Dissimilatory Fe(III) and Mn(IV) reduction.

Organic acids, 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. A full assessment of their role in these processes, however, cannot be determined unless the exact mechanisms of plant organic acid release and the fate of these compounds in the soil are more fully understood. This review therefore includes information on organic acid levels in plants (concentrations, compartmentalisation, spatial aspects, synthesis), plant efflux (passive versus active transport, theoretical versus experimental considerations), soil reactions (soil solution concentrations, sorption) and microbial considerations (mineralization). In summary, the release of organic acids from roots can operate by multiple mechanisms in response to a number of well-defined environmental stresses (e.g., Al, P and Fe stress, anoxia): These responses, however, are highly stress- and plant-species specific. In addition, this review indicates that the sorption of organic acids to the mineral phase and mineralisation by the soil's microbial biomass are critical to determining the effectiveness of organic acids in most rhizosphere processes.

Organic acids in the rhizosphere: a critical review

Nitrate attenuation in groundwater: a review of biogeochemical controlling processes.

Chromium is a highly toxic non-essential metal for microorganisms and plants. Due to its widespread industrial use, chromium (Cr) has become a serious pollutant in diverse environmental settings. The hexavalent form of the metal, Cr(VI), is considered a more toxic species than the relatively innocuous and less mobile Cr(III) form. The presence of Cr in the environment has selected microbial and plant variants able to tolerate high levels of Cr compounds. The diverse Cr-resistance mechanisms displayed by microorganisms, and probably by plants, include biosorption, diminished accumulation, precipitation, reduction of Cr(VI) to Cr(III), and chromate efflux. Some of these systems have been proposed as potential biotechnological tools for the bioremediation of Cr pollution. In this review we summarize the interactions of bacteria, algae, fungi and plants with Cr and its compounds.

http://envismadrasuniv.org/Bioremediation/pdf/Interactions%20of%20chromium%20with%20microorganisms.pdf

Interactions of chromium with microorganisms and plants

Butanol is an aliphatic saturated alcohol having the molecular formula of C4H9OH Butanol can be used as an intermediate in chemical synthesis and as a solvent for a wide variety of chemical and textile industry applications Moreover, butanol has been considered as a potential fuel or fuel additive Biological production of butanol (with acetone and ethanol) was one of the largest industrial fermentation processes early in the 20th century However, fermentative production of butanol had lost its competitiveness by 1960s due to increasing substrate costs and the advent of more efficient petrochemical processes Recently, increasing demand for the use of renewable resources as feedstock for the production of chemicals combined with advances in biotechnology through omics, systems biology, metabolic engineering and innovative process developments is generating a renewed interest in fermentative butanol production This article reviews biotechnological production of butanol by clostridia and some relevant fermentation and downstream processes The strategies for strain improvement by metabolic engineering and further requirements to make fermentative butanol production a successful industrial process are also discussed Biotechnol Bioeng 2008;101: 209-228 © 2008 Wiley Periodicals, Inc

https://onlinelibrary.wiley.com/doi/pdf/10.1002/bit.22003

Fermentative butanol production by clostridia

THE presence of synthetic and naturally occurring chelating agents in nuclear and toxic-metal wastes is a major concern because of their potential to enhance mobilization of metal ions away from the disposal sites1–3. Of particular interest is citric acid, which is present in low-level and transuranic radioactive wastes4,5 and in domestic and industrial wastes (as washing fluids, for instance), as well as being found naturally. Citrate ions form multidentate, stable complexes with a variety of toxic metals and radionuclides; but biodegradation of these complexes, precipitating the metal ions as insoluble hydroxides, oxides or other salts, may retard migration. Here we report a study of the biodegradation of citrate complexes of Ca, Fe(n), Fe(m), Cd, Cu, Ni, Pb and U. Several of these complexes were not readily degraded by bacteria, and the biodegradability depended on the chemical nature of the complex, not on the toxicity of the metal to the bacteria. This resistance to biodegradation implies that citrate complexation may play an important part in migration of these hazardous wastes.

Biodegradation of metal citrate complexes and implications for toxic-metal mobility

Speciation of uranium in cultures of Clostridium sp. by X-ray absorption near-edge spectroscopy (XANES) at the National Synchrotron Light Source and by X-ray photoelectron spectroscopy (XPS) showed that U(VI) was reduced to U(IV). In addition to U(IV), a lower oxidation state of uranium, most probably U(III), was detected by XANES in the bacterial cultures. Reduction of uranium occurred only in the presence of growing or resting cells. Organic acid metabolites, the extracellular components of the culture medium, and heat-killed cells failed to reduce uranium under anaerobic conditions. The addition of uranyl acetate or uranyl nitrate (>210[mu]M U) to the culture medium retarded the growth of the bacteria as evidenced by an increase in the lag period before resumption of growth, by decreases in turbidity, and in the total production of gas and organic acid metabolites. These results show that uranium in wastes can be stabilized by the action of anaerobic bacteria. 31 refs., 2 figs., 3 tabs.

XPS and XANES Studies of Uranium Reduction by Clostridium sp.

Laboratory batch and column experiments were conducted to investigate the role of microbial exudates, e.g., exopolymeric substance (EPS) and alginic acid, on microbial Cr(VI) reduction by two different Pseudomonas strains (P. putida P18 and P. aeuroginosa P16) as a method for treating subsurface environment contaminated with Cr(VI). Our results indicate that microbial exudates significantly enhanced microbial Cr(VI) reduction rates by forming less toxic and highly soluble organo-Cr(III) complexes despite the fact Cr(III) has a very low solubility under the experimental conditions studied (e.g., pH 7). The formation of soluble organo-Cr(III) complexes led to the protection of the cells and chromate reductases from inactivation. In systems with no organic ligands, soluble organo-Cr(III) end products were formed between Cr(III) and the EPS directly released by bacteria due to cell lysis. Our results also provide evidence that cell lysis played an important role in microbial Cr(VI) reduction by Pseudomonas bacteria due to the release of constitutive reductases that intracellularly and/or extracellularly catalyzed the reduction of Cr(VI) to Cr(III). The overall results highlight the need for incorporation of the release and formation of organo-Cr(III) complexes into reactive transport models to more accurately design and monitor in situ microbial remediation techniques for the treatment of subsurface systems contaminated with Cr(VI).

Chromium(VI) bioremoval by Pseudomonas bacteria: role of microbial exudates for natural attenuation and biotreatment of Cr(VI) contamination.

Decontamination of metal surfaces contaminated with low levels of radionuclides is a major concern at Department of Energy facilities. The development of an environmentally friendly and cost-effective decontamination process requires an understanding of their association with the corroding surfaces. We investigated the association of uranium with the amorphous and crystalline forms of iron oxides commonly formed on corroding steel surfaces. Uranium was incorporated with the oxide by addition during the formation of ferrihydrite, goethite, green rust II, lepidocrocite, maghemite, and magnetite. X-ray diffraction confirmed the mineralogical form of the oxide. EXAFS analysis at the U L(III) edge showed that uranium was present in hexavalent form as a uranyl oxyhydroxide species with goethite, maghemite, and magnetite and as a bidentate inner-sphere complex with ferrihydrite and lepidocrocite. Iron was present in the ferric form with ferrihydrite, goethite, lepidocrocite, and maghemite; whereas with magnetite and green rust II, both ferrous and ferric forms were present with characteristic ferrous:total iron ratios of 0.65 and 0.73, respectively. In the presence of the uranyl ion, green rust II was converted to magnetite with concomitantreduction of uranium to its tetravalent form. The rate and extent of uranium dissolution in dilute HCl depended on its association with the oxide: uranium present as oxyhydroxide species underwent rapid dissolution followed by a slow dissolution of iron; whereas uranium present as an inner-sphere complex with iron resulted in concomitant dissolution of the uranium and iron.

Association of uranium with iron oxides typically formed on corroding steel surfaces

An anaerobic N{sub 2}-fixing Clostridium sp. solubilized Cd, Cr, Ni, Pb, and Zn coprecipitated with goethite ({alpha}-FeOOH) by (i) direct action due to enzymatic reduction of ferric iron and the release of metals associated with iron and (ii) indirect action due to metabolic products. The extent of dissolution depended upon the nature of the association of the metals with goethite. Substantial amounts of Cd and Zn, which were closely associated with iron, were released due to direct action. Nickel was solubilized by direct and indirect actions, while a small amount of Cr was solubilized only by direct action. The nature of association of Pb in the coprecipitate was not affected by the presence of other cations and it was solubilized by indirect action. In the presence of bacteria, the concentration of soluble Pb decreased due to biosorption. These results show that there could be significant remobilization of toxic metals coprecipitated with iron oxides in wastes, contaminated soils, and sediments due to microbial iron reduction.

Cleveland J. Dodge

Papers

Biodegradation of metal citrate complexes and implications for toxic-metal mobility

XPS and XANES Studies of Uranium Reduction by Clostridium sp.

Chromium(VI) bioremoval by Pseudomonas bacteria: role of microbial exudates for natural attenuation and biotreatment of Cr(VI) contamination.

Association of uranium with iron oxides typically formed on corroding steel surfaces

Anaerobic microbial remobilization of toxic metals coprecipitated with iron oxide