Previous studies indicate that DMS exhibits antioxidant function in marine algae (Sunda et al., 2002). We proposed that MsrA may increase the antioxidant activity of DMS. By luminol chemiluminescence assays in a Fenton reaction system, it was shown that H2O2‐induced luminescence was not inhibited by 0.1 mm DMS (97 ± 6% of control, shown in Fig. S3A). Neither MsrA nor nonactive MsrA alone (1 μm) had a perceptible effect on the luminescence intensity (Fig. S3B). Meanwhile, cotreated with 0.1 mm DMS and MsrA (1 μm), but not nonactive MsrA, remarkably reduced the luminescence signal intensity to 52 ± 5% (P < 0.01, Fig. S3C). In electron paramagnetic resonance (EPR) assay, hydroxyl radical was induced by Fenton reaction and trapped by DMPO to form a stable spin adduct DMPO‐OH. The signal intensity of DMPO‐OH adducts was not changed by MsrA or nonactive MsrA alone (1 μm, Fig. S4). However, cotreatment with 0.1 mm DMS and MsrA (1 μm) decreased the DMPO‐OH signals to 73 ± 1% when compared to the group cotreated with 0.1 mm DMS and nonactive MsrA (Fig. 1C), which strongly suggested that MsrA increased DMS‐mediated reduction in OH˙. Then, we monitored the effect of MsrA on the reaction between ROS and DMS in vitro by detecting the reaction product. Increased amount of DMSO could be detected by GC‐MS, which supported by the catalytic activity of MsrA (Fig. 1D). MsrA facilitated the formation of DMSO from 3.3 ± 1.6% to 46.4 ± 4.3% (P < 0.01).