Team:SCAU-China/Description

MESEG

Description

Problems


The rapid global industrialization over the past century has led to various environmental problems, one of which is the heavy metal pollution. The heavy metals are commonly defined as elements with relatively high density compared to water (Chaney, 1991). Nowadays, the term "heavy metal" has been used to describe the metallic chemical elements and quasi metals that are toxic to the environment and humans (Briffa et al., 2020). Heavy metals interact with DNA and nuclear proteins, causing DNA damage and toxicity to various cellular organelles that may lead to apoptosis or carcinogenesis (Tchounwou et al., 2012).

Figure 1: Heavy metal pollution has dangerous effects on humans. (Godwell et al., 2019).


There are many sources of heavy metal pollution, mainly contaminating the water and soil.


Table 1: Background concentration for heavy metals in soil and river water (Vareda et al., 2019).

Heavy Metal Upper Crust/mg·kg−1 Surface Soil/mg·kg−1 River Water/μg·L−1
Arsenic 1.8 4.7 0.13-2.71
Cadmium 0.1 0.41 0.06 × 10−2–0.61
Chromium 35 42 0.29–11.46
Copper 14 14 0.23–2.59
Lead 15 25 0.007–308
Mercury 0.07 0.07 -
Nickel 19 18 0.35–5.06
Zinc 52 62 0.27–27



Current Solutions


Remediation of heavy metals from the environment requires proper attention to protect soil quality, the ecosystem, and human health. At present, the commonly used methods to deal with heavy metal pollution are mainly through physical, chemical, and bioremediation technologies.

Leaching is the most widely used physical remediation technology to remove the heavy metals in soil, through which heavy metals are dissolved by leaching agents and mixed with chemical additives via chemical reactions. Heavy metal pollution in water can be treated by chemical strategies such as electrolysis and chemical precipitation, or physical methods such as ion exchange resin absorption, inert absorbent (such as activated carbon), coagulation, flocculation, and membrane filtration (Fu and Wang, 2011). However, physical and chemical technologies are very expensive, often destructive to the local ecosystem, and require handling of a large amount of hazardous waste (Roy Chowdhury et al., 2018).

On the other hand, the bioremediation of heavy metals from the environment uses plants such as reed (Phragmites australis), and reed mace (Typha latifolia), or microorganisms that absorb heavy metals (Jiang, 2007; Ding and Liu, 2021). Compared with other methods, bioremediation technologies are more environment friendly and less expensive, and have great potential.




Our Idea


Molecular Magnet Super Environmental Guard (MESEG)


The previous iGEM teams have designed various projects in the bioremediation of heavy metal pollution, such as expressing Ni ion channels in yeasts to absorb Ni (2019-Hubei University of Technology), or using heavy metal binding proteins (metallothionein, MT) (2015-UMBC-Maryland and 2014-Penn). In our project (MESEG), we engineer Chlamydomonas reinhardtii, a type of algae which has been used in bioremediation, via the autophagy pathway to absorb three representative heavy metal ions (cadmium, copper, and zinc) in water. C. reinhardtii has been shown to absorb heavy metal ions through several chemical groups on the cell wall, such as carboxyl, hydroxyl, amino, sulfhydryl, and phosphate groups (Kong et al., 2017). However, heavy metal ions enter the algae through ion channels on the cell membrane, and inevitably cause stress to C. reinhardtii and thus shorten its life for bioremediation (Jiang, 2007).


Video 1: Heavy metal ions enter Chlamydomonas reinhardtii through ion channels on the cell membrane.


Therefore, we construct a fusion protein expressing in C. reinhardtii, like an intelligent molecular magnet, to attract and capture heavy metal ions and transport them to the storage place like the vacuole for degradation, thus reducing the toxicity of heavy metal ions to the algae. This molecular magnet has been constructed by combining the heavy metal binding protein metallothionein (MT), the fluorescent protein for visualization, and the Atg8 interacting motif (AIM). The AIM portion functions as a binding site to the autophagosomes that later can be transported to the vacuole via autophagy.Please click on Project Design for details.




Fusion Protein Model


A fusion protein is a protein consisting of at least two domains that are encoded by separate genes that have been joined so that they are transcribed and translated as a single unit, producing a single polypeptide (Riggs, 2013), which is not constructed as easily as lining up the building blocks. In our project, we need to find out whether the order of the three protein domains is appropriate and whether the linker peptides are needed to ensure the normal function of the fusion protein. The protein domains chosen is based on the function of the desired fusion protein, which is relatively simple in most cases. However, it is difficult to choose the appropriate linkers. The functional domains joined directly without the linkers would lead to some adverse results, such as misfolding, low yield, or impaired activity of the fusion protein. Therefore, the rational design and selection of the linkers are very important for the construction of fusion proteins (Li et al., 2015). Combined with the Wet Test, we established the molecular model of the desired fusion protein with the appropriate order of the three protein domains and the types of linkers. Please click on Fusion Protein Model for details.


Figure 2: Schematic diagram of the intelligent molecular magnet (MESEG) that absorb heavy metal ions and transport them to the vacuole. MT represents metallothionein, AIM represents Atg8 interacting motif, and mCherry is a red fluorescent protein.





Recovery time

We hope that eventually the engineered C. reinhardtii can be put into practice. This raises the questions of when and how the engineered algae are recovered. The recovery time relates to the work efficiency of the whole device. An indicator is needed to show that the engineered algae are about to be saturated with the heavy metal ions. Then we can recover the device for further treatments. Please click on Project Model for details.




Biosafety

For biosafety, we use sodium alginate + PVA cell fixation and embedding method and semi permeable membrane to filter the engineered algae cells. We also design a device to comprehensively use physical methods to prevent the leakage of engineered algae cells. Please click on Hardware and Safety page for details.




Innovation

We are the first team in iGEM history to design the intelligent molecular magnet (MESEG) and use autophagy as a tool for the bioremediation of heavy metals from the environment. Theoretically, any type of harmful substances in eukaryotes, even pathogens, can be directed for degradation or storage through autophagy via the AIM adaptor, which makes our project has great potential as a super environmental guard.




References

  1. Briffa, J., Sinagra, E., and Blundell, R. (2020). Heavy metal pollution in the environment and their toxicological effects on humans. Heliyon 6, e4691.
  2. Chaney, R.L.(1991). The Heavy Elements: Chemistry, Environmental Impact, and Health Effects. J ENVIRON QUAL 20, 876.
  3. Ding Y, and Liu X. (2021). Remediation technology and prospect of soil heavy metal pollution. Resources Economization & Environmental Protection, 77-78.
  4. Fu, F., and Wang, Q. (2011). Removal of heavy metal ions from wastewaters: A review. J ENVIRON MANAGE 92, 407-418.
  5. Godwill Azeh Engwa, Paschaline Udoka Ferdinand, Friday Nweke Nwalo and Marian N. Unachukwu (2019). Mechanism and Health Effects of Heavy Metal Toxicity in Humans, Poisoning in the Modern World - New Tricks for an Old Dog?, Ozgur Karcioglu and Banu Arslan, IntechOpen.
  6. Jiang Y. Study on Bioremediation of Cadmium-contaminated Water by Chlamydomonas Reinhardtii [D]. Capital Normal University, 2007.
  7. Kong X, Li B, and Yang J. (2017). Research progress in microalgae resistance to cadmium stress. Microbiology China 44, 1980-1987.
  8. Li J, Wang C, and Wu M. (2015). Design of Linker Peptides and Its Application in Fusion Protein. Journal of Food Science and Biotechnology 34, 1121-1127.
  9. Riggs, P. (2013). Fusion Protein. In Brenner's Encyclopedia of Genetics (Second Edition), S. Maloy and K. Hughes, eds (San Diego: Academic Press), 134-135.
  10. RoyChowdhury, A., Datta, R., and Sarkar, D. (2018). Chapter 3.10 - Heavy Metal Pollution and Remediation. In Green Chemistry, B. Török and T. Dransfield, eds (Elsevier), 359-373.
  11. Tchounwou, P.B., Yedjou, C.G., Patlolla, A.K., and Sutton, D.J. (2012). Heavy metal toxicity and the environment. Experientia supplementum (2012) 101, 133-164.
  12. Vareda, J.P., Valente, A.J.M., and Durães, L. (2019). Assessment of heavy metal pollution from anthropogenic activities and remediation strategies: A review. J ENVIRON MANAGE 246, 101-118.