# Green Macrolgae Biomass Upcycling as a Sustainable Resource for Value-Added Applications
## 3 Results and Discussion
### Antioxidant Activity (DPPH and ABTS Assays)  
Antioxidant activity in _Ulva_ extracts, as assessed by DPPH and ABTS assays, showed solvent-dependent profiles, which is indicative of different classes of active compounds found in the extracts (Table 4). Overall, methanolic and ethanolic extracts showed higher radical scavenging activity in DPPH assays, consistent with their superior flavonoid content. Indeed, water-based extraction demonstrated a low antioxidant capacity (1.66 \(\pm\) 0.001%), while ethanol (16.42 \(\pm\) 1.52%) and methanol (21.24 \(\pm\) 2.35%) extracts showed higher activities (Table 4). The similarity between ETA and MET indicates that both solvents effectively extract antioxidants, possibly due to their moderate polarity, which is compatible with phenolic compounds found in the sample. These findings are consistent with those reported by [PERSON] et al. [1] for _Caulerpa_ sp. extracts, where methanol showed approximately 25% DPPH inhibition, slightly better than ethanolic extracts. Additionally, [PERSON] et al. [76] reported DPPH results ranging from 5% to 20% for methanolic (70%) extracts of _Ulva_ sp. at concentrations four times higher than those used in this study, suggesting a lower antioxidant potential. The results also align with [PERSON] et al. [77], who observed approximately 10% inhibition of DPPH radical scavenging activity in ethanolic extracts for _Caulerpa lentitillera_. In contrast, ABTS activity was more pronounced in ethanolic extracts (9.39 \(\pm\) 1.57). These significant differences (\(p<0.05\)) suggest solvent-specific efficiency, with ethanol showing broader solubility for antioxidant compounds. Different from our results, [PERSON] et al. [43] reported that ethanolic (70%) extracts of _Ulva_ sp. yielded lower antioxidant activity in the ABTS assay, with approximately 4 \(\upmu\)mol TE/g. Similarly, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Samples** & **DPPH (\% of Inhibition)** & **ABTS (\(\upmu\)mol TE/g)** \\ \hline WAT & 1.66 \(\pm\) 0.001  & 7.00 \(\pm\) 1.50  \\ ETA & 16.42 \(\pm\) 1.52  & 9.39 \(\pm\) 1.57  \\ MET & 21.24 \(\pm\) 2.35  & 3.57 \(\pm\) 0.73  \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Antioxidant activity of aqueous (WAT), ethanolic (ETA), and methanolic (MET) extracts derived from oven-dried green algae _Ulva_ sp. (UODP). [PERSON] et al. [78] found ABTS values for water extracts of _Ulva lactuca_ ranging from 0.15 to 8.14 (mg ascorbic acid equivalents/g), depending on extraction conditions. These findings align with the 7.00 \(\upmu\)mol TE/g reported here for water-based extraction. The differences in the results obtained by the DPPH and ABTS methods can be attributed to distinct antioxidant-related mechanisms involved in each assay. The DPPH method is based on electron transfer and estimates the ability of an antioxidant to reduce an oxidant that undergoes a color change during the reduction process. In contrast, the ABTS method involves both electron transfer and hydrogen atom transfer [79, 80]. For all extracts, ABTS values were higher than in DPPH analysis, showing that the ABTS method detects a broader or different set of compounds compared to the DPPH assay [43]. Overall, the TFC results are associated with the antioxidant activity observed in the DPPH assay, following the descending order of methanol > ethanol > water. This suggests that flavonoids play a major role in the antioxidant activity of the extracts. However, the differing trends between TPC and antioxidant activity indicate that compounds other than phenolics, such as non-phenolic pigments, fatty acids, and polysaccharides, might contribute significantly to the antioxidant effects observed in the extracts [71, 81]. Moreover, solubility plays a role in the observed differences between DPPH and ABTS results [82].