Exudation of low-molecular-weight-organic acids by Kandelia candel (L) Druce roots and implication on heavy metal bioavailability in mangrove sediments
秋茄(Kandelia candel (L))根系分泌低分子量有机酸及其对重金属生物有效性的影响

Low molecular weight organic acids (LMWOAs) such as malate, citrate and oxalate, have been proposed to be involved in many processes operating in the rhizosphere, including nutrient acquisition and metal detoxification, alleviation of anaerobic stress in roots, mineral weathering and pathogen attraction. Therefore the aims of this study was to investigate the Cu, Pb, Zn and Cd concentrations in mangrove sediment and to identify a variety of LMWOAs exuded by Kandelia candel (L) Druce at two sites with different sediment texture (sandy and muddy) in Zhangjiang estuary mangrove swamp to examine whether specific differences in sediment texture influenced plant exudation. The identified organic acids were consequencely used as extractants of metals from sediments and the results were compared with sequential extraction of Community Bureau of Reference (BCR) in parallel. The results showed that muddy sediment accumulated more heavy metal than sandy sediment. And heavy metal concentrations in rhizosediment (sediments in contact with the plant roots and rhizomes) were higher than those in sediment. A predominant portion (36%-71%) of Cu, Zn, Pb and Cd was strongly associated with Fe/Mn (hydr) oxides fractions at both sides. Except for Pb (4%-5%), a important part of Cu (16%-21%), Zn (15%-19%), and Cd (16%-20%) were associated with exchangeable and carbonate-bound fractions. Copper Cd and Zn would use to be the main potential risk in mangrove sediments. Formic, butyric, malic, citric and lactic acids were found and quantified in the root exudates, with the last two acids being predominant. Total amount of LMWOAs at the sandy site was higher than that at the muddy site. Furthermore, different sediment texture significant influenced the amounts of malic, citric and lactic acids (P<0.05). The percentage of exchangeable and carbonate-bound fraction heavy metal extracted by LMWOAs following the pattern: Citric acid> Mixture acids> Malic acids> Lactic acids> Acetic acids. This indicate that LMWOAs released by K. candel acts as complexing agents to the heavy metals and therefore organic exudates should be accounted for when dealing with mangrove swamp sediment quality.