Implication of tannin changes in two mangrove plants, Kandelia obovata and Bruguiera gymnorhiza under PBDE contamination

Mangrove plants are known to be tolerant to many types of pollutants, including heavy metals and persistent organic pollutants (POPs). Polybrominated diphenyl ethers (PBDEs), due to their strong hydrophobicity, persistence and toxicity, are a group of ubiquitous POPs in mangrove ecosystems. The tolerance of mangrove plants could be due to their antioxidant systems, including enzymatic and non-enzymatic antioxidants, to counterbalance the negative effect of reactive oxygen species (ROS) caused by these environmental stresses. Tannins, a group of phenolic compounds with a large number of phenol sub-structures and high redox potentials, are well-known of their antioxidant activity. The response of tannins in mangrove plants to PBDE contamination and their antioxidant potential for the tolerance are still unknown. The present study therefore aims to reveal the changes of tannins, both in concentration and chemical structure, in two mangrove plants, namely Kandelia obovata (Ko) and Bruguiera gymnorhiza (Bg) under PBDE stress, as well as evaluate the antioxidant potential of mangrove tannins. 
An environmentally relevant contamination level of mixed PBDEs in mangrove sediment was prepared by adding a commercial penta-BDE mixture (TBDE-71X) and BDE-209 at a ratio of 1:4 (20 mg Kg^-1). The sediments were planted with either Ko or Bg, and the effects of PBDEs on the growth and changes of tannins in these plants were investigated over twelve months. The mixture of TBDE-71X and BDE-209 showed significant adverse effects on the growth of Ko during the first three months, with significant decreases in dry biomass of leaf, stem and root; however, no significant difference in growth was found between Ko planted in PBDE contaminated sediments and control (without PBDEs) at the end of twelve months. Tannins in the leaf, stem and root of PBDE-treated Ko decreased significantly to counter-balance the toxic oxidative stress of PBDEs during the experiment, especially under the 12-months long-term treatment. Compared with Ko, changes of biomass and tannins in Bg were less significant, suggesting that Bg was a more tolerance mangrove species than Ko. The structural information of tannins obtained by HPLC-ESI-MS and MALDI-TOF-MS techniques revealed that procyanidins (PC) and prodelphinidins (PD) were the major types of condensed tannins for Ko and Bg, respectively. The numbers of hydroxyl groups (OH) on the B-ring of phenol sub-structures were two and three for PC from Ko and PD from Bg, respectively. Condensed tannins of Bg with more OH exhibited higher antioxidant activity than those of Ko, thus Bg was more tolerant to PBDE stress. This study firstly showed the direct evidence that both the concentration-level and structure-level of tannins play important roles in the tolerance of mangrove plants to PBDE contamination.