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      Characterization of the enhancement of zero valent iron on microbial azo reduction

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          Abstract

          Background

          The microbial method for the treatment of azo dye is promising, but the reduction of azo dye is the rate-limiting step. Zero valent iron (Fe 0) can enhance microbial azo reduction, but the interactions between microbes and Fe 0 and the potential mechanisms of enhancement remain unclear. Here, Shewanella decolorationis S12, a typical azo-reducing bacterium, was used to characterize the enhancement of Fe 0 on microbial decolorization.

          Results

          The results indicated that anaerobic iron corrosion was a key inorganic chemical process for the enhancement of Fe 0 on microbial azo reduction, in which OH −, H 2, and Fe 2+ were produced. Once Fe 0 was added to the microbial azo reduction system, the proper pH for microbial azo reduction was maintained by OH −, and H 2 served as the favored electron donor for azo respiration. Subsequently, the bacterial biomass yield and viability significantly increased. Following the corrosion of Fe 0, nanometer-scale Fe precipitates were adsorbed onto cell surfaces and even accumulated inside cells as observed by transmission electron microscope energy dispersive spectroscopy (TEM-EDS).

          Conclusions

          A conceptual model for Fe 0-assisted azo dye reduction by strain S12 was established to explain the interactions between microbes and Fe 0 and the potential mechanisms of enhancement. This model indicates that the enhancement of microbial azo reduction in the presence of Fe 0 is mainly due to the stimulation of microbial growth and activity by supplementation with elemental iron and H 2 as an additional electron donor. This study has expanded our knowledge of the enhancement of microbial azo reduction by Fe 0 and laid a foundation for the development of Fe 0-microbial integrated azo dye wastewater treatment technology.

          Electronic supplementary material

          The online version of this article (doi:10.1186/s12866-015-0419-3) contains supplementary material, which is available to authorized users.

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          Bacterial iron homeostasis.

          Simon C. Andrews, Andrea Robinson, Francisco Rodríguez-Quiñones (2003)
          Iron is essential to virtually all organisms, but poses problems of toxicity and poor solubility. Bacteria have evolved various mechanisms to counter the problems imposed by their iron dependence, allowing them to achieve effective iron homeostasis under a range of iron regimes. Highly efficient iron acquisition systems are used to scavenge iron from the environment under iron-restricted conditions. In many cases, this involves the secretion and internalisation of extracellular ferric chelators called siderophores. Ferrous iron can also be directly imported by the G protein-like transporter, FeoB. For pathogens, host-iron complexes (transferrin, lactoferrin, haem, haemoglobin) are directly used as iron sources. Bacterial iron storage proteins (ferritin, bacterioferritin) provide intracellular iron reserves for use when external supplies are restricted, and iron detoxification proteins (Dps) are employed to protect the chromosome from iron-induced free radical damage. There is evidence that bacteria control their iron requirements in response to iron availability by down-regulating the expression of iron proteins during iron-restricted growth. And finally, the expression of the iron homeostatic machinery is subject to iron-dependent global control ensuring that iron acquisition, storage and consumption are geared to iron availability and that intracellular levels of free iron do not reach toxic levels.
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            Metals, minerals and microbes: geomicrobiology and bioremediation.

            G M Gadd (2010)
            Microbes play key geoactive roles in the biosphere, particularly in the areas of element biotransformations and biogeochemical cycling, metal and mineral transformations, decomposition, bioweathering, and soil and sediment formation. All kinds of microbes, including prokaryotes and eukaryotes and their symbiotic associations with each other and 'higher organisms', can contribute actively to geological phenomena, and central to many such geomicrobial processes are transformations of metals and minerals. Microbes have a variety of properties that can effect changes in metal speciation, toxicity and mobility, as well as mineral formation or mineral dissolution or deterioration. Such mechanisms are important components of natural biogeochemical cycles for metals as well as associated elements in biomass, soil, rocks and minerals, e.g. sulfur and phosphorus, and metalloids, actinides and metal radionuclides. Apart from being important in natural biosphere processes, metal and mineral transformations can have beneficial or detrimental consequences in a human context. Bioremediation is the application of biological systems to the clean-up of organic and inorganic pollution, with bacteria and fungi being the most important organisms for reclamation, immobilization or detoxification of metallic and radionuclide pollutants. Some biominerals or metallic elements deposited by microbes have catalytic and other properties in nanoparticle, crystalline or colloidal forms, and these are relevant to the development of novel biomaterials for technological and antimicrobial purposes. On the negative side, metal and mineral transformations by microbes may result in spoilage and destruction of natural and synthetic materials, rock and mineral-based building materials (e.g. concrete), acid mine drainage and associated metal pollution, biocorrosion of metals, alloys and related substances, and adverse effects on radionuclide speciation, mobility and containment, all with immense social and economic consequences. The ubiquity and importance of microbes in biosphere processes make geomicrobiology one of the most important concepts within microbiology, and one requiring an interdisciplinary approach to define environmental and applied significance and underpin exploitation in biotechnology.
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              Novel mode of microbial energy metabolism: organic carbon oxidation coupled to dissimilatory reduction of iron or manganese.

              Andrew Phillips, Derek Lovley (1988)
              A dissimilatory Fe(III)- and Mn(IV)-reducing microorganism was isolated from freshwater sediments of the Potomac River, Maryland. The isolate, designated GS-15, grew in defined anaerobic medium with acetate as the sole electron donor and Fe(III), Mn(IV), or nitrate as the sole electron acceptor. GS-15 oxidized acetate to carbon dioxide with the concomitant reduction of amorphic Fe(III) oxide to magnetite (Fe(3)O(4)). When Fe(III) citrate replaced amorphic Fe(III) oxide as the electron acceptor, GS-15 grew faster and reduced all of the added Fe(III) to Fe(II). GS-15 reduced a natural amorphic Fe(III) oxide but did not significantly reduce highly crystalline Fe(III) forms. Fe(III) was reduced optimally at pH 6.7 to 7 and at 30 to 35 degrees C. Ethanol, butyrate, and propionate could also serve as electron donors for Fe(III) reduction. A variety of other organic compounds and hydrogen could not. MnO(2) was completely reduced to Mn(II), which precipitated as rhodochrosite (MnCO(3)). Nitrate was reduced to ammonia. Oxygen could not serve as an electron acceptor, and it inhibited growth with the other electron acceptors. This is the first demonstration that microorganisms can completely oxidize organic compounds with Fe(III) or Mn(IV) as the sole electron acceptor and that oxidation of organic matter coupled to dissimilatory Fe(III) or Mn(IV) reduction can yield energy for microbial growth. GS-15 provides a model for how enzymatically catalyzed reactions can be quantitatively significant mechanisms for the reduction of iron and manganese in anaerobic environments.
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                Author and article information

                Contributors
                Yun Fang: fangyun1987@126.com
                Meiying Xu: xumy@gdim.cn
                Wei-Min Wu: wei-min.wu@stanford.edu
                Xingjuan Chen: xingjuan_chen@aliyun.com
                Guoping Sun: guopingsun@163.com
                Jun Guo: guojun@gzb.ac.cn
                Xueduan Liu: xueduanliu@yahoo.com
                Journal
                Journal ID (nlm-ta): BMC Microbiol
                Journal ID (iso-abbrev): BMC Microbiol
                Title: BMC Microbiology
                Publisher: BioMed Central (London )
                ISSN (Electronic): 1471-2180
                Publication date (Electronic): 10 April 2015
                Publication date PMC-release: 10 April 2015
                Publication date Collection: 2015
                Volume: 15
                Electronic Location Identifier: 85
                Affiliations
                [ ]School of Minerals Processing and Bioengineering, Central South University, 410083 Changsha, China
                [ ]Guangdong Provincial Key Laboratory of Microbial Culture Collection and Application, Guangdong Institute of Microbiology, 510070 Guangzhou, China
                [ ]State Key Laboratory of Applied Microbiology Southern China, 510070 Guangzhou, China
                [ ]Key Laboratory of Biometallurgy of Ministry of Education, 410083 Changsha, China
                [ ]Department of Civil and Environmental Engineering, Center for Sustainable Development and Global Competitiveness, Codiga Resource Recovery Center, Stanford University, Stanford, CA 94305 USA
                Article
                Publisher ID: 419
                DOI: 10.1186/s12866-015-0419-3
                PMC ID: 4428006
                PubMed ID: 25888062
                SO-VID: 629037a8-add9-4576-b809-706e80e61423
                Copyright © © Fang et al.; licensee BioMed Central. 2015
                License:

                This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver ( http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

                History
                Date received : 12 August 2014
                Date accepted : 27 March 2015
                Categories
                Subject: Research Article
                Custom metadata
                issue-copyright-statement © The Author(s) 2015

                ScienceOpen disciplines: Microbiology & Virology
                Keywords: azo reduction,shewanella decolorationis s12,zero valent iron (fe0)
                Data availability:
                ScienceOpen disciplines: Microbiology & Virology
                Keywords: azo reduction, shewanella decolorationis s12, zero valent iron (fe0)

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