# Seasonal Monitoring of Volatiles and Antioxidant Activity of Brown Alga _Cladosteplus spongiosus_
## 4 Conclusions  
HS-SPME showed that aliphatic compounds dominated in all samples. Hydrocarbons were more than twice as high in the dry samples (except in May). Aliphatic alcohols were present in large amounts and were more abundant in the fresh samples. Hexanal, heptanal, nonanal, and tridecanal formed the majority of aldehydes. Aliphatic ketones were more abundant in the fresh samples. Benzene derivatives were predominant in the fresh samples of May and August. Mono- and sesquiterpenes (mainly (_E_)-verbenol and _p_-cymen-8-ol) were the most abundant in the May dry sample.  
HD obtained four groups of compounds: aliphatic compounds (e.g., heptadecane, pentadecanal, (_E_)-heptadec-8-ene and (_Z_)-heptadec-3-ene with the highest percentage in August), sesquiterpenes (germacrene D, epi-bicyclosesquiphellandrene, gleenol), diperenes (phytol, pachydictyl A, (_E_)-geranyl geraniol, cembra-4,7,11,15-tetraen-3-ol) and others. Except for the July fresh sample, terpen were the most abundant. The lowest percentage of diterpenes was always in fresh samples and the highest in dry samples.  
Seasonal variations in antioxidant activity of _C. spongiosus_ extracts obtained with MAE and UAE were confirmed. All MAE extracts showed higher activity. The highest peroxyl radical inhibition and reducing activity was observed in the May samples. Therefore, the ethanolic extract from May was analysed by UHPLC-ESI-HRMS. A total of 17 fatty acid derivatives, 9 pigments and derivatives, and 2 steroid derivatives were found. The highest content of pheophorbide \(a\) and fucoxanthin and the presence of other pigment derivatives could be related to the observed antioxidant activity.  
Further research should focus on exploring the seasonal variations of the non-volatile compounds of this alga and the potential use of the extracts obtained.  
Conceptualisation, I.J. and V.S.; methodology, S.R., I.J., M.C. and V.S.; formal analysis, S.R. and M.C.; investigation, S.R., M.C., V.S. and I.J.; resources, I.J. and V.S.; data curation, S.R. and M.C.; writing--original draft preparation, S.R., I.J., V.S. and M.C.; writing--review and editing, S.R., I.J., V.S. and M.C.; supervision, I.J. and V.S.; funding acquisition, I.J. and V.S. All authors have read and agreed to the published version of the manuscript. [PERSON] and [PERSON] have shared co-first authorship.  
**Funding:** This research was funded by the Croatian Government and the European Union (European Regional Development Fund--the Competitiveness and Cohesion Operational Programme--KK.01.1.1.01) through project Bioprospecting of the Adriatic Sea (KK.01.1.1.01.0002) granted to The Scientific Centre of Excellence for Marine Bioprospecting-BioProCro and supported by the PRIMA program under project BioProMedFood (Project ID 1467). The PRIMA programme is supported by the European Union.  
**Institutional Review Board Statement:** Not applicable.  
**Data Availability Statement:** Data are contained within the article.  
**Acknowledgments:** We would like to thank the Croatian Government and the European Union (European Regional Development Fund--the Competitiveness and Cohesion Operational Programme--KK.01.1.1.01) through project Bioprospecting of the Adriatic Sea (KK.01.1.1.01.0002) granted to The Scientific Centre of Excellence for Marine Bioprospecting-BioProCro and to the PRIMA program under the project BioProMedFood (Project ID 1467), which is also supported by the European Union).  
**Conflicts of Interest:** The authors declare no conflict of interest.