If the topic in these Superfund Research Program (SRP) monthly summaries falls within “bioremediation,” these GEITP pages believe this is close enough to ‘gene-environment interactions’ to warrant the summary as an important, exciting, and valuable teaching item.
Siderophores Reduce Asbestos Toxicity in Soil
Researchers have discovered that natural compounds released from bacteria and fungi in soil, known as siderophores, can decrease the toxicity of asbestos fibers. According to the authors, their results support the feasibility of asbestos bioremediation, or using organisms such as bacteria to degrade contaminants at waste sites.
Asbestos fibers are highly toxic and have been linked to serious health conditions –– including mesotheliomas and lung and stomach cancers. This toxicity is mostly attributed to the long shape of asbestos fibers, which makes it harder for cells of the immune system to remove them from the lung.
Immune cells called macrophages generally destroy foreign materials, such as bacteria and particles like asbestos, by either engulfing the particles or producing reactive oxygen species (ROS) to destroy the material. Because asbestos fibers are often too large to be engulfed, macrophages produce excessive ROS, which can lead to inflammation and DNA damage and may eventually lead to tumor development. Previous studies have shown that the presence of iron, which is often on the outer layer of asbestos, can increase ROS production.The broad goal of the research is to determine whether alteration of asbestos particles by plants or fungi may be useful for bioremediation of asbestos-contaminated sites. Another mechanism for ROS formation by long thin needles is that they generate free electrons at the point of the solid needle form; the free electrons simply invade the surrounding cells at the needle point and create ROS formation.
Based on this connection between iron and asbestos toxicity, researchers explored whether natural soil concentrations of siderophores, which have an affinity to bind to iron, can remove iron from asbestos and thus lower its toxicity. Plants, bacteria, and fungi all release siderophores to bind to iron in soil, facilitating iron uptake. The study, supported by the University of Pennsylvania Superfund Research Program (Penn SRP), was led by Jane Willenbring, PhD, currently at the Scripps Institution of Oceanography, University of California, San Diego.
Iron Removal and Toxicity
The researchers measured the release of iron from asbestos fibers in the presence of natural soil concentration levels of a fungal and a bacterial siderophore and compared that with specific organic acids, which are also released by organisms to bind to iron. They treated solutions containing asbestos with siderophores or organic acids, they extracted samples from those solutions at regular intervals for up to 16 days, and they measured the total dissolved iron. They found that both fungal and bacterial siderophores removed iron from asbestos fibers. The organic acids, on the other hand, proved ineffective.
In collaboration with Penn SRP Center project leader Melpo Christofidou-Solomidou, PhD, the researchers then measured the toxicity of the treated asbestos fibers using a new method developed by the research team. They quantified ROS generated by macrophages from mice exposed to asbestos fibers with and without siderophore treatment and observed a decrease in the amount of ROS generated, and thus decrease in the toxicity of asbestos, when fibers were treated with siderophores. They also discovered that the fungal siderophores were more effective than the bacterial siderophores in diminishing ROS levels.
Potential Site Applications
Because of its fiber strength and heat resistance, asbestos has been used in many materials produced for building or home construction –– such as shingles, ceiling and floor tiles, and attic and pipe insulation –– particularly if they were manufactured prior to 1980. In the U.S., nearly a thousand environmental waste dump sites are contaminated with either asbestos-containing materials or naturally-occurring asbestos minerals. The current standard for treatment of asbestos-contaminated sites is to physically remove the asbestos from the site or to cap the site with soil, both of which are expensive strategies.
According to the authors, this study indicates that siderophores in the soil environment can decrease asbestos fiber toxicity and possibly lower health risks. Removal of iron by siderophores could potentially form the basis of a low-cost bioremediation strategy, which should be explored as a viable approach for managing asbestos-contaminated sites.
For more information, contact:
Jane Willenbring, PhD