Project Inspiration and Design

Motivation

In recent years, incidents of heavy metal pollution have been reported several times. One of the most well-known is rice contaminated with excessive amount of heavy metals in mining areas of Chenzhou, Hunan province, which seriously harmed people's health and caused huge economic losses. As high school students, we should have the responsibility and obligation to do something good for the society. Among the reported heavy metals, we choose arsenic, a metalloid, known as a human carcinogen, one the most toxic substance. So this year we plan to carry out a project on arsenic treatment in water.

Arsenic, the most toxic heavy metal

With rapid industrialization and urbanization, a large number of industries release their untreated toxic wastes and contaminate our environment. In the industrial waste, heavy metal pollution is a major concern. These heavy metal ions and metalloid such as lead (Pb), cadmium (Cd), mercury (Hg), and arsenic (As), etc. are very toxic to living organisms in a variety of ways. Among of these, arsenic is one of the most toxic substance as it is a known human carcinogen, which ranks the 1^st on the environmental protection agency (EPA) priority list of drinking water contaminant. Exposure to excessive arsenic may cause skin harms, color change, spots on hands and feet skin, if untreated, leading to skin cancer in the end. Arsenic contamination also cause hypertension, diabetes and other disorders of reproduction.

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Arsenic contamination of groundwater remains a serious global issue

Arsenic is a natural toxicant for groundwater. Arsenic contamination of groundwater is a global issue. It estimated that over 170 million of people are under the threat of drinking arsenic contaminated water in a variety of regions worldwide including South and Southeast Asia, especially in Bangladesh, where more than 70 million people possibly exposed to arsenic contaminated water alone. In China, especially arid regions in northern provinces, such as, Xinjiang, Inner Mongolia and Shanxi province, arsenic contamination of groundwater is a crucial problem due to lack of clean water for cooking and drinking water sources other than groundwater. It has been predicted that 5.6 million people are exposed to high levels of arsenic in drinking water above 50 μg/liter, and about 14.7 million are exposed to arsenic concentrations of above the World health organization guideline 10μg/liter.

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Fig. 1. Location of known and potential arsenic-affected basins in China. Areas with high levels of arsenic (As) are generally characterized by Holocene sediments (green), where large basins may be affected.

Arsenic contamination of food raised public concern

Arsenic is also an environmental food chain contaminant. Contaminated groundwater irrigation was the primary reason which leads to the accumulation of arsenic in crops. Grain crops like rice take arsenic easily so people who consume a lot of rice are at a high health risk. In recent years, news about excessive heavy metals in rice from heavy-metal contaminated areas has been reported in various media. For example, the “toxic rice incident” occurred in Chenzhou, Hunan province, which seriously affects people's health and causes huge economic losses.

Facing the increasingly severe arsenic pollution, therefore, techniques to removal contaminated arsenic are urgently need.

Conventional treatment of As have many limitations

Some of the conventional methods for As and other metals removal include membrane filtration, physical approach, adsorption, ion exchange, and chemical method. Like other metals, arsenic can be removed through membrane filtration. For example, nano-filtration membrane was used to remove arsenic in drinking water. However, the technology used has some limitations, such as high-cost, not eco-friendly, complex and secondary contamination. Therefore, in order to clean up the environmental better, new techniques are needed to deal with the threat of arsenic contamination.

Bioremediation is a promising technique for as removal

In recent years, bioremediation has been regarded as an environmentally friendly and effective method for arsenic removal.

One kind of protein is metallothionein (MT), which is a kind of ubiquitous protein with low molecular weight, high cysteine content and high metal content. These proteins trap not only essential metal ions such as zinc and copper, but also toxic heavy metals such as cadmium. and mercury. MT is expressed in many organisms, including vertebrates, fungi, plants, algae, and metal-resistant bacterial. Recently, a newly identified MT from an arsenic-tolerant marine alga, Fucus vesiculosus (fMT), has been cloned and stably expressed as a fusion protein (24) in Escherichia coli and has been shown to bind arsenite with high affinity in vitro.

Another metal-binding protein is phytochelate protein (PC), a natural peptide found in plants and fungi that has a high affinity for arsenite. PCs have a general structure of (gammar-Glu-Cys-)n-Gly (n=1-11)with higher affinity than MTs. PCs are synthesized by phytochelatin synthase(PCS) using glutathione (GSH). PCS have been expressed in E. coli with effective arsenic accumulation.

The aim of our project

In this project, we aimed to establish a system of As detection and bio-sorbent. Firstly, we used a mature asenic biosensor form previous iGEM projects to build our biosensor to as dection. Secondly, we put our emphasis on the development of as bio-sorbents for As removal. A highly-selective metallothion fMT was surface-displayed on the cell membrane of E. coli to remove the arsenic in water. In addition, a recombinant bacteria expressing a phytochelatin synthase (PCS) ,which can be used for the synthesis of phytochelatin was constructed to accumulate As intracellularly. Finally, we will design an engineered device for the application of as removal in As contaminated groundwater regions.

References

1. Smith A H , Lingas E O , Rahman M . Contamination of Drinking-Water by Arsenic in Bangladesh: A Public Health Emergency[J]. Bulletin of the World Health Organization, 2000, 78.

2. Argos M , Kalra T , Rathouz P J , et al. Arsenic exposure from drinking water, and all-cause and chronic-disease mortalities in Bangladesh (HEALS): A prospective cohort study[J]. The Lancet, 2010, 376(9737):252-258.

3. Rodriguez-Lado L , Sun G , Berg M , et al. Groundwater Arsenic Contamination Throughout China[J]. Science, 2013, 341(6148):866-868.

Norton G , Deacon C , Mestrot A , et al. Arsenic Speciation and Localization in Horticultural Produce Grown in a Historically Impacted Mining Region[J]. Environmental Science & Technology, 2013, 47(12):130529080645002.

4. Mallick I , Hossain S T , Sinha S , et al. Use of Indigenous Bacteria from Arsenic Contaminated Soil for Arsenic Bioremediation[J]. Management of Natural Resources in A Changing Environment, 2015:155-165.

5. Singh S , Mulchandani A , Chen W . Highly Selective and Rapid Arsenic Removal by Metabolically Engineered Escherichia coli Cells Expressing Fucus vesiculosus Metallothionein[J]. Applied and Environmental Microbiology, 2008, 74(9):2924-2927.

6. Singh S , Kang S H , Lee W , et al. Systematic engineering of phytochelatin synthesis and arsenic transport for enhanced arsenic accumulation in E. coli[J]. Biotechnology & Bioengineering, 2010, 105(4):780-785.