# Seasonal Variability of Volatilome from _Dictyota dichotoma_
## 1 Introduction  
The genus _Dictyota_, the richest genus of the Dictyotaceae family, contains more than 40 species, but in the Mediterranean in general, only the species _D. dichotoma_ and _D. linearis_ are found [1].  
Brown macroalgae of the Dictyotaceae family produce a significant number of secondary metabolites, especially diterpenes, with three types of carbon skeletons: xenicanes, dolabellanes and extended sesquiterpenes including unique cyclic diterpenes [1, 2, 3, 4]. Most of these diterpenes exhibit significant biological activities, such as antiviral, cytotoxic and chemical defensive activities [5]. Different secondary metabolites of the brown alga _D. dichotoma_ were isolated and their chemical structures were elucidated including diterpenes isopachydictyloal [1], pachydictylols B and C [6], dictyols A-D, dictyl B acetate [7], dictylO-2-\(\beta\)-acetate [8], dictylO E [6] and others [5]. Sulfated polysaccharides, fructose, mannose, xylose and uronic acids and other fucidans were found in _D. dichotoma_[9]. Fatty acids were also investigated in _D. dichotoma_[10]. The result of _D. dichotoma_ phytochemical screening showed the presence of alkaloids, terpenoids, steroids, tannins, flavonoids, phenols, coumarins, quinones and glycosides [11]. The extracts of _D. dichotoma_ from Croatia were analysed for total phenolics, flavonoids and tannins [12].  
In the course of our investigations towards the bioprospecting of the Adriatic Sea and chemical profiling of secondary metabolites from different marine sources, we investigated the volatile organic compounds (VOCs)--volatile chemical composition of the brown alga _D. dichotomous_ collected from the Adriatic Sea. There are limited or scarce data on _D. dichotomous_ chemical composition from the Adriatic Sea [12; 13; 14]. In our previous research [14], sesquiterpenes were found among _D. dichotomous_ headspace VOCs, predominantly germacrene D followed by other cadinenyl (abundant), muurolenyl and amorphenyl structures. The headspace VOCs in _D. dichotomous_ from the Yellow Sea of China were also characterised as sesquiterpenes (predominantly germacrene D) and diterpenes [15]. However, there is a gap in the research on less volatile compounds of _D. dichotomous_ as well as about VOC seasonal variability that could be also important for the algal chemotaxonomy or biological activity. Therefore, the goals of our study are to: (a) isolate the headspace VOCs from fresh _D. dichotomous_ (HS-FrDd) by headspace solid-phase microextraction (HS-SPME) and by hydrodistillation (HD; first-time research); (b) analyse VOCs by gas chromatography and mass spectrometry (GC-MS); (c) investigate the impact of air-drying (first time report) to chemical profiles of dried _D. dichotomous_ obtained by HS-SPME (HS-DrDd) and HD (HD-DrDd); (d) compare the chemical profiles obtained by HS-SPME and HD within the seasons; (e) determine the variability of the VOCs composition from May to September from fresh and air-dried samples, and to elaborate the data by principal component analysis (PCA); (f) provide new insights for the algal chemotaxonomy regarding moderate and less volatile compounds and their seasonal changes, or the impact of air-drying.