IGEM:IMPERIAL/2008/Projects/Water filter/Background

Cause of the problem

Cadmium occurs naturally, particularly within lead and zinc ores. Recently, we have seen an increase in the levels of cadmium within soil, water and the food chain. This can be predominantly put down to the following:

• Agricultural use of phosphate fertilizers,
• Sewage sludge also has high concentration,
• Industrial such as for Nickel-Cadmium batteries,

In europe negative response to the potential health risks of cadium has lead to the Restriction of Hazardous Substances Directive (RoHS) that bans the use of cadmium in electrical and electronic equipment products, that was implemented in July 2006. However, there is still risk from unregulated use such as those in Nickel-Cadmium batteries. In addition accidential spills still do occur, for example, in 2005 a smelting plant in China released high levels of Cadmium into the Beijiang River resulting in no drinking water for many major cities.

Current technology for removing heavy metals from water

Ion exchanger method: basically the harmful cation metal ions are exchanged for harmless cations such as potassium, sodium...

Health Risk

Cadmium exposure can result in numerous toxic effects for humans. These include the following:

• Diabetic renal complications,
• Hypertension,
• ostroporosis,
• leukaemia,
• Cancer in lungs, kidney, bladder, pancreas, breast and prostate [1].

By far the best studied effects of cadmium is the impairment of renal tubular re-absorption function. This results in increased secretion of numerous small molecular weight compounds. This can result in tubular dysfunction and exposure to cadmium has been shown to correlate to the mortality rates. more short term exposure to cadmium can result in stomach pains, vomiting and diarrhoea.
Various studies have looked at exposure levels of cadmium that can result in problems. Studies have estimated that a typical exposure to cadmium from food sources alone can be 30<micro>g per day and in extreme cases 90µg per day. The World Health Organisation recommend a daily intake of 70µg per day. However, research has shown that exposure of 30-50µg per day has shown significant increase in the risk to several of the toxic effects mentioned above.

Areas of Problem

On the global scale the major areas of contamination are in Japan, Austrailia, India and China. However, there are numerous examples of when leakages and poor regulation have resulted in high exposure to cadmium. One of the most recent events was in China were several tonnes of Cadmium were released into the Beijiang River. On the local scale, contamination from agricultural sites, sewage and land fill sites can create problems for local residence.

Useful Biology

Many organisms have developed mechanisms for metal ion resistance. These include:

• Compartmentalization,
• Complexes,
• Synthesis of binding proteins, such as Metallothioneins (MTs) and phytochelatins (PCs),

Metallothioneins

• These are cysteine rich proteins that can bind to metal ions. The only prokaryotic MT is from cyanbacterial strains from the genus Synechococcus by the gene smtAgene

Genetically engineered E. coli JM109, which simultaneously expressed a cadmium transport system and MT (namely M4) was evaluated for its ability to accumulate Cd2+ from aqueous solutions. M4 showed resistance to the toxicity of cadmium and could accumulate 63.26 mg/g Cd2+,

[2]

Chassis

Psychrophiles and psychrotrophs are a class of microbes defined as Extremophiles, with the ability to grow in cold environments. Psychrophiles have a maximum growth temperature of 20 °C, an optimum growth temperature of 15 °C or lower, and a minimum growth temperature of 0°C or lower and psychrotrophs normally have a warmer optimum.

Bacillus subtilis

Is a gram positive bacterium

Disadvantages

• Chimeric plasmids for infection of B. subtilis have been difficult to prepare, and if E. Coli is used as an intermediate host to provide plasmid forms suitable for Bacillus transformation, the B. subtilis treats any E. coli-propagated DNA as foreign and preferentially attacks the insert portion of the plasmid. This attach results in loss of cloned genes and limits the use of B. subtilis as a cloning system.

Reference

1. Satarug S, Baker JR, Urbenjapol S, Haswell-Elkins M, Reilly PE, Williams DJ, and Moore MR. A global perspective on cadmium pollution and toxicity in non-occupationally exposed population. Toxicol Lett. 2003 Jan 31;137(1-2):65-83. DOI:10.1016/s0378-4274(02)00381-8 |
3. Deng X, Yi XE, and Liu G. Cadmium removal from aqueous solution by gene-modified Escherichia coli JM109. J Hazard Mater. 2007 Jan 10;139(2):340-4. DOI:10.1016/j.jhazmat.2006.06.043 |
4. Herbert RA. A perspective on the biotechnological potential of extremophiles. Trends Biotechnol. 1992 Nov;10(11):395-402. DOI:10.1016/0167-7799(92)90282-z |

5. Adaption of Bacillus subtilis to growth at low temperature: a combined transcriptomic and proteomic appraisal.

6. Adaptations in Psychrophilic Bacteria:

   Potential for Biotechnological Applications