be used as chemotaxonomic markers for species identification of genus _Dictyota_, but it is worth exploring this through the VOC approach.