Industrialized nations' passing flirtation with asbestos has left a nasty legacy. Exposure to airborne asbestos fibrils causes in fact asbestosis and malignancies such as bronchogenic carcinoma and pleural mesothelioma.
Although extraction and use of asbestos have been banned in many countries, substantial amounts of fibres have remained on sites previously occupied by dismissed asbestos industries. Cleaning up contaminated soil around defunct asbestos mines and factories is a lot harder than removing asbestos sheets from buildings, and the widely-dispersed fibres can easily become airborne if disturbed.
The toxic effects of inhaled asbestos fibres rely on both physical and chemical factors. The surface properties of asbestos fibres, and in particular iron, play a role in pathogenicity ( ). Crocidolite, one of the most potently carcinogenic forms of asbestos, contains up to 29 % iron, that originates highly reactive centres when occurring at the surface in poorly co-ordination state. This can cause the formation of highly reactive free radicals that damage DNA and eventually trigger cancers.
Experiments in vitro demonstrate that iron removal makes the asbestos considerably less hazardous by reducing their potential to generate radicals and to damage DNA ( ).
Most soil microorganisms need iron for their own metabolism, so some have very effective ways of scavenging it from their environment. Plants, bacteria and fungi can release potent chelators such as siderophores and polycarboxylic acids, that grab hold of iron atoms in soil minerals and bring it into soluble forms.
We have focused on fungi because these organisms display interesting features for bioremediation: they are found in all ecosystems and, through the formation of extensive hyphal networks, they explore large soil volumes. In addition, their biodiversity provides a huge reservoir of genes and functions. We thus investigated growth of some soil fungi in the presence of crocidolite fibres and their ability to release iron from this material. We demonstrated that fungi can steal considerable amounts of iron from crocidolite (Figure 1). The best iron-gatherers among the species tested were Fusarium oxysporum, Mortierella hyalina and Oidiodendron maius, a mycorrhizal fungus. Some species were still sucking up iron after more than seven weeks.
In addition, fungal hyphae can form a web of thin strands that bind asbestos fibres, making them less liable to escape into the air (Figure 1, inset).
The action of fungal chelators contribute s to modify the fibre surface in vitro, depriving it of the active sites likely involved in the triggering of the carcinogenic mechanisms. After exposure to fungi, in fact, the iron-stripped fibres could not generate reactive free radicals ( ).
We suggest, as also reported on Nature Science Update (http://www.nature.com/nsu/ 030120/030120-2.html), that if iron could be progressively extracted from the fibres dispersed in the environment, by native or introduced fungi, consequent changes in the nature of the fibre surface could likely result in a decrement in their carcinogenic potential.
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