Phenolic root exudates enhance Avicennia marina tolerance to cadmium under the mediation of functional bacteria in mangrove sediments

This study was carried out to demonstrate the mechanism of phenolic root exudates affecting microbial-mediated cadmium (Cd) speciation transformation thus enhancing the Avicennia marina tolerance to Cd. A rhizo-box experiment was conducted including eight treatments with four Cd levels (0, 1, 2, and 4 mg Cd kg−1) and two phenol levels (0, 15 mg kg−1). The results showed that the addition of phenols increased the pH, reduced the number of iron-reducing bacteria (IRB) and sulfur-oxidizing bacteria (SOB) in the rhizosphere sediments, meanwhile promoted the transformation of Cd to low activity speciation. Furthermore, the sulfate accumulation and synthesis of flavonoid phenols in plants were also enhanced. The results indicated that phenolic root exudates inhibit functional bacteria-mediated Fe and S cycles and promote the immobilization of Cd in the sediments. In conclusion, the mitigation of Cd phytotoxicity induced by phenolic root exudates enhanced the Cd tolerance of A. marina.