PowerLeaf - a bacterial solar battery

ENERGY STORED IN CELLULOSE • LIGHT-SENSING DORMANCY SYSTEM • OPTIMISED NANOWIRES

Abstract

The PowerLeaf introduces a novel solution, for long-term storage of solar energy, thus becoming an alternative to solar cells. This is accomplished without the use of environmentally harmful resources. The device is designed to resemble a plant leaf, which is meant to provide a nature-in-city ambience. This hypothetical implementation of the PowerLeaf in an urban environment, was developed through public engagement and collaboration. The bacterial solar battery is composed of an energy storing unit (1), and an energy converting unit (2). The energy storing unit (1) is defined by a genetically engineered Escherichia Coli, which fixates carbon dioxide into the chemically stable polymer cellulose. A light sensing system activates dormancy during nighttime, to reduce energy lost by metabolism. The energy converting unit (2) uses genetically engineered Geobacter Sulfurreducens to consume the stored cellulose. Retrieved electrons are transferred by optimised nanowires to an anode resulting in an electrical current.

About Our Wiki

Functionality on the Wiki

Hi there, and welcome to our iGEM experience. I’m the creator of this wiki, and this year I decided to make it a one-page wiki - to make it stand out from the crowd. All you have to do is keep scrolling, and you will be guided through the project. You will always be able to follow your process through the wiki, by looking at the navigation bar. Whenever you want, you can click on one of the tabs, to get an overview of the sections associated with it. If you know where you want to go, you can click the section-links in each of the tab menus, and they will get you there immediately.
Since it’s a one page wiki, all of our text will be displayed on this page, which might seem overwhelming at first. Don’t worry though, to avoid unnecessary time consumption, the front page of the wiki will only contain the strictly necessary information. Whenever you need this additional information, you can always click a ‘read more’-link, which will display the information as a pop-up window. Once you have finished reading the in-depth details, you simply close the pop-up window, and continue right where you left off. We have prepared an example for you right here. Throughout the wiki, you might stumble upon this icon (book icon), which if you hover over it with the mouse, will show the references used for the associated statements.
I am fully aware of our readers being in a constant race against time, and I definitely identify with those who just want to read the essentials of the wiki as fast as possible. So, I went ahead and highlighted some of the important segments, making the entire wiki easy to quickly skim-read. Whenever you need my highlights, just go ahead and enable them in the top right corner. Go ahead and give it a try, it makes this wiki-nonsense so much easier to read. ;-)

A Green Wiki

Our project is all about ensuring a greener and more sustainable future for ourselves and the coming generations. This of course meant, that our wiki had to to follow this pursue. CO₂NeutralWebsite sponsored our wiki with a CO₂ quota equal to the amount of CO₂ produced, by having the wiki running until 31-Oct-2018. This does not mean the wiki is CO₂ neutral, but that the quote equal to its pollution is bought. Buying a quota means, that other companies won’t be able to buy this CO₂ quota, thus, forcing them to improve their environmental policies if they wish to become CO₂ neutral.

Introduction

Welcome to our wiki! We are the IGEM team from the University of Southern Denmark. We have been waiting with great anticipation, for the chance to introduce you to our project, and to tell you our story.
It all began with a meeting between a group of strangers, each with their own disciplinary origin. Despite our differences, we had one thing in common; a shared interest in synthetic biology. Soon after our first meeting, we were herded off to a weekend in a cottage - far away from our regular lives. It was a place to bond, and to discuss project ideas. It immediately became apparent, that being an interdisciplinary team, was going to be our strength. Each member had unique qualities, that enabled them to efficiently tackle different aspects of the iGEM competition. So we made it our goal to take advantage of these qualities.
We decided to make a proof-of-concept project. Specifically, we wanted to use bacteria as a novel and greener solution for solar energy storage. This project was later dubbed the PowerLeaf – a bacterial solar battery. There is so much to tell you about our bacterial solar battery and its’ amazing features.
Since it’s a one-page wiki, you will be taken on a journey, and introduced to our story throughout the wiki-page. Just keep scrolling, and we will gently guide you through our project.

Achievements

Bronze Requirements 4/4

Register and attend - Our team applied 2017-03-30 and got accepted 2017-05-04. We had an amazing summer and are looking very much forward to attending the Giant Jamboree!
Meet all the deliverables requirements - You’re reading the team wiki now, so that’s one down. You can find all attributions made to the project in the credits section of the wiki, here. The team poster and team presentation is all ready to go for the Giant Jamboree. We also filled the safety form(link), the judging form (link) and all our parts were registered and submitted (link) in time.
Clearly state the Attributions - All attributions made to our project has been clearly credited in our credits section at the end of the wiki.
Improve and/or characterize an existing Biobrick Part or Device - pending

Silver Requirements 3/3

Validated part/contribution - pending
Collaboration - We have collaborated with several teams through project discussions, meetups, answering questionnaires and even hosted our first meetup ever, for our fellow Danish teams. You will get to read all about it in the credits section of our wiki.
Human Practices - Our philosopher, historian and biologist have discussed the bioethical (link) aspect of our project in great detail. In extension to their work, we have been working extensively with public engagement and education (link).

Gold Requirements 4/4

Integrated Human Practices - We have been in close relationship with city planners from our hometown on the development and implementation of the device (link). This regards anything from size, shape, color, plastic types and locations to implement it.
Improve a previous part or project - pending
Model your project - There has been made extensive modelling of our light sensing system, which proved to be highly informative. Through this we learned that it was possible to control the bacterial dormancy, by insertion of both the toxin and the anti-toxin where only the anti-toxin is regulated. You can read all about the project modelling here.
Demonstrate your work - pending

World Situation

A Global Problem

The planet’s ecosystem is currently on a path out of balance, caused by our extreme abuse of its natural habitat and valuable resources, for the advancement of our society. An ecosystem in imbalance can have a severe impact on our everyday lives. The unique diversity of the Earth’s inhabitants, will not be the only consequence of such an imbalance. As a more extreme example; scientists believe that humankind, could go extinct, as our fervor drives us to further better our ways of life. Restoring this balance should be the highest priority, in the advancement of our civilization.
One of the most recognized approaches to restore this balance, is the advancement and implementation of technologies for renewable energy. However, there are currently certain limitations on the existing solutions for renewable energy; the intermittency- and the diluteness problem. The intermittency problem describes the discontinuous energy production, with inefficient storage. Whereas the diluteness problem is described as the resource demanding production of technical devices, such as solar cells and batteries.

In a Local Environment

We are a team of young adults, raised to be aware of climate changes and the potential limitations to the continuation of our way of life. We are also a generation that appreciates open source and shared information. A generation, that has been encouraged to constantly challenge the ideas of our peers. With this in mind, we decided the best solution to the eventual energy crisis, would be to seek out experts, the general public, even children, in order to rethink the current notion; That the only way to save our planet, is to compromise our living standards.
Thankfully, through interaction with local experts, we learned that a great deal of people share our belief; that we ought to pursue the advancement of low energy cities with a high quality of life. In fact, we even discovered that our own hometown – Odense – wants to be the greenest, most renewable city in Denmark by 2050 https://www.odense.dk/borger/miljoe-og-affald/klima.
We decided to pursue this goal, by taking on the challenge to create a truly green solution, which will provide both an environmentally friendly source of energy, as well as green aesthetics and naturalistic ambience to compliment a high quality of city-life.
Please keep scrolling if you wish to read more about our solution, or go straight to bioethics, if you wish to read about why we not only could, but ought to do something about the current energy crisis.

Inspiration

Our early ideas were reviewed after attending the Danish Science Festival, where we met several young minds with creative and inspiring ideas. The children would come to our booth with their parents to learn about bacteria, GMO, ethics and iGEM. After which, they would attend our “Draw-a-Bacteria”-competition. While drawing their own unique bacteria, they would present us with detailed stories about their design.

See a selection of their amazing drawings here.

Through this, we felt inspired and decided to revise our ideas. At this point, our project was starting to take shape. They even inspired the physical design for our final product; the PowerLeaf.

Our Solution

Our Solution

The bacterial solar battery we envision, is composed of an energy storing- and an energy converting unit. The energy storing unit is defined by a genetically engineered Escherichia Coli (E. Coli). The E. Coli uses solar energy for ATP production, to fixate carbon dioxide into the chemically stable polymer, cellulose, which essentially is the battery. A light sensing system activates dormancy during nighttime, in order to reduce energy lost by metabolism. The energy converting unit uses genetically engineered Geobacter Sulfurreducens to consume the stored cellulose, using an inducible switch. Retrieved electrons are transferred by optimised nanowires to an anode, resulting in an electrical current. The complete system will be combined into a single device containing a compartment for each of the two units. Details about the construction and device will be discussed in the Integrated Practices section.
The device is designed to resemble a plant leaf, which is meant to provide a nature-in-city ambience. This hypothetical implementation of the PowerLeaf in an urban environment, was developed through public engagement and collaborations. We even worked together with local city planners from our hometown, Odense.
Our vision was clear and ambitions were high, probably too high, considering the limited timeframe. So, at an early stage, we decided to focus on the following features:

Light sensing system to regulate dormancy
Converting CO₂ to glucose
Production and secretion of cellulose
Converting cellulose to glucose
Optimising the nanowires

It will then be up to prospective iGEM teams to continue on the development of the PowerLeaf. We would love to see our project become a reality one day, and have therefore created a special page for future iGEM teams. This page includes suggestions for further development of the project.

Project & Results

We have throughout the project worked on the development of 2 units for our device, an energy storing and an energy converting unit. Each of the systems we worked on for the units can be seen here:
Energy storing (E. Coli)

Light-sensing dormancy
CO₂ fixation
Cellulose production and secretion

Energy converting (G. Sulfurreducens)

Breakdown of cellulose
Optimised nanowires

Once you reach each of the 5 systems in the 'Project Design'-section, you will first be given a short introduction to the underlying theory, which you will be able to expand on, by pressing “read more”. Which in turn will open a pop-up window with the additional information. After the theory, you will be given the approach used in each of the respective systems for the project. Before continuing on to the next system. To make things easier on you, we have developed icons to each of the above systems which will be used throughout the rest of the wiki.

Project Design

lets gooo

hello my friend

Light Sensing System

Theory

Cyanobacteria contain signal transduction systems, thereby making them capable of sensing and respond to light Bussell AN, Kehoe DM. Control of a four-color sensing photoreceptor by a two-color sensing photoreceptor reveals complex light regulation in cyanobacteria. Proceedings of the National Academy of Sciences of the United States of America. 2013;110(31):12834-12839. doi:10.1073/pnas.1303371110.. This ability gives the organisms the opportunity, to adapt and optimize their metabolism to a circadian rhythm. Photoreceptors in the plasma membrane, of which phytochromes are especially abundant and well described, are responsible for this property http://www.sciencedirect.com/science/article/pii/S1084952100902064?via%3Dihub. In 2004 the Austin iGEM team made a light response system consisting of a photoreceptor combined with an intracellular indigenous regulator system https://www.ncbi.nlm.nih.gov/pubmed/16306980. EnvZ and OmpR makes up the two-component system naturally found in E. coli. The photoreceptor from phytochrome known as Cph1 was isolated from the cyanobacteria Synechocytis PCC6803. Cph1 has functional combination sites, which combined with the kinase EnvZ forms a two-domain receptor, known as Cph8. Activation of Cph8 is mediated by the phycocyanobilin, PCB that is sensitive to red light.
When not exposed to light the photoreceptor PCB activates the phytochrome Cph1, thereby promoting kinase activity through the EnvZ kinase. When the the transcription factor OmpR is phosphorylated by EnvZ, expression of genes regulated by the OmpR-regulated promoter is initiated. Excitation of PCB by red light, results in a situation where the transcription factor OmpR is not regulated. The absence of phosphorylated OmpR leads to no activation of the OmpR-regulated promoter, thereby preventing gene expression.

Using the photocontrol device to set up a A toxin-antitoxin system is a system composed of two gene products, of which one specifies a cell toxin and the other an antitoxin, which neutralizes the toxic effect caused by the toxin. In E. coli K-12 the cytotoxin RelE and antitoxin RelB comprise such a system https://www.ncbi.nlm.nih.gov/pubmed/9767574. Expression of the cytotoxin RelE inhibits translation in the cells, due to its ability to cleave mRNA found in the A-site of the ribosome. The indigenous cause of RelE expression is amino acid starvation https://www.ncbi.nlm.nih.gov/pubmed/12526800. Whether the cell lie dormant in response to expression of RelE depends on the ratio of antitoxin RelB and RelE present in the cell. Several studies have shown that RelB:RelE forms a complex with RelB:RelE stoichiometry of 2:1 https://www.ncbi.nlm.nih.gov/pubmed/19747491https://www.ncbi.nlm.nih.gov/pubmed/18532983. When the RelB:RelE stoichiometric-ratio is lowered to 1:1, studies show that RelB is not able to protect the cells against the RelE-caused translational inhibition https://www.ncbi.nlm.nih.gov/pubmed/18532983. Read more here

Theory

To introduce an efficient energy harvesting system, the E. coli should lie dormant during nighttime, thereby reducing the energy used for the bacteria’s basic metabolism. For this purpose a photocontrol system created by a former iGEM team is used in combination with a toxin-antitoxin system found in E. coli, to make the bacteria able to respond to the environmental exposure of light.
Plants and several photosynthetic microorganisms, such as cyanobacteria, contain signal transduction systems, which makes them capable of reacting to light [12]. This ability gives the organisms the opportunity, to adapt and optimise the regulation of their metabolic rate in response to sunlight. This property is achieved by photoreceptors incorporated in their plasma membrane, of which phytochromes are the most abundant and well described photoreceptors. [1]
Several two-component signal transduction systems evolved in E. coli enables it to respond to various external conditions, such as osmotic stress, lack of metabolites and other external stress factors. Nothing indicates that light initiates such a two-component signal transduction pathway in wild type E. coli [9]. The Austin iGEM team from 2004 applied the light sensing property of phototrophs to an E. coli [10][11]. By aligning different phytochromes with the intrinsic kinase EnvZ from E. coli they revealed a way to create a two-component system consisting of a photoreceptor with an intracellular indigenous regulator system found in E. coli. By establishing this system the bacteria acquired the ability to respond to red light. [3] The photoreceptor from phytochrome known as Cph1 was isolated from the cyanobacteria Synechocytis PCC6803. Cph1 has a fusion site, which can be used to combine it with the kinase EnvZ, from the EnvZ-OmpR kinase-regulator system, to form a two-domain receptor known as Cph8. The phycocyanobilin, PCB, which is sensitive to red light and only expressed in genetically engineered E. coli, can, in combination with the light receptor Cph8, be used to form a light-sensitive circuit, making E. coli able to respond to red light. [2]
In situations of no exposure to light, the photoreceptor PCB activates the phytochrome Cph1, thereby promoting kinase activity through the EnvZ kinase. Illustrated in figure #. When the the transcription factor OmpR is phosphorylated by EnvZ, expression of genes regulated by the OmpR-regulated promoter is initiated. Excitation of the PCB by red light, results in a situation, where EnvZ won’t be able to phosphorylate the transcription factor OmpR. The lack of phosphorylated OmpR leads to no activation of the OmpR-regulated promoter, thereby preventing gene expression by this promoter.
A toxin-antitoxin system is a system composed of two gene products, of which one specifies a cell toxin and the other an antitoxin, which neutralizes the toxic effect caused by the toxin. In E. coli K-12 the cytotoxin RelE and antitoxin RelB comprise such a system. [4] Expression of the cytotoxin RelE inhibits translation in the cells, due to its ability to cleave mRNA found in the A-site of the ribosome. [5] RelB neutralize the toxic effect of RelE through interaction between the two proteins. In situations of amino acids starvation, it is appropriate for the bacteria to halt the translation, in order to avoid errors owing to absent amino acids. Consequently, one of the exciting factors for the expression of RelE is conditioned by amino acid starvation [5].
Whether the cell lie dormant in response to expression of RelE depends on the ratio of RelB and RelE present in the cell. Several studies have shown that RelB:RelE forms a complex with RelB:RelE stoichiometry of 2:1. [6][7] When the RelB:RelE stoichiometric-ratio is lowered to 1:1, studies show that RelB is not able to protect the cells against the RelE-caused translational inhibition. [7] To prevent free RelE circulating and discharging toxic effects in the cells under favorable conditions, studies in vivo have shown that RelB is present in 10x higher concentrations than RelE. [6]

Approach