Heavy metal pollution is a long-term problem in the field of environmental protection. Worldwide, more than 30 million tons of heavy metals have entered the natural environment, mainly lead, chromium, mercury, cadmium, and other heavy metals. Due to the strong toxicity and bioaccumulation of heavy metals, it is easy to accumulate in organisms, leading to harmfulness to human health and the ecological environment. The agricultural acreage polluted by heavy metals in China has reached 20 million hectares. Current treatment methods are complicated, difficult for applications on a large scale, and unaffordable for agricultural industries.
Squirrel-Shanghai utilizes the self-actuated biological ability of Escherichia coli to synthesize nanomaterials which enables it to detect and respond to the heavy metal ions present.
ABSTRACT
Sustainable development acts as an essential goal for humans [1] and soil is one of the most common types of land resources in our lives. Heavy metal pollution is a long-term problem in the field of environmental protection. Worldwide, more than 30 million tons of heavy metals have entered the natural environment, mainly lead, chromium, mercury, cadmium, and other heavy metals. Due to the strong toxicity and bioaccumulation of heavy metals, it is easy to accumulate in organisms, leading to harmfulness to human health and the ecological environment.
The agricultural acreage polluted by heavy metals in China has reached 20 million hectares. Current treatment methods are complicated, difficult for applications on large scale, and unaffordable for agricultural industries. Squirrel-Shanghai focuses on the track of bioremediation and utilizes the self-actuated biological ability of E. coli[2] to synthesize nanomaterials which enables it to respond and detect the heavy metal ions, especially Cd2+[3] present. We developed a few parts in order to modify the gene expression of E. coli[4]. We interviewed experts to gain feedback on the design of our project collected methods for further research and visited farms to learn certain policies on crop farming and the impact of our project.
DEFINING THE PROBLEM AND THE CURRENT BACKGROUND
The Origin of Heavy Metal Ions
Soil heavy metals are mainly affected by human activities, and their sources can be summarized as sewage irrigation, atmospheric sedimentation, pesticide fertilization, solid waste, and disposal. Through sampling and analysis[5] of the contents of heavy metals Cu, Zn, Pb, and Cd in soil and plants (hemlock, legumes, corn, pepper, and sugarcane) in sewage irrigation areas, it is found that the heavy metal content in hemlock far exceeds the standard value. However, the content in other plants is also very high.
Taking the farmland of Fulong Village, Huanjiang County, Guangxi as the research object[6], the research results showed that the contribution rates of atmospheric deposition (dry and wet), soil, irrigation water, and fertilizer to lead in topsoil were 37.5%, 43.8%, 12.4%, and 6.35%, respectively.
The Harmful Effects of Heavy Metal Ions[7]
- Biological hazards: The soil contains a large number of organisms, and the availability of the soil is closely related to the quality of internal microbial activities. If the soil contains a large number of heavy metal ions, it will limit the normal activities of the microorganisms inside the soil, and the heavy metal ions are difficult to decompose, and the fertility of the soil will gradually reduce in the long run.
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Plants:
Heavy metals in the soil will have a certain toxic effect on plants, causing changes in plant height,
taproot length, surface area of lamina, and a series of physiological characteristics. Mainly because the
heavy metals absorbed into the plant can induce some substances (H₂O₂, C₂H₂, etc.) that have toxic effects
and adverse effects on enzymes and metabolism in the body. Heavy metal pollution stress can harm the roots
of plants, causing the absorption capacity of roots to weaken, resulting in a plant nutrition deficit.
The growth of plants is inseparable from the soil. Once the soil contains a large number of heavy metal ions, it will also make the plants absorb too much of that. The precipitation of heavy metal ions in the plant body may produce some harmful substances, which will affect its own growth, and also affect its absorption capacity of other nutrients, resulting in the loss of internal nutrients, which cannot guarantee its own growth and nutrient requirements. -
Human:
After heavy metals enter the soil, they will not only cause soil quality degradation, resulting in reduced
crop yield and quality, but also enter the human body through direct contact with or food chain. If people
eat plants that grow in heavy metals in the soil or drink water that contains large amounts of heavy metal
ions, it will affect their health, leading to symptoms such as renal failure or cancer.
When the human body ingests excessive Cd2+,[8] it will cause organ lesions in the body, which can lead to bone effects characterized by the decrease of bone mineral density and the increase of fracture.
OUR APPROACH
We designed our E. coli to be able to respond to heavy metal ions in the outside world to achieve rapid detection of heavy metal ion levels [9] in agricultural land and it can be combined with heavy metal ions. Through a certain process, the E. coli can be separated from the soil. At the same time, we added killing mechanisms to E. coli to prevent them from escaping under certain circumstances.
- Response: Since the burden of expressing the protein is too great for E. coli, we added the bivalent cadmium ion sensor. The cadmium-responsive sensor consists of the repressor protein CadR and its corresponding pCasA promoter, where CadR binds to pCadA, which binds to CadR and changes its conformation when cadmium bivalent is present in the environment. At this point, CadR no longer binds to pCadA, thus turning on the expression of downstream genes of pCadA. Since the signal of the cadmium-ion sensor is low, we need to enlarge it. The commonly used signal amplification system is T7 polymerase-T7 promoter.
- Adsorption[10]: CsgA is a subunit protein, MBP is Metal metal-binding protein, which is involved in the composition of bacterial peritroflagellum. After the fusion expression of the two, bacteria can attach heavy metal ions. In order to enhance the adsorption capacity of bacteria, we increased the production of H2S in bacteria, so that it can react with cadmium bivalent ions in soil to produce cadmium sulfide, which can be captured and fused by MBP.
- Killing mechanisms: We use sodium salicylate as a condition for the killing mechanisms because sodium salicylate is a crop-friendly and even growth-promoting substance, and if it is used as a variable in our process, even if a small amount of solution leaks into the soil, it will not affect agricultural production. We chose the hok gene as the suicide gene, which causes the bacteria to lyse and die.