Saturday, February 25, 2012

Heavy metals and radioactivity in wild mushrooms

I'd like to begin by saying that this post will certainly not be exhaustive. The topic of radioisotope and heavy metal accumulation in wild mushrooms is an extensive one.  In 1964, Grüter first recognized that wild mushrooms preferentially accumulate radioactivity, primarily in the form of Cesium-137. Some ten years later, Stegnar (1973) recognized that certain mushroom species accumulate heavy metals, such as cadmium, copper, zinc, and lead.

One way to think about bio-accumulation is by using a transfer factor, equal to the concentration of the element in the mushroom, divided by the concentration of the element in the substrate. This transfer factor tells us how effectively the mushroom is at extracting the element from its surroundings, and thus, in the case of harmful elements, how much of a health risk the mushroom poses.

We now know that transfer factors are highly variable (see Pavel Kalač's 2001 review), depending on mushroom species, soil horizon from which a species takes nutrients, moisture, and contamination level of soil (which, in the case of radioactive elements, relates to the time since the nuclear disaster.)

That said, there are some noteworthy patterns. To begin with, I'll note the dangerous elements that mushrooms do not concentrate.

Elements that mushrooms do NOT concentrate

  • Lead (Gast, 1987), although mushrooms do incorporate lead, and thus, in areas of lead pollution, mushrooms may have high lead concentrations (Kalac, 2000). 
  • Strontium (Kalac, 2001)
  • Ruthenium (Kalac, 2001)
  • Plutonium (Kalac, 2001) 

Conversely, mushrooms appear to concentrate a number of radioisotopes and heavy metals. 

Elements that mushrooms DO concentrate
  • Cesium-137 (man-made radioisotope) (Kalač, 2001). 
  • Potassium (natural radioisotope). In general, mushrooms concentrate K with a factor of 20 to 40. (Kalač, 2001). 
  • Mercury. Common puffball and King bolete concentrate Hg at factors of 137±78 and 160±120 respectively (Falandysz et. al., 2002). 
  • Arsenic, though it is primarily in organic compounds, which are significantly less dangerous to humans than inorganic arsenic (Vetter, 2004). 
  • Cadmium, though absolute levels are generally low (Gast, 1987). 

Furthermore, there is a gradient of potassium concentration within the mushroom fruit-body, with the concentration greatest in the cap, then stipe, then sporophore (typically gills or tubes) (Kalač, 2001). Perhaps the same relationship holds for other radioactive elements and heavy metals. 

Figure from Kalač, 2001 showing schematic physiology of a mushroom. 

Likewise, there is a gradient of accumulation among different species, with the following table from Stachowiak and Regula, 2011, who borrowed from Kalac, 2001, illustrating the difference in radio-cesium accumulation among different species (note that Xerocomus is so close to Boletus that it is now Boletus). 

There are a number of limitations of the current literature. Firstly, almost all studies are located in Europe, partially on account of the proximity to the radio-isotope-producing Chernobyl disaster. How mushroom bio-accumulation of dangerous elements varies on a global basis is unconstrained. Secondly, concentrations and transfer factors have standard errors approaching 100% (for example the transfer factor of Mercury in king bolete is 160±120). This wide variation likely reflects both the complexity of the natural processes and analytical shortcomings. 

Stay tuned for radioisotopes and heavy metals in cultivated mushrooms (concentrations are much lower)! Let me know if you have any questions! 

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