Team:Sorbonne U Paris/Implementation

IGEM

Implementation / Perspectives

Our project focuses on the protection of Chlamydomonas reinhardtii against radiations. As we observed by studying organisms that are naturally radioresistant, various strategies exist. Those mechanisms are converging into two trends :

  • A strong DNA repair system to overcome heavy damage
  • A highly efficient DNA and protein protection strategy[1][2].

In order to successfully protect C. reinhardtii, we chose to work on protection against damage induced by ionizing radiations. Those damages can directly affect macromolecules, for example by causing double-strand breaks (DSBs), or indirectly owing to the production of reactive oxygen species as a result of radiolysis of water molecules[3](figure1).

Figure 1: Illustration of the damage caused by ionising radiation. Based on a figure by Jonathan Roseland[4]. [image description: Direct ionization causes DNA damage (double-strand breaks) and Indirect ionisation causes a redox of water molecules, which lose a hydrogen atom and form reactive oxygen species (ROS) that damage DNA.]

The Chlamy’N Space project consists of the expression of an exogenous antioxidant gene in Chlamydomonas reinhardtii. This small antioxidant is a rationally-designed peptide inspired by small antioxidant peptides coming from the extremophile Deinococcus Radiodurans, naturally resistant to radiations. It was already tested and showed great efficiency in the radioprotection of mice and human cells[5].

But why do we want Chlamydomonas reinhardtii to become radioresistant?

When designing our project we thought about the different applications it could have. We identified 3 of them:

Using radioresistant C. reinhardtii for long-term space travels
Use of ROS-protected Chlamydomonas reinhardtii for the decontamination of irradiated areas and the water filtration
Global advancement in the understanding of oxidative-stress solutions

Radioresistant Chlamydomonas reinhardtii for long-term space travel

Long-duration space travel poses new technological challenges. Bioregenerative life support systems (BLSS) provide a solution to the constraints of waste generation and lack of food or oxygen resources. They are loop systems based on the principle of matter cycles (carbon and nitrogen cycles) and allow for the continuous recycling of system resources[6].

ESA (European Space Agency) is one of the agencies that have decided to take up the challenge, with the MELiSSA (Micro-Ecological Life-Support System Alternative) project[7]. The system consists of five chambers containing different types of bacteria performing their own functions such as nitrification, oxygen production or biomass. One of these chambers is a photobioreactor in which the photosynthetic activity of the microorganisms grown in the chamber allows them to consume CO2 and produce O2.

Figure 2: MELiSSA project. [image description: This illustration shows the cycle of the matter inside of the BLSS. The crew is providing microorganisms with urine and feces that are treated to produce nutrients that are then going to be used by plants to grow. Both the crew and microorganisms provide carbon dioxide that is converted into oxygen during photosynthesis. Phototrophic organisms are providing microorganisms and the crew with oxygen and clean water. The plants are eaten by the crew.]

Chlamy’N Space emerged from our common interest in these loop systems.

Christophe Lasseur, responsible for the MELiSSA program, made a conference in 2017 where he reported the relevance of using microalgae in closed systems[8]. Like all photosynthetic organisms, the microalga Chlamydomonas reinhardtii has the ability to convert carbon dioxide into sugar and oxygen to provide energy for the alga and to enable the respiration of living things. It is therefore a perfect candidate to integrate the photobioreactor of a BLSS. It could act on several levels such as oxygen production and carbon dioxide absorption or in the filtration of liquid waste. Moreover, C. reinhardtii is a well-known organism, and a model for many years[9].

Despite this, space remains a hostile environment. We know that humans suffer from excessive radiation exposure, but this is also true for algae. Without the protection of the magnetosphere and Earth's atmosphere, C. reinhardtii will be exposed to a lot of cosmic radiation that will have a deleterious effect on its cellular and genetic integrity. If space agencies wish to maintain a constant production of oxygen, it is necessary to protect them throughout the flight. The use of radioresistant modified Chlamydomonas reinhardtii could be an effective solution in this context.

Figure 3: Radioresistant modified Chlamydomonas reinhardtii. [Image description: This illustration shows how the manganese and the peptide form a complex that surrounds the ROS to convert it into water. This happens inside of Chlamydomonas reinhardtii’s cytoplasm. On the image, there is a Chlamydomonas reinhardtii on the side and a focus on its cytoplasm.]

Use of ROS-protected Chlamydomonas reinhardtii for the decontamination of irradiated areas and the water filtration

Radiation is not only present in space, there are many examples on Earth. Some areas are heavily polluted with radioactive compounds and the clean-up of these places is a real challenge. Microalgae are already being used as a tool for bioremediation, i.e. decontamination of an environment with chemical compounds by a microorganism[10]. The next step would be to use Chlamydomonas reinhardtii to accumulate radionuclides in irradiated areas and thus clean up the water around radioactive sources. However, these radioactive areas induce the production of intracellular ROS which damage the cells of the microorganisms and can lead to the death of the organism. Our Chlamy'n Space project can provide a solution to this type of problem. We can imagine a closed system for filtering water, in which there would be a compartment containing our manganese-resistant Chlamydomonas reinhardtii.

More generally, Chlamydomonas reinhardtii is known for its filtration capacities. It is for this reason that our predecessors, the iGEM team of Sorbonne University 2020 had designed Chlamydomonas reinhardtii to retain and degrade a toxic chemical compound: Atrazine[11]. Our project can be a continuation of theirs. Indeed, we can imagine combining the two to optimise the filtering capacities of Chlamydomonas reinhardtii by protecting it from stress-induced pollutants.

SEMiLLA IPStar is also developing applications on Earth of the closed system of ESA's MELiSSA project. One of their projects is a water filtration circuit for the University of Kenitra in Morocco to filter nitrates from groundwater. Our alga Chlamydomonas reinhardtii could therefore also be included in this closed system in order to achieve this filtration while being protected from oxidative stress.

Global advancement in the understanding of oxidative-stress solutions

The previous possible uses of our project mainly took advantage of our chassis as a microalga. It is the use of the capabilities of microalgae, under stress conditions, that justifies our implementation. But we can focus more on the mechanism of response to oxidative stress itself. Because even if some research teams focus a large part of their research on the understanding of this mechanism in Chlamydomonas reinhardtii[12], oxidative stress is universal and there is an incalculable number of subjects of study on this subject on all types of living organisms.

  • In humans, oxidative stress is involved in cell ageing[13].
  • Studies are also being carried out on the regulation of dormancy by ROS in the Arabidopsis thaliana seed[14].
  • Last year, the UNSW Australia 2020 team carried out a project on the protection of corals by the neutralization of reactive oxygen species.

Oxidative stress is everywhere! The use of a ROS scavenger complex such as the one in our project is part of this context of research on oxidative stress. Our research provides additional data on the understanding of this phenomenon and, above all, on the potential solutions that could be envisaged in the context of mediating oxidative stress. We also provide additional data on the knowledge of prokaryotic gene transfer in a eukaryotic organism.

Bibliography

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