Iron-sulfur distribution and its environmental significance in three typical areas of western Lake Taihu

Abstract

以太湖西岸聚藻区、清淤区和开阔水域3种典型区域为对象，对沉积物中硫铁的分布进行研究.结果显示，各区域沉积物中铁还原均比硫还原活跃，硫铁氧化还原菌的分布表明硫铁循环主要集中于0~15 cm表层沉积物.开阔水域未发现明显硫离子（dS^2-）释放与酸性可挥发性硫化物（AVS）沉积，硫还原较弱；二价铁离子（dFe²⁺）平均浓度为dS^2-的5.2倍，说明沉积物仍以铁还原为主导.清淤区硫酸盐还原菌（SRB）相对丰度与dS^2-含量均为所有区域中最低，且未见明显dFe²⁺释放，表明清淤抑制了硫铁还原的进行.聚藻区表层沉积物SRB相对丰度达到2.7%，且出现dS^2-和AVS的显著升高，说明沉积物中发生着强烈的微生物硫还原.在表层沉积物也出现了明显的dFe²⁺释放，反映出强烈的铁还原，但沉积物中铁还原菌（FeRB）的平均丰度仅为0.6%，这一数值与其他区域相似，显然无法解释聚藻区与其他区域巨大的dFe²⁺差异，因此微生物铁还原并不是铁还原的主要途径.考虑到硫铁化合物的沉积与Fe（Ⅲ）氧化物的消耗一致，沉积物中的铁还原是由ΣS^2-诱导的化学铁还原主导.dFe²⁺平均浓度为dS^2-的4.8倍，监测培养沉积物的dS^2-、dFe²⁺变化所得的铁还原速率是硫还原速率的7.4倍，说明铁还原仍比硫还原活跃.但由于AVS的沉积对ΣS^2-的消耗，真实硫还原速率应高于测量值.;Geochemical characteristic of sulfur and iron was investigated in three types of sediments including algae accumulated sediment, dredged sediment and sediment away from lake shore. It was found that iron reduction dominated in all sediments rather than sulfate reduction. Both reduction concentrated in 0-15 cm surface sediments according to the distribution of microorganisms of iron and sulfur species. Accumulation of dS^2- and acid volatile sulfide(AVS)was not observed in sediment away from lake shore, indicating that sulfate reduction is minor. Iron reduction was more active than sulfate reduction as the mean concentration of dFe²⁺ was 5.2 folds of dS^2-. Sulfate reducing bacteria (SRB) abundance and dS^2- content in dredged sediments were the lowest and dFe²⁺ release was not found, suggesting the depressed iron and sulfate reduction by dredging. SRB abundance in algae accumulated sediment reached to 2.7% and intensive dS^2- release and AVS deposition were found, indicating the strong sulfate reduction. Active iron reduction was also found with high dFe²⁺ concentration in algae accumulated sediment. However, the mean abundance of iron reducing bacteria (FeRB) was only 0.6%, which was similar to other sediments and was obviously not able to explain the huge difference of dFe²⁺ concentrations. Thus, microbial iron reduction was not the main pathway of iron reduction. Considering the corresponding dissolution of Fe(Ⅲ) oxides and deposition of iron sulfides, sulfide-mediated chemical iron reduction was the dominating pathway. The mean concentration of dFe²⁺ was 4.8 folds of dS^2-, and iron reduction rate was 7.4 folds of sulfate reduction rate by monitoring dS^2- and dFe²⁺ variations, suggesting iron reduction was more active than sulfate reduction. However, as AVS generation could consume dS^2-, the real rate of sulfate reduction was higher than the measured rate. Transition of iron and sulfur cycling would cause various negative effects and even extreme cases including dead zones in marine and black bloom in freshwater lakes. Although similar phenomenon was only observed in algae accumulated sediment, concern should still be raised.