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

应用电感耦合等离子体质谱(ICP-Ms)与高效液相色谱(HPLC)分别对福建漳江口红树林湿地不同土壤结构(砂质与泥质)根际与非根际沉积物中重金属(Cu,Pb,Cd,Zn)含量,以及生长于砂质与泥质滩涂上的红树植物秋茄(Kandelia candel(L)Druce))幼苗根系分泌物中的低分子量有机酸进行测定。在室内模拟秋茄根系分泌的低分子量有机酸,作为重金属提取剂提取沉积物中可溶解态与碳酸盐结合态重金属,并与欧盟标准物质局(BCR)连续提取法相比较,探讨红树根系分泌的低分子量有机酸对红树林沉积物重金属生物有效性的影响。研究结果表明:漳江口红树林泥质沉积物中重金属含量大于砂质沉积物,根际沉积物大于非根际沉积物。两样地沉积物中重金属的赋存形态主要以铁锰氧化物结合态为主,根际沉积物中可交换态与碳酸盐结合态重金属含量均大于非根际沉积物。秋茄根系分泌低分子量有机酸为甲酸,丁酸,苹果酸,柠檬酸,乳酸。不同土壤结构对秋茄根系分泌的苹果酸,柠檬酸,乳酸有显著影响(P<0.05)。以低分子量有机酸作为提取剂对沉积物中可溶解态与碳酸盐结合态重金属的提取率表现为:柠檬酸>混合酸>苹果酸>乳酸>乙酸,低分子量有机酸对红树林沉积物重金属的生物有效性有促进作用。

Exudation of low-molecular-weight-organic acids by Kandelia candel (L) Druce roots and implication on heavy metal bioavailability in mangrove sediments

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.