## 2 Results and Discussion
### Possible Biosynthetic Origin of the Major Identified VOCs  
Sesquiterpenes, dominant in the _T. atomaria_ headspace and volatile oil, are C\({}_{15}\)-compounds containing the assembly of three isoprenoid units. The large number of sesquiterpenoid carbon skeletons arises from the common precursor, farnesyl pyrophosphate (FPP), by various modes of cyclizations usually followed by skeletal rearrangement (Figure 1). Selinane and cadinane are two main types of sesquiterpenes reported in brown algae [36]. According to the currently accepted hypotheses, in addition to farnesyl pyrophosphate (FPP), nervyl pyrophosphate (NPP) may be a precursor in sesquiterpenes formation (Figure 1). It can be assumed [37] that in the cyclisation process (after pyrophosphate (PP) loss) germacrenyl cations A and B are formed, and after hydride migration, cations C and D are formed (Figure 1). For the biosynthesis of cadinene-type sesquiterpenes, two alternative pathways were suggested [38]. The primarily formed cation A may be transformed to cation C and after deprotonated to germacrene D (1b). The subsequent change from _cis_- structure to _trans_- 1b is crucial for the formation of cadinane type sesquiterpenes (with \(Z\) double bond in the ring) followed by protonation and rearrangement to cation D. As an alternative, NPP may serve as a substrate for cadinene-type sesquiterpene biosynthesis. In this case, the cyclization to the cadinanes would proceed via cations B and D. Moreover, it can be assumed that germacrene D and germacrenyl cation D are the biosynthetic precursors of the major isolated sesquiterpenes from _T. atomaria_. Major sesquiterpenes of this alga with cadinane skeleton that could be derived from germacrene D were (Figure 1): cadina-1(6),4-diene, \(\delta\)-cadinene, cadina-3,5-diene, 4-epi-bicyclosesquiphellandrene; ylangene; cubebol, \(\beta\)-cubebene, \(\alpha\)-cubebene, zonarene, and gleenol. Bicyclogermacrene is probably derived from germacrenyl cation C.  
Dimethyl sulfide (DMS), a major compound of the _P. padina_ headspace, results from an enzymatic decomposition of dimethyl-\(\beta\)-propiothetin [33], a metabolite of methionine that is fairly widespread in marine plants. Formed dimethylsulfoniopropionate (DMSP), a tertiary sulfonium compound, is the precursor of DMS. Recently, the algal enzyme responsible for the formation of DMS from DMSP was found and characterised in alga _Emiliania luxleyi_[39]. In general, the overall mechanism of enzymatic lipid degradation is identical in terrestrial plants [40] and algae [41]. The enzyme cascade is initiated by activated phospholipase, followed by lipoxygenase and hydroperoxide lyase. However, the particular enzymes are highly species- and sometimes even strain-specific [41], and this can explain the large biodiversity of lipid degradation VOCs (e.g., carbonyl compounds, alcohols, and hydrocarbons). Marine algae contain unsaturated fatty acids, and they can produce C\({}_{18}\), C\({}_{20}\) and C\({}_{22}\) fatty acid hydroperoxides. Following the general concept of lipid peroxidation, and subsequent oxidative cleavage of the carbon skeleton, the biosynthesis of C\({}_{8}\)-hydrocarbons from _P. paonica_ could start from the polyunsaturated fatty acid substrate that could be activated [42] either by 9-lipoxygenase or by 12-lipoxygenase producing 9- or 12-hydroperoxides that further cleave oxidatively to produce C\({}_{8}\)-compounds. Octanal could originate from \(\omega\)9 mono-unsaturated fatty acids (MUFAs) and also from \(\omega\)6 poly-unsaturated fatty acids (PUFAs) such as linoleic acid [43]. Figure 1: Possible biosynthetic correlations [37; 38] among the main identified compounds in _T.