Team:Valencia/Modeling

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Overview

Our aim in this part of the Project is the development of a model which describes how the different phenotypes of yeasts interact with the prion switch dynamics. We have considered very interesting to take advantage of the model proposer by Watson and Lovelock (1983) Daisyworld, and improve the modeling incorporating the dynamic of the prion switch, in order to see how the system behaves and finally how the stabilization of the planetary temperature is made when the system reaches a steady state.

Also we will incorporate in this section, some modeling we had to perform in the previous stage of constructing our Testing Technologies: The Red House, and the Microbial Albedo Recorder. Because they uses similar temperature sensors and acquisition systems, we are going try to make some modeling concerning those systems.

Modeling Mad Yeast On Mars

Introduction

The real system we want to model is formed by three basic but different elements:

• Physical element: Mars, the planet we want to terraform. It is defined by a series of physical parameters. Mars is the environment, the frame in which the terraforming takes place.
• Biological element: Saccharomices cereviseae, the yeast we used as chassis or vector to colonize the planet. The yeasts are the vector, the way it has been used to achieve this terraforming goal.
• Molecular element: prionic circuit that works as a switch between yeasts that express different proteins. The prionic switch is the key mechanism that allows the terraforming process itself.

A model is not just a tool that science and engineering use to allow us to understand the reality that surrounds us. Moreover, it helps us to explain and predict the physical principles of universe we live in. Modeling is by definition simplifying. We use simply it because it is useful.

There are some climate models that intend to explain the global physical characteristics of Mars. These kinds of models are extremely complex. Hundreds of different variables are connected by equations and networks. Honestly, they are far beyond any iGEM team could achieve with some months of working. It is simply too complex. In fact, they are not really efficient in predicting anything, not even the ones on Earth. Global climate is something still too complex to be represented in a model with a minimum of confidence.

Therefore, what did we do? Well the answer is not really so difficult. We focused primarily in the very definition of the term model. This way, we develop a group of mathematical equations that allow us (and more important, you!) to understand how this hypothetical terraforming process would eventually move on. We used as inspiration a model known as Daisyworld developed by Watson and Lovelock (1983) for other totally different purpose, which was to support the Lovelock Gaia Hypothesis. Daisyworld is a static (mind this!) and very simple model of a planet which is able to regulate its own temperature using the living organisms living in it, in this case, daisies. We named our model in a similar way to pay tribute to that excellent piece of work.

Welcome to the Yeastworld: theoretical background

Yeastworld is a simplistic version of Mars. Some input parameters of the model derive from the real values on Mars: solar luminosity, distance from the Sun, lack of atmosphere, initial temperature and background albedo. At the beginning in the Yeastworld there is no life at all (Figure 1).

Then, black pigmented yeasts (or [psi ^-] yeasts) appear thanks to human action. These black yeasts are able to grow under the medium-low temperatures that mimic the conditions of a partially terraformed Mars. As they start to spread, they progressively expand and cover an important portion of the planet surface. Due to the albedo effect reduction (see Microbial Albedo Recorder for more details), the entire planet starts to warm up (Figure 2).

As we explained in the prionic switch section, even in this conditions some white ([PSI ⁺]) colonies will appear. However, their presence will become relevant as the temperature increases. The prion-based switch allows the change from black to white yeast by stopping the pigment expression. Together with this molecular underlying process, the [PSI ⁺] yeasts will be selected in favour in these warm conditions. This is due to their intrinsic protection against overheating and their ability to reduce the local temperature. Therefore, the white yeasts will finally cover a big portion of the Yeastworld surface. This will importantly increase the albedo effect of the planet and reduce the global temperature (Figure 3).

In this situation, the opposite process will take place. At low temperatures the black yeasts will have a higher growing rate because of the elevated inner temperature at which their cellular processes will take place, so they will again displace the white cells. Together with the fact that the prion activation will not be highly promoted, this will result in a major abundance of the black [psi ^-] yeasts, thus increasing again the temperature.

This cycle of white and black yeasts goes on and on and finally ends in a steady state (Figure 4) in which the amount of each kind of yeast is constant and so it is the global temperature (and albedo). Therefore, we can regulate the Yeastworld temperature by using our engineered yeasts. This capability of regulate the temperature of an entire planet is one of the most important points of our proposal of Terraforming process.

It is also interesting to remember that our yeasts could express LEA protein aswell so their resistance to extreme temperatures is improved. Moreover, note that the key parameter studied and modeled here is not the temperature but the albedo. The temperature is only the physical consequence that arises from modifying the albedo of the planet.

It is important to keep in mind that the black and white yeasts and the Yeastworld background albedo are different and that this fact is the key concept that allows the process to work. For checking the differences in albedo and temperature in the different color cultures check the Microbial Albedo Recorder section.

Yeastworld Mathematics

Red House and Microbial Albedo Recorder modeling