Project Description

MercuLess - a cyanobacteria-based bioremediation tool for purifying mercury-contaminated waters


Figure 1. Visual overview of the project. Methylmercury (MeHg) accumulates in bodies of water and causes damage to the environment and organisms. By using modified cyanobacteria (Synechocystis sp. PCC6803), our team aims in developing a water purification system to help mercury contaminated ecosystems. The cyanobacteria is modified by adding two genes, merA and merB, that can convert extremely toxic methylmercury into elemental mercury (Hg) which is much less harmful to organisms. Figure has been created with BioRender.

Our project, MercuLess, is about utilizing cyanobacteria to bioremediate mercury-contaminated waters (Figure 1). Using the tools of synthetic biology, our team will enhance the ability of cyanobacteria Synechocystis sp. PCC6803 to transform methylmercury into elemental mercury which is then gathered up. In the lab, our team modifies cyanobacteria to express merA and merB genes that code for proteins called mercuric reductase and organomercury lyase, respectively (Figure 2). Together, these proteins can decrease the toxicity of methylmercury by first cleaving the bond between the mercury atom and the carbon atom in a methylmercury molecule and then reducing the released mercury ion into elemental mercury. To create the gene construct containing merA and merB we use a method called One-pot Golden Gate, where multiple DNA-fragments can be assembled together in a single reaction and the construct can be optimized. The produced functional plasmids are then transformed into Synechocystis whose function and mercury modifying ability we will study more in the lab.

Figure 2. Cartoon visualization of the modification of methylmercury to elemental mercury by using the proteins coded by the genes merA and merB.

To implement our bioremediating cyanobacteria, our idea is to build an on-site photobioreactor in which the modified cyanobacteria purify natural waters contaminated with toxic methylmercury (Figure 3). Since cyanobacteria are autotrophic they can utilize light as their energy source and carbon dioxide as their carbon source making them a sustainable and scalable solution. Hence the bioreactor is transparent to let light through and includes air inputs to provide Synechocystis carbon dioxide. The external energy needed for our system is provided by solar panels, ideally team Edinburgh's cyanobacteria powered solar panels, making our system 100 % cyanobacteria powered. To secure the biosafety of our photobioreactor, the reactor contains filters to ensure that no modified cyanobacteria are released to the environment and the purified water is clean and safe for us and the environment.

Figure 3. Visualization of the implementation of MercuLess: a mobile bioreactor. The purification container is transported on-site with a truck. The container will be open during the day to let sunlight in. The water from the contaminated water will go through the bioreactor containing the bioremediating cyanobacteria and the purified water returns back to the environment with pumps. You can read more about the implementation here.

The problem of methylmercury

This year our ABOA team’s project is about cleaning methylmercury out of contaminated waters. Methylmercury is hazardous for the environment and individual organisms in multiple ways. The toxicity of methylmercury is caused by its high affinity for sulfhydryl groups. The sulfhydryl-methylmercury compound can bind into proteins, disrupting their functions (Christakis et al., 2021). In ecosystems, methylmercury accumulates in large carnivores such as large predator fish and mammals. The methylmercury distracts the normal development of the fetus and young individuals, decreases fertility and causes nervous disorders (U.S. Environmental Protection Agency, 2022). Many of these larger animals such as seals and large predator fish are used as food for humans resulting that the methylmercury enriches also in us causing these same issues. These problems endanger the balance of ecosystems and harm biodiversity.

The origin of the mercury is most often fossil fuels or other mercury containing material that is burned (U.S. Environmental Protection Agency, 2023). The vaporized mercury then gets into the atmosphere and precipitates down with rain into bodies of water. In bodies of water microbes methylate the mercury into methylmercury.

In the bodies of water with high concentrations of biomass, especially marshlands, the biomass accumulates the hydrophobic methylmercury efficiently. These aged fenlands may contain decades worth of methylmercury. As they are ditched for agriculture or forestry, the sulfates in the soil react with the oxygen in the atmosphere and produce acids that dissolve the accumulated mercury. Hence the mercury that has been stored in the biomass for years, spreads into the environment with the runoff water.

Background and inspiration

In the beginning of this project our team got inspired by our native marshlands and lakes. Indeed our country is named after its many fens: fen + land = Finland, which also holds true in the Finnish language: suo + maa = Suomi. After some research on the topic we came to realize our idea could be utilized in promoting the wellbeing of our beloved Baltic Sea as well as purifying the waters of drained marshlands and mercury contaminated lakes in Finland and the rest of the world to restore them.

The Finnish marshlands have been long used for commercial forestry and agriculture and hence have been drained. As the drained waters flow downstream, the methylmercury released from the depths of the marshlands ends up in one of the many isolated small lakes there are in Finland harming its ecosystem.

The Baltic Sea has been in a horrible condition for decades although many actions have been made in order to make it a safer habitat for flora, fauna and inhabitants of its coasts. Yet, our solution could be the one that makes a sustainable impact in her health.

All in all our project has a versatile impact on the wellbeing of nature. Developing a sustainable and scalable solution for the remediation of the state of our national bodies of water will be all the more important as fresh water becomes a more scarce resource globally. There are also rising threats to food security therefore it’s important to ensure we can use fish and other sources of food safely and sustainably. Also, by improving the state of these bodies of water and protecting these ecosystems we can maintain biodiversity by providing safe habitats for species and decreasing fertility and development issues.

In addition, industries that handle heavy metals could benefit from our solution. There are for example artisanal mines in which the people and the environment come into contact with heavy metals. The waters used in these mines or some factories that produce heavy metal contaminated sewage could purify the water with a on-site purification system.

Our solution to bioremediate methylmercury-contaminated waters

Our project aims to use cyanobacteria to bioremediate mercury-contaminated waters. To achieve that, our team will enhance the ability of cyanobacteria to transform toxic organic mercury into less harmful elemental mercury as it is less hazardous to organisms and the environment. This conversion is significant as compared to the conversion of other organomercurials into elemental mercury (Dash & Das, 2012).

To reach our goal our team will genetically modify cyanobacteria, specifically Synechocystis sp. PCC6803, by overexpressing a merA- and incorporating a merB -gene into its genetic material with self-replicating plasmids. Together, MerA and MerB work synergistically to enhance the bioremediation of mercury. MerA reduces inorganic mercuric ions to elemental mercury, while MerB aids in the degradation of organic mercury compounds. Detoxifying methylmercury begins when merB-gene codes for a protein called alkylmercury lyase whose function is to cleave bonds between a mercury atom and a carbon atom in organic mercury compounds. The mercuric reductase enzyme, coded by the merA-gene, reduces mercury ions into elemental mercury. The elemental mercury can then be transported out of the cell with diffusion (Christakis et al., 2021).

Synechocystis PCC6803 has naturally a merA-gene. By transforming the cyanobacteria with a merB-gene and overexpressing the merA-gene we try to develop the modified Synechocystis into an effective tool in bioremediation of mercury. To create the construct, we are going to use the One-pot Golden Gate system, where multiple DNA-fragments can be assembled together in a single reaction. The system utilizes BbsI enzyme which generates compatible DNA overhangs that can be ligated into a plasmid. First we will transform the created inserts into E. coli and after that into Synechocystis. After transforming the Synechocystis, we will study the acclimation capacity to mercury induced stress and the mercury-modifying and removing abilities of the modified strains.

The current solutions for removing mercury from the environment have disadvantages such as high energy consumption and for that reason we are working on a solution that utilizes the tools of synthetic biology. Synthetic biology has been used for mercury remediation before but our novelty is to use Synechocystis: an organism that is capable of photosynthesis. Our solution would be sustainable and scalable as it utilizes phototrophic cyanobacteria, which both is a carbon sink and uses free sunlight as energy. The system would be cheap to use especially in areas with a stable source of light and heat like the Equator or regulated spaces. Our primary natural targets, the marshlands, small lakes and the Baltic Sea are all isolated bodies of water and the parameters can be set easily. As our organism would not be freed to the environment and the reactions would be done in a closed system, it is possible to use our bioremediating cyanobacteria in various areas and produce clean and safe water for all.

To read about the implementation of our solution in more detail, see our Implementation page.

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