Low-molecular-weight organic acids exuded by Mangrove ( Kandelia candel (L.) Druce) roots and their effect on cadmium species change in the rhizosphere

Abstract

Cadmium (Cd) accumulation has been found in large areas of estuaries due to emissions from municipal waste incinerators, car exhausts, residues from metalliferous mining and the smelting industry, and the use of sludge or urban composts, pesticides and fertilizers. In these areas, mangroves have been observed to possess a tolerance to high levels of Cd and it is hypothesized that low-molecular-weight organic acids (LMWOAs) produced at the soil–root interface (rhizosphere) may play an important role in the availability of Cd to these plants. Changes in both LMWOAs and Cd bioavailability, directly or indirectly related to the Cd stress were studied in the laboratory. A rhizobox technique was used for 6 months under growth in air-conditioned greenhouse with natural illumination and the relative humidity of 85%, the temperature ranging from 26 to 32 °C, in increasing Cd concentration stress conditions (0, 5, 10, 20, 30, 40 and 50 ppm). Six-month-old Kandelia candel (L.) Druce seedlings which grown in the rhizoboxes were selected to examine their root exudates. The results showed that monocarboxylic acids (formic, acetic, lactic, butyric and propionic acids), and di- and tricarbonxylic acids (maleic, fumaric, citric and l-tartaric acids) were found in root exudates. Citric, lactic and acetic acids being dominant took up 76.85–97.87% of the total LMWOAs in root exudations. Fumaric acid was only found where mangroves were growing on 20 ppm Cd. Root exudates reduced pH by 0.2–0.5 pH units in the rhizosphere compare to the bulk soil. The proportion of exchangeable Cd and Cd bound to carbonate had a positive correlation to total LMWOAs in the rhizosphere soil. Root exudates induced changes in soil Cd species under control conditions, consisting of lower exchangeable Cd compared with increasing stress. Results indicate that the measurement of LMWOAs may be included as early biomarkers in a plant bioassay to assess the phytotoxicity of Cd-contaminated soils on mangrove plants.