## 3 Results and Discussion
### Change of Total Phlorotannin Contents through UVA and Heat Exposure  
In Table 2, among the extract and three solvent fractions, the chloroform fraction showed the highest TPC (59.42 \(\pm\) 2.25 PGE/g dw), followed by the ethyl acetate fraction (20.188 \(\pm\) 1.07), hexane fraction (11.30 \(\pm\) 1.20), and ethanol extract (5.05 \(\pm\) 0.24). Despite a tenfold difference in TPC levels at the beginning of the treatment, one-hour exposure to UVA increased the TPC levels in the ethanol extract and three fractions in a similar proportion (235-255%). In addition, more prolonged UVA exposure did not further alter the TPC levels. A notable increase in the TPC in response to high temperatures was also observed in the ethanol extract, hexane fraction, and ethyl acetate fraction, but not the chloroform fraction (Table 2). In addition, thermal treatment at 75 \({}^{\circ}\)C increased the TPC in the ethanol extract and hexane fraction at temperatures higher than at 55 \({}^{\circ}\)C.  
Contrary to UVA-induced TPC elevation, the increase in the TPC upon thermal treatment occurred in a solvent-dependent and time-dependent manner. Polyphenols are synthesized in nature as a response to harmful environmental stimuli such as UV irradiation. Short-term UV-induced biosynthesis of kelp phenolic compounds in various cultured seaweed has been reported [24; 25; 26]. However, the changes in the TPCs in the extracts upon UV radiation have not been reported. In the current study, we discovered a 2.5-fold increase in the TPCs in the ethanol extract and solvent fractions upon UVA irradiation, indicating that such a change is likely to be an ex vitro chemical reaction independent from the coexisting compounds. To the best of our knowledge, this is the first time such an increase has been reported. The unique structure of phlorotannins, the polyphenol discovered only in brown seaweed, may be critical for such a UVA-induced increase. Phlorotannins, oligomers of phloroglucinol (1,3,5-trihydroxybenzene), have been identified as the predominant phenolic compound in various brown seaweed species [27; 28]. The isomerization and monomerization of phlorotannins could cause the increased TPC which was observed. Oxidative reactions could produce a phlorotannin isomer by the structural recombination of molecules, most likely through intra- or intermolecular bonding [29]. Although the different TPCs between different phlorotannin isomers are unclear, polymer  
Figure 4: The concentration of fucoxanthin in the (**A**) ethanol extract, (**B**) hexane fraction, (**C**) chloroform fraction, and (**D**) ethyl acetate fraction at 75 \({}^{\circ}\)C.  
ized phlorotannins showed lower TPC values than the monomeric unit phloroglucinols under the Folin-Ciocalteu method [30]. The Folin-Ciocalteu method, widely used in phenolic compound studies, including the current study, was designed to measure the total concentration of phenolic hydroxyl groups in the extract and fractions based on an oxidation-reduction reaction [31]. Different phenolic compounds show different reducing powers under such text. Therefore, the changes in the TPC suggested a structure change of the phenolic compounds in our extract and fractions induced by UVA. When the ethanol extract and solvent fractions were heated at 55 and 75 \({}^{\circ}\)C, the TPCs significantly increased, except for in the chloroform fraction. The increased temperature during the extraction process enhanced the recovery of phenolic compounds from kelp [32; 33]. However, the aforementioned increase in the TPC may have been caused by the increased solubility of phenolic compounds, since a high temperature promotes the release of bound phenolic compounds from cellular components [34; 35]. On the contrary, the increased TPCs in the extraction and fractions in our study suggest that certain chemical reactions catalyzed by high temperatures could increase the phenolic compound activity.