Could the rhizoplane biofilm of wetland plants lead to rhizospheric heavy metal precipitation and iron-sulfur cycle termination?
Could the rhizoplane biofilm of wetland plants lead to rhizospheric heavy metal precipitation and iron-sulfur cycle termination?
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湿地植物根面生物膜能否导致根际重金属沉淀和铁硫循环终止？
DOI：
10.1007/s11368-019-02343-1
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发表时间：
2019-11
影响因子：
3.6
通讯作者：
Wang Shizhong
中科院分区：
文献类型：
--
作者：
Li Yuanyuan;Feng Wenling;Chi Haochun;Huang Yunxi;Ruan Dishen;Chao Yuanqing;Qiu Rongliang;Wang Shizhong
目的湿地植物通过径向失氧(ROL)形成的铁斑(IP)已被公认为是根际重金属固定的主要原因。然而，生活在潜在的根面生物膜基质中的微生物的相同贡献还没有得到全面的阐明。本文提出了湿地植物根际生物膜的概念模型，总结了湿地植物根际生物膜中重金属沉淀和铁硫循环终止的可能途径。材料和方法在介绍IP和微生物在重金属污染湿地修复中的作用后，阐述了湿地植物根际细菌的分布和不同的金属形态。基于对根际微生物区系和IP的研究，结合ROL的性质，我们提出了一种具有特殊结构的根面生物膜的存在，这种生物膜可能通过产生重金属沉淀物(金属硫化物)来终止根际的铁硫循环。结果和讨论ROL导致氧从根面(根面)向土壤的扩散梯度递减，从而形成了由好氧内层和厌氧外层组成的非常规根面生物膜。因此，好氧细菌，如铁氧化细菌(IOB)和硫氧化细菌(SOB)，以及厌氧细菌，如铁还原细菌(IRB)和硫酸盐还原细菌(SRB)，分别有利于根面生物膜的内层和外层。在内层，硫化亚铁被IOB和SOB氧化成Fe~(3+)和SO_4~(2+)−。然后，Fe3+与氧结合成铁(氢)氧化物，聚集成铁膜屏障，用于重金属的隔离和O2的扩散。扩散到外层的过量SO42−被硫酸盐还原为S2−，形成足够的金属硫化物沉淀物，一方面固定H+释放的重金属离子，另一方面起到阻止硫化亚铁与O2接触的屏障作用。从而阻止了硫化亚铁的进一步氧化。结论这种根面生物膜与IP共存有助于根际元素的循环。进一步研究和论证其组成、结构和功能，将有助于我们更好地解读洪涝矿区湿地植物的生存策略和生物修复潜力，如热带、亚热带雨量充沛地区的尾矿库。
PurposeIron plaque (IP) formation via radial oxygen loss (ROL) of wetland plant has been predominantly recognized as the reason for heavy metal sequestration in the rhizosphere. However, the same contribution of the microbes living in a potential rhizoplane biofilm matrix has not been comprehensively elucidated. In this review, we proposed a conceptional model of the wetland plant rhizoplane biofilm and summarized the possible pathways therein for heavy metal precipitation and iron-sulfur cycle termination.Materials and methodsAfter an introduction of the effects of IP and microbes on the phytoremediation of heavy metal-contaminated wetland, the distribution of rhizospheric bacteria and different metal speciations resulted from wetland plant ROL were demonstrated. Based on the studies of microflora in the rhizosphere and IP, coupled with the ROL nature, we proposed an existence of rhizoplane biofilm with a special structure that could contribute to the rhizospheric iron-sulfur cycle termination by the production of heavy metal precipitates (metal sulfides).Results and discussionThe ROL leads to the diffusion of oxygen with a decreasing gradient from the root surface (rhizoplane) to the bulk soil, allowing the formation of an unconventional rhizoplane biofilm comprising an aerobic inner layer and an anaerobic outer layer. Thus, aerobic bacteria, e.g., iron-oxidizing bacteria (IOB) and sulfur-oxidizing bacteria (SOB), as well as anaerobic bacteria, e.g., iron-reducing bacteria (IRB) and sulfate-reducing bacteria (SRB), are favored in the inner layer and outer layer of the rhizoplane biofilm, respectively. In the inner layer, ferrous sulfide is oxidized by IOB and SOB to Fe3+and SO42−. Fe3+is thereafter bound with oxygen into iron (hydro)oxides, aggregating into a barrier of iron plaque for heavy metal sequestration and O2diffusion. Excessive SO42−diffused to the outer layer is reduced to S2−by SRB, forming sufficient metal sulfide precipitates that on one hand immobilize those heavy metal ions released by H+, and on the other hand serve as a barrier for preventing the contact of ferrous sulfide from O2. Hence, further oxidization of ferrous sulfide is terminated.ConclusionsThis rhizoplane biofilm co-existing with IP contributes to the rhizosphere element circulation. Further investigation and demonstration of its composition, structure, and function will help us better interpret the survival strategy and bioremediation potential of wetland plants in flooded mining areas, such as mine tailing ponds in tropical and sub-tropical regions with abundant rainfall.
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