Fungi that eat lead

19 Nov

This post was chosen as an Editor's Selection for ResearchBlogging.orgBioavailability = solubility (for the most part)

To discuss heavy metals (lead, nickel, mercury, cadmium, silver, copper, and so on) and how they can be detrimental to the environment or toxic to people, plants, or animals, we have to first posses a vague understanding of bioavailability and bioaccessibility.  These terms describe whether or not the substance in question can cross an organism’s cellular membrane, which allows the organism’s internal system access to the substance.

This might sound a little confusing at first, but if the toxin can’t get inside the organism, it can’t do damage (i.e. if the small child didn’t eat the paint flakes, he wouldn’t suffer from lead poisoning).  Whether or not a toxic heavy-metal can enter into the cells of a plant, microbe, and even a human, depends on the solubility of that metal, and the solubility of any given metal will change according to what it’s chemically bound with. For an entertaining visual demonstration of chemical bonding, I thought you might enjoy this video …

But seriously, bioavailability…

The child eating lead based paint is actually a good illustration of what I’m trying to convey here.  The lead typically found in lead based paint is lead carbonate or lead chromate, which means the lead is chemically bound with carbon and oxygen, or chromium and oxygen.  Humans cannot absorb that type of lead through our skin and therefore it isn’t toxic unless ingested, in which case our digestive processes dissolve the different lead-compounds and we absorb the lead into our systems.

However, lead nitrate is another story altogether.  In lead nitrate, the lead is bound with nitrogen and oxygen and is extremely toxic because it is highly soluble in water; we can absorb it through our skin fairly easily, so we don’t have to eat it for it to become bioaccessible.

Most of the time, bioaccessabilty and bioavailability, relate directly back to the solubility of a substance.  If it’s not soluble, it’s not bioavailable, and vice versa.  The more bioavailable a toxin is, the more dangerous it is, to us and the environment at large.

But what about the microbes…?

With all that in mind, I find it particularly interesting to know that various microorganisms found in the soils, sediments, and waters of contaminated sites (mines, smelters, spill sites, etc.) have been discovered over the last 50 years capable of converting insoluble metal compounds (such as lead carbonate) into more soluble forms.   These microbes are able to convert metals from relatively inert forms into readily available (often bioavailable) forms and have received much attention for their biotechnological applications, such as dissolving useful metals from metal ores (termed “bioleaching”).

In fact, in 1995, Geoffrey Gadd and his colleagues at the University of Dundee (Dundee, U.K.) developed a particularly useful method to screen soil fungi for their potential ability to solubilize heavy metals in the lab (Gadd et al, 1995).  They were able to incorporate any one of a variety of insoluble metal compounds into the agar of a Petri dish and measure the rate at which the fungus could dissolve the metals.  They tested the method using aluminum, cobalt, manganese and zinc and found many fungi from a common “garden soil” sample were able to dissolve insoluble zinc and cobalt just as quickly as they could grow and expand across the Petri dish.

A common fungus dissolves the highly insoluble lead carbonate as it grows across a Petri dish.

In my own experiments recently, I’ve tested this method and have a nice photo of a fairly common fungus (Aspergillus niger) literally dissolving lead carbonate, just like the lead carbonate found in lead-based paints. (photo at right)

This may initially seem like a bad thing… soil organisms turning relatively insoluble and inert metals into the more soluble, and consequently more bioavailable, forms.  But learning about the natural process and understanding how it all works actually affords science a new and developing tool in the ongoing struggle against contamination.  We can apply our knowledge to clean up efforts and remediation work on contaminated sites, and even biotechnology for lower-impact mining techniques.

Of course, we have a long way to go with the organisms I’m working on right now, but along the way they help us understand which contamination sites can cause the most dangerous types of human exposure and why.  It all comes back to how the toxins interact with the microbes in the soil!

So, back to the lab I go this afternoon to see how my other fungi are breaking down lead (or not) and converting metals from insoluble forms to the bioavailable forms (after I watch that chemical bonding video one more time for laughs!)

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ResearchBlogging.orgSayer, J., Raggett, S., & Gadd, G. (1995). Solubilization of insoluble metal compounds by soil fungi: development of a screening method for solubilizing ability and metal tolerance Mycological Research, 99 (8), 987-993 DOI: 10.1016/S0953-7562(09)80762-4

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2 Responses to “Fungi that eat lead”

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  1. Tweets that mention Fungi that eat lead « Microbial Modus -- Topsy.com - November 19, 2010

    […] This post was mentioned on Twitter by ResearchBlogging.org, Flipboard Science. Flipboard Science said: Fungi that eat lead http://bit.ly/dzVEbj […]

  2. ResearchBlogging.org News » Blog Archive » Editor’s Selections: Omega 3 fatty acids and heart disease, who writes the health news, and fungi that eat lead - November 26, 2010

    […] found in the soils, sediments, and waters of contaminated site have been discovered that can convert insoluble metal compounds such as lead carbonate into more soluble forms.  These microbes have received much attention for […]

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