Individual Presentation: Unlocking Red Seaweed Phenolics with Cold Plasma–Treated Water and Enhancing Antioxidant Activity

Red seaweeds are increasingly recognized as promising sources of bioactive phenolic compounds. However, a major proportion of their phenolics remains locked in insoluble-bound form, limiting extraction efficiency, biological utility, and industrial application. This study investigates cold plasma–treated water (CPTW) as a green and chemical-free extraction medium to liberate bound phenolics from Mazzaella japonica and enhance antioxidant functionality. Free (F), esterified (E), and insoluble-bound (IB) phenolic fractions were extracted following CPTW exposure for 10, 20, and 30 minutes. Individual phenolics were identified via LC-QTOF-MS/MS, while bioactivity was evaluated using DPPH, ABTS, metal chelation, and Caco-2 intracellular reactive oxygen species (ROS) inhibition assays.CPTW treatment significantly enhanced the total phenolic content of the free fraction, peaking at 20 minutes (+24.2%; 240.4 → 298.6 mg gallic acid equivalents (GAE)/100 g). In contrast, both esterified and insoluble-bound phenolics steadily declined with longer plasma exposure, indicating cleavage and mobilization of bound compounds. IB phenolics decreased by ~20% at 20 minutes and ~26% at 30 minutes, supporting a mechanism of bond disruption and redistribution. Importantly, at 20 minutes, the total phenolic pool (F + E + IB) increased significantly relative to the control, while the IB/F ratio decreased by ~36% (0.694 → 0.445), demonstrating conversion of bound phenolics into bioavailable free forms rather than degradation. A similar trend was observed by HPLC quantification.A total of 27 phenolic compounds, including phenolic acids, flavonoids, and phlorotannins, were identified for the first time in M. japonica, with CPTW-20 extracts showing the strongest antioxidant performance across assays. The cellular ROS model confirmed concentration-dependent antioxidant activity with >90% cell viability (MTT assay), indicating safety. These results position CPTW as an eco-friendly and scalable strategy to maximize phenolic recovery and functional performance in marine biomass, with strong implications for sustainable ingredient development, nutraceuticals, and food applications.