Monitoring of microbial metal transformations in the environment
Introduction
Metals and radionuclides contaminate vast tracts of land in the USA as a result of industrial activities and nuclear weapons manufacturing. Although regulations have reduced the input of new metals to the environment, the old sites continue to leach toxic metals. A driving force behind cleaning up these sites is the high degree of public awareness regarding metal contamination and the potential consequences toward human health. Not only did the 2000 movie ‘Erin Brockovich’ garner an Oscar for Julia Roberts, it also educated the public about the dangers of chromate [Cr(VI)] in the water supply. (Throughout this manuscript the chemical symbol is used when speciation is known; the generic names for metallic elements, such as chromium, is used when speciation is unknown or when relating to the element in general.) This trend is not likely to reverse, as there is an increasing availability of products that bring the diagnosis and treatment of heavy metal poisoning out of the realm of physicians and environmental health specialists directly to the consumer. Websites such as http://www.extremehealthusa.com and http://www.Awakennutrition.com offer inexpensive hair analysis kits, and subsequently sell the client home chelation therapy. Although there is no doubt that true metal poisoning is dangerous to human health — mercury causes renal damage and severe developmental delays in children and Cr(VI) is a potent carcinogen — chelation therapy is offered to a naive public as a cure for many poorly defined maladies, such as attention deficit disorder, autism, hypoglycemia, and chronic fatigue syndrome. Far from remaining in the realm of the desperate and the wacky, self-diagnosis of metal poisoning became fashionable this past fall when Greenpeace teamed up with Aveda hair salons to offer mercury hair tests to people who had come in for a cut and color [1, 2]. As more voters become convinced that many ailments they suffer from are a direct result of metal poisoning, there will be less tolerance for trace amounts of metals in drinking water, and increased pressure to tackle even the most intractable problems.
Metals provide several unique challenges for remediation. Unlike organic contaminants, they are elements and cannot be degraded into innocuous products. Additionally, metals in subsurface environments, which can leach into ground water aquifers and contaminate drinking water, are unreachable by the many methods available to decontaminate surface soils. One option currently being explored is to immobilize metals by microbial transformations to insoluble states [3]. Specifically, oxidized forms of chromium, technetium and uranium are water soluble, but upon reduction form solid precipitates with lower mobility. By contrast, oxidized inorganic mercury Hg(II), although water soluble, sorbs onto sediment particles and forms ligands with organic matter. The reduced form, Hg(0), is a volatile gas with poor water solubility that can be transported to the atmosphere (Figure 1). To harness microbial transformations to immobilize metals in the environment, information on metal concentrations and speciation and the status of the microbial community in the treated environment are needed. This review focuses on the methods and the challenges involved in monitoring microbial metal transformations in environmental samples, specifically for chromium, technetium, uranium and mercury, for which bioremediation strategies are in development.
Section snippets
Controlled microbial metal transformations as a bioremediation strategy
The following is a brief description of microbial metal transformations and their potential in environmental remediation. The interested reader may find more comprehensive discussion of these issues in recent reviews cited herein.
Microbial transformations of mercury have long been of interest, because they control the production of the potent neurotoxin methylmercury (CH3Hg) in the environment [4, 5]. Very low concentrations of CH3Hg are biomagnified and bioaccumulated more than a million times
Monitoring metal transformations in natural waters
To a large extent, our ability to measure soluble, oxidized metals in natural waters is well developed, and numerous techniques are available. However, most analytical methods do not distinguish between metal oxidation states and manipulations that separate metals according to their chemical form are needed before the analysis of environmental samples. For example, highly sensitive quantitation of mercury is achieved using fluorescence spectrometry of Hg(0). To distinguish various chemical
Detection of insoluble metals in soils and sediments
Analysis of immobilized metals in their solid phase is essential for the basic understanding of the immobilization process and the potential for remobilization as conditions and microbial activities change. In addition, many microbes precipitate metals within their biomass and the chemical form of the cell-associated metal and its ligands provide evidence for the precipitation processes. However, quantitation and speciation of metals in the solid phase presents many challenges to the
Microbial community analysis as evidence of biotic metal transformations
Relating changes that occur in microbial community structure during biostimulation to the dynamics of metal transformation facilitates optimization and the safe application of remedial approaches [19••]. Thus, monitoring microbial communities is an essential part of metal bioremediation and the application of methods that do not depend on culturing have been of high priority. These methods are based mostly on the analysis of macromolecules such as DNA, RNA or phospholipid fatty acids (PLFA)
Future perspectives
One major challenge with using established methods to monitor microbial transformations is that these methods were originally designed with other goals in mind. For example, determining the bioavailability of trace elements has offered a treasure trove of techniques that can be used to monitor metal transformations, provided the investigator has a thorough understanding of the strengths and weaknesses of each technique. It is therefore beneficial to examine existing methods and their possible
Conclusions
Metal transformations are gaining importance as a tool for the environmental management of metal-contaminated environments and as part of remedial strategies that seek to control metal mobility. The need to monitor immobilization processes is driving the development of new methods for the detection of metals in their various oxidation states and their interactions with the transforming microbes. Future developments in the application of state-of-the-art methodologies, such as high-resolution
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
We thank Sharron Hicks for bringing references to our attention. Work on metals and their interactions with microorganisms in our laboratory is supported by the National Science Foundation and the US Department of Energy.
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Interactive effects of multiple stressors revealed by sequencing total (DNA) and active (RNA) components of experimental sediment microbial communities
2018, Science of the Total EnvironmentCitation Excerpt :Bacteria (Sun et al., 2012) and microbial eukaryotes (Chariton et al., 2010) are sensitive to sediment stressors. In addition, bacteria have the ability to break down a variety of chemical substances (Wiatrowski and Barkay, 2005; Antizar-Ladislao, 2010; Das and Chandran, 2011; Mason et al., 2014). This makes them ideal for biomonitoring; their response to chemical stressors should be rapid and stressor-specific.
Determination of chromium (VI) in primary and secondary fertilizer and their respective precursors
2017, ChemosphereCitation Excerpt :All these matrices contain significant amounts of organic matter, this includes struvite 2 that was precipitated from digested sludge, in contrast to struvite 1 that was crystallized from sludge liquor and thus contains no organic matter. It is known that microbial activity (Wiatrowski and Barkay, 2005; Al Hasin et al., 2010; Dhal et al., 2013), sulfur-containing compounds (Barrera-Díaz et al., 2012), sorption of Cr to lignocellulosic material (Miretzky and Cirelli, 2010) and reaction with organic compounds can lead to reduction or removal of Cr(VI). Thus, we can assume that the respective samples not only contain no Cr(VI) but have the potential to reduce more Cr(VI).
Production of bioemulsifiers by Amycolatopsis tucumanensis DSM 45259 and their potential application in remediation technologies for soils contaminated with hexavalent chromium
2013, Journal of Hazardous MaterialsCitation Excerpt :In the field of bioremediation, the application of bioemulsifiers as natural alternatives to synthetic production is an efficient strategy for removing hydrocarbons from contaminated soils and sediments [5–7]. However, remediation of heavy metal contamination brings up several unique challenges, since these metals cannot be degraded to innocuous products [8]. In addition, metals tend to be strongly absorbed on the matrix of soils and sediments, which limits their solubility and hinders their subsequent removal.
Chapter 1 Influence of Coupled Processes on Contaminant Fate and Transport in Subsurface Environments
2008, Advances in AgronomyCitation Excerpt :Direct microbial reduction of Cr(VI) is also possible if the contaminant concentration does not exceed a toxic effect on the organism (Bank et al., 2007; Cummings et al., 2007; Middleton et al., 2003; Sani et al., 2002). Indirect microbial reduction of Cr(VI) by subsurface dissimilatory bacteria is more common in mixed systems whereby biogenic Fe(II), formed from microbial‐induced Fe(III)‐oxide reduction, serves to reduce Cr(VI) to Cr(III) (Hansel et al., 2003; Wielinga et al., 2001; Wilkins et al., 2007). Hazen and Tabak (2005) performed field biostimulation investigations at a Cr(VI) contaminated site at the Hanford 100 H area in Richland, WA, USA, where the vadose and saturated zones were contaminated with Cr(VI) due to historical reactor operations.
A review of isotopic composition as an indicator of the natural and anthropogenic behavior of mercury
2006, Applied Geochemistry