Combined effects of tidal regimes and polybrominated diphenyl ether BDE-47 (a persistent organic chemical) on growth, enzymatic and non-enzymatic antioxidants in Kandelia obovata

The present study examined the combined effects of polybrominated diphenyl ethers (PBDEs, represented by 2,2’,4,4’-tetrabromodiphenyl ether, BDE-47) and tidal flooding regimes on the growth, antioxidative enzymes (superoxide dismutase, SOD; peroxidase, POD; catalase, CAT) and non-enzymatic antioxidants (total polyphenols and extractable condensed tannins) in Kandelia obovata (Ko) seedlings. Greenhouse experiments were conducted with one-year-old Ko seedlings, which were exposed to three tidal frequencies (no tidal flooding, every two-day tidal flooding and every day tidal flooding) and five BDE-47 levels (0, 0.1, 1, 5, 10 mg kg⁻¹ dry weight). Results showed that Ko tolerated BDE-47 up to 10 mg kg⁻¹ dry weight, with no significant differences in tissue biomass or chlorophyll contents vs. control. Daily tidal flooding (highest frequency) promoted growth (higher biomass and leaf number) and increased the activity of three key antioxidant enzymes (SOD, POD, CAT) in roots and leaves, relative to the less frequent flooding. Notably, 10 mg kg⁻¹ BDE-47 reduced total polyphenols and extractable condensed tannins in roots and leaves, with no significant differences among tidal regimes. In conclusions, our findings confirmed that the tolerance of Ko to combined BDE-47 and tidal flooding stress was mediated by a clear mechanistic link: Enzymatic antioxidants (SOD, POD, CAT) adapt to tidal-induced oxidative stress, while non-enzymatic antioxidants (total polyphenols and extractable condensed tannins) mitigate BDE-47 damage. These findings offer critical insights into mangrove adaptive strategies under co-occurring stresses, informing conservation and restoration of mangrove ecosystems facing tidal dynamics and pollution.