P a r t t w o
Heavy metals and radiation
Soil and underground water contamination with heavy metals is a serious ecologic issue (1). Among the main contaminating heavy metals are: zinc, copper, arsenic, magnsium, calcium, cadmium, lead, silver and mercury.
Industrial and mining waste water, car emissions, phosphatated fertilizers, the geological singularities of a region, together with phenomena like volcano eruptions, are known sources of heavy metals. Some of them -like cadmium- cannot be metabolized by living organisms, so they accumulate in their tissues and can make their through the trophic chain to human consume, causing toxicity. This has been mainly observed in seafood and land crops that have been treated with contaminated water (2, 3, 4 and 5).
One heavy metal bioremediation procedure that has been developed in the past few decades, is phytoremediation, i.e., the use of plants for bioremediation (6, 7 and 8).
Some plants can accumulate high amounts of heavy metals and are known as “hyperaccumulators”; nevertheless, this plants show a low growth rate, limiting their application in bioremediation schemes. (9). But in the other hand, edible crops have also been explored, because of their higher growth rate, giving place to a procedure called “biofortification“, which is basically using this metal accumulation properties of edible crops to capture important metals for human diet, like zinc and magnesium.
There are also some microorganisms capable of transforming heavy metals to less toxic or more easily extractable states (10 and 11).
One of the most interesting examples are the Geobacter bacteria, which can bioremediate uranium by reducing uranium(IV) to uranium(VI), a form that is less soluble in water that can be then extracted. Moreover, Geobacter bacteria can also be used to generate bioelectricity and their conductive properties make them useful for electric circuits. The Geobacter Project has generated a number of studies about applications of Geobacter; their papers can be found in ther web page.
Another interesting bacterial group are the Shewanella bacteria. They can also generate electricity from a great variety of electron acceptors, making the useful for bioremediation schemes, specially for some radionuclides and halogenated organic compounds. However, in some cases the activity of Shewanella bacteria can be counterproductive, as it is for arsenate and mercury(II), which they transform to more toxic states (12).
Another kind of places that are usually contaminated with heavy metals are radioactive wastelands. But the issue here is that common microorganisms cannot be used to bioremediate this metals, as they do not survive at such high radiation levels.
What can be done? Well, there is a general procedure to find useful microorganisms out of environmental samples and is basically finding a way to exert a selective pressure that can be overcome by only those organisms with the characteristics we are looking for. So, if we are looking for organisms capable of metabolizing aromatic compounds or surviving high heavy metal concentrations, it is precisely through the exposure to these environmental challenges what would lead us to identify those useful microorganisms.
Radiation-resistant microorganisms are not the exception. In fact, a report states that the relative ease with which these organisms have been isolated -the procedure consists on making a culture of environmental samples in the presence of high radiation doses- and the authors even encourage readers to send them environmental samples that they can process (13).
One of these organisms are the Deinococcus bacteria, some of which can stand radiation up to 15,000 Gy (1,500,000 Rads).
To have a better idea of the awesomeness of Deinococcus we can compare to them to other organism’s radiation resistance: a 5 Gy radiation dose is lethal for a human, while 3,000-5,000 Gy for other resistant organisms -like Caenorhabditis elegans, Ustilago maydis and some archea- (14 and 15).
Nevertheless, Deinococci do not naturally show exceptional heavy metal bioremediation qualities, so they have been subject to a number of studies, trying to genetically engineer them with bioremediation-useful genes that come from other kind of microorganisms that cannot stand high radiation levels.
Phytoremediation and the different microorganisms with special traits make possible that heavy metal bioremediation schemes may be developed for industrial tanks but also in situ in impacted places.
We’ll continue talking about biosorption and some interesting iGEM projects about bioremediation.