Monitoring of microbial metal transformations in the environment

The biotransformation of metals is an exciting, developing strategy to treat metal contamination, especially in environments that are not accessible to other remediation technologies. However, our ability to benefit from these strategies hinges on our ability to monitor these transformations in the environment. This involves monitoring metals in both solid and aqueous samples, distinguishing between different chemical states, and obtaining information on the activities of specific microbial taxa in communities that inhabit the treated site. Accomplishing these goals requires cooperation among scientists from various disciplines and would benefit from both new, innovative approaches and the tailoring of established methods to control metal mobility 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.