## 2 Materials and Methods
### Chemical Characterization  
GC-TOF-MS Analysis  
A carboxen/divinylbenzene/polydimethylsiloxane (CAR/DVB/PDMS) fiber for autosamplers--1 cm, 50/30 \(\upmu\)m film thickness (d\({}_{I}\)) (Supelco, St. Louis, MO, USA)--was used for HS-SPME extractions. Fiber blanks were run periodically to ensure the absence of contaminants and/or carryover. The thall from the seaweeds were removed from three different packages (the same ones for which the composition of headspace gases was measured), combined and ground using liquid nitrogen. HS-SPME extraction was performed according to the following procedure: 3 g of each fresh seaweed sample were introduced in a 20.0 mL headspace vial and sealed with a PTFE/silicone septum and a magnetic screw cap (Supelco, USA). The vial was equilibrated for 10 min at 50 \({}^{\circ}\)C and then extracted for 30 min at the same temperature. Thermal desorption of the analytes was carried out by exposing the fiber in the GC injection port at 260 \({}^{\circ}\)C for 3 min in splitless mode.  
The analyses were performed on a GC-TOF-MS system consisting of a GC 8890 System (Agilent Technologies, Santa Clara, CA, USA) with a BenchTOF-Select detector (Markes International, UK). An automatic sampler injector--CTC Analysis autosampler PAL-System (SepSolve Analytical, Peterborough, Cambs, England, UK)--was used and the data were acquired and analyzed with TOF-DS 4.1 (Markes International, Bridgend, MGM, Wales, UK). Chromatographic separation was achieved on a DB-WAX (60 m \(\times\) 0.25 mm I.D. \(\times\) 0.25 \(\upmu\)m d\({}_{I}\)) polyethylene glycol capillary column (Agilent Technologies, Santa Clara, CA, USA). The oven temperature program began at 50 \({}^{\circ}\)C, held for 2.5 min, raised at 3 \({}^{\circ}\)C\(\cdot\)min\({}^{-1}\) up to 90 \({}^{\circ}\)C, at 6 \({}^{\circ}\)C\(\cdot\)min\({}^{-1}\) up to 140 \({}^{\circ}\)C, 2 \({}^{\circ}\)C\(\cdot\)min\({}^{-1}\) up to 180 \({}^{\circ}\)C and finally 20 \({}^{\circ}\)C\(\cdot\)min\({}^{-1}\) up to 240 \({}^{\circ}\)C and held for 20 min to ensure column cleanliness [22]. The MS transfer line and source temperatures were set at 250 \({}^{\circ}\)C. Helium was used as the carrier gas.  
Spectra were matched using the NIST MS Search Program, version 2020. To determine the retention times and characteristic mass fragments, electron impact ionization (EI) at 70 eV was used and mass spectra of the analytes were recorded at full scan, from 30 to 400 Da. The linear retention index (LRI) values were calculated through the analysis of an \(n\)-alkane standard solution (C8-C20) (Supelco, St. Louis, MO, USA), using the same chromatographic conditions [53]. The volatile compounds were first identified by matching mass spectra with the spectra of reference compounds in the NIST mass spectral library, also taking into consideration structure and molecular weight, and by comparing the calculated LRIs with those described in the literature. Additionally, 37 analytical standards (Sigma-Aldrich/Merck, Darmstadt, HE, Germany) were diluted to 100 ppm in hexane PA and 1 \(\upmu\)L was injected, using a 1:20 split rate and the same ramp program, to make a more accurate identification of the main volatile organic compounds (VOCs) present in the seaweeds. The relative amount of each compound was calculated as the percent ratio of the respective peak area relative to the total peak area of the chromatogram and expressed as a percentage (%).