Bacteria such as E. coli have a bad reputation. They breed in unsanitary places and love to feed on excrement and human waste; however, this love of waste could prove to be the ultimate source of green energy as some bacteria produce electricity when they feed.
The idea of using bacteria to generate electricity – electromicrobiology – has been around for more than 100 years. E. coli, Clostridium, Chlorobium, Shewanella and Geobacter are all candidates, as scientists seek to take advantage of the low-temperature catalysts – enzymes – found in living cells.
So how do you go about getting electricity from bacteria? The trick is to harvest electricity from biochemical activities that take place in bacteria during cellular respiration, when energy is released from food substances, such as glucose. Respiration occurs in all living organisms and produces excess electrons in the process. In most animals, the excess electrons produced by respiration are soaked up by oxygen molecules. However organisms in environments without oxygen – such as bacteria living in mud sediments – have developed other ways around this problem. Some pass their electrons to mineral ions inside their cells, while others push the electrons out of their cells, producing a small electric current.
Of all the possible bacterial candidates for energy generation, Derek Lovley at the University of Massachusetts, US, believes that Geobacter hold the most promise. Lovley and his team first discovered the mud-loving microbe on the banks of the Potomac River in 1987, and found that they were able to ‘breathe’ iron and other metals instead of air.1
During this process, the researchers observed that an electrical charge was released, as electrons were transferred to the metals in the mud. In 2005, Lovley discovered the mechanism behind the charge:2 the bacteria have thousands of tiny molecular wires called pili embedded in their surface that allow electrons to travel along them, much like a copper wire. The pili create a thin biofilm that conducts electrons from the organism to iron in the mud or wastewater. According to Lovley, Geobacter also pass electrons to electrodes, making them ideal for use in microbial fuel cells.
Bacteria actively grow
In fact, experiments by Lovley3 show that Geobacter will actively choose to grow on electrodes. He constructed fuel cells in the lab by placing graphite electrodes into mud sediments – acting as an anode – and connecting them in an electrical circuit to another electrode – the cathode. He also constructed similar fuel cells ‘in the field’ in salt marsh sediments in New Jersey and estuarine sediments in Oregon, US. After three months, all of the energy harvesting anodes were populated by thriving communities of Geobacter.
‘Geobacter make electrically conductive filaments and these are essential for high density current production,’ says Lovley. ‘[It] directly transfers electrons to electrodes, and microorganisms closely related to Geobacter are typically found to be highly enriched on electrodes harvesting electrons from wastes and sediments. Geobacter species produce the highest current out of any microbes known (10A m-2 of electrode surface).’
What is more, Lovley and his team have also bred a strain of Geobacter that produced more pili by deleting some of their genes; the bacteria were five times more conductive than normal strains.4
Tom Clarke at the University of East Anglia, UK, meanwhile, has led studies into Shewanella oneidensis. This bacteria is found in faeces, as well as other environments that often contain heavy metals. Shewanella microbes are also capable of transmitting an electric current to metals in the surrounding environment, but, unlike Geobacter, they do not contain molecular wires. Instead, the electrons are moved from the inside of the cell to the outside by hopping across proteins called ‘multi-haem cytochromes’.5 The centre of these proteins contain metal and therefore allow the conductance of electricity; by lining up these centres, the proteins act as stepping stones, allowing electrons to hop through an otherwise electrically insulating structure.
‘In terms of current production there is still a long way for Shewanella, or any other bacteria, to go in energy production to rival a conventional battery, although it is possible to use them to charge a battery or to power a continuous, low power, device,’ Clarke acknowledges. ‘It makes them useful for remote devices, but limited as a replacement for conventional batteries.’
According to Ioannis Ieropoulos, professor of bioenergy at the Bristol BioEnergy Centre in the University of the West of England’s Bristol Robotics Laboratory, UK, one possible small scale application may be in wastewater treatment plants, whereby the bacteria could simultaneously clean up the waste by breaking down the organic content in the water and generate power to feed back to the plant. Another use, pioneered by Lovley, could be in bioelectric sensors to detect toxins and pollutants in the environment. As the bacteria feeds off these contaminants, an electric current is generated, registering a positive result.
However it is not just waste and sewage water that could be used as a power source. Batteries could be made to feed off human fluids, such as urine and blood. One line of enquiry, for example, is to harness biobatteries to power pacemakers, with the microbes getting the energy they need to produce electricity from the glucose in human blood. Such a limitless supply of power means that there would be no need for regular operations to replace batteries. A team at the University of British Columbia in Vancouver, Canada, moved a step closer to developing such a device in 2009, by creating microbial fuel cells6 consisting of a colony of yeast cells locked inside a flexible capsule made from polydimethylsiloxane (PDMS).
The prototype made by the Vancouver team is just 15mm2 and 1.4mm thick and contains Saccharomyces cerevisiae, the same yeast that is used in brewing and baking. The fuel cells can generate electricity from a single drop of human blood plasma. A compound called methylene blue effectively steals the electrons that are produced from the metabolism of glucose by the yeast, passing them to the anode part of the cell and generating a small current. At the cathode, hydrogen ions from the yeast cells combine with oxygen to form water.
The cell produces about 40 nanowatts of power, which is much less than the microwatt of power used by a typical wristwatch battery. However, this problem could be solved by pairing the cell with a capacitor, the researchers say – or by genetically engineering the yeast to produce more power.
Dealing with waste
One remaining challenge, however, is how to deal with the waste produced by yeast, which would have to be removed before it could get into the human bloodstream.
Another human waste, urine, has also been harnessed in biobatteries. In July 2013, Ieropoulos and colleagues at Bristol produced the world’s first urine-powered smartphone by passing urine through a cascade of microbial fuel cells (MFCs).7 The microbes fed on the organic compounds in the urine, and produced an electric charge as a byproduct, which was then used to charge the mobile phone. According to Ieropoulos, the microbial fuel cells gave the smartphone enough power to send SMS phone text messages, browse the web and make a brief phone call.
The process needs to be refined to power the phone for longer periods as the electricity output from MFCs is still relatively small. One single MFC will produce half a volt at best, says Ieropoulos. This means that in order to power 3V or 5V devices you need to connect more than one to multiply the voltage output. The scientists have only been able to store and accumulate low levels of energy into capacitors or supercapacitors for short charge/discharge cycles. Nevertheless, being able to directly charge the battery of a mobile phone is a breakthrough. The scientists believe that in future the technology could be installed in domestic bathrooms to harness urine to produce sufficient electricity to power showers, lighting or razors as well as mobile phones.
‘This unique feature of turning organic waste into electricity, and the fact that we can tune the size of microbial fuel cells, means that there is a potential to use the fuel cells as low power generators in remote locations,’ says Ieropoulos. ‘If they were made to be small and portable then they could act as a portable power supply for charging mobile phones when camping, or to power devices in places where there is a lack of basic facilities or utilities, such as in areas struck by natural disasters or rural areas in the developing world.’
However, the ability of bacteria like Shewanella and Geobacter to pass electrons onto metals could also be harnessed for other purposes. By reversing the process and pushing electrons back into the bacteria through their wires – say, by placing them on electrodes connected to solar panels –the electrons could be used as a power source, driving the bacteria to produce carbon compounds such as biofuels from CO2.
This process is known as microbial electrosynthesis, and is similar to the natural process of photosynthesis in plants where energy from sunlight is used to convert CO2 into complex carbohydrates. However, as bacteria often live in environments with no sunlight, they have evolved a different method where organic compounds are manufactured using energy derived from reactions involving inorganic chemicals, such as hydrogen gas, hydrogen sulphide or methane as a source of energy. Bacteria can already make some small carbon molecules by this process, but they could be genetically engineered to produce more complex molecules such as fuels or polymers. Unlike traditional bacteria that are widely used to manufacture proteins and even drugs, these microbes would not have to be provided with a costly food source such as sugar. All they would need is a source of water and CO2 to feed on, which could be provided through industrial waste, and a supply of electrons provided by photovoltaic sun panels.
‘The potential to make bacteria produce biofuels is very exciting,’ says Clarke. ‘By applying a current to the bacteria we can disrupt their metabolism and drive them to make new products - this is the new direction that this field is moving in.’
‘The best candidate bacteria for making biofuels are species of Clostridium,’ says Lovley. ‘Shewanella does not have the appropriate enzymes and is also very poor in accepting electrons from electrodes. However, we have not only demonstrated that Clostridium can accept electrons from electrodes, but have also successfully re-engineered the bacteria to produce new products from carbon dioxide.’
Cheap energy for the future
So, will waste loving bacteria provide the cheap green energy of the future? On the small scale, researchers say they may well carve out a niche as biosensors and portable biobatteries for small devices. Ieropoulos has already shown that by stacking microbial fuel cells together you can charge a mobile phone and power a digital wristwatch.
Despite the limitations, he is confident that microbial powered biobatteries will be used in the future to power devices. ‘I certainly believe so,’ he says. ‘We’ve seen mobile phones being charged by 12 fuel cells for 24 hours and this gave us 20 minutes of operation. If we scale it up, then we can accelerate this. Preliminary data shows that it’s feasible, it’s just a way of finding the appropriate tunings.
One way of increasing the power output of MFCs, says Ieropoulos, is to make them bigger; however his group is focusing on scaling down the size of the fuel cells whilst maintaining the power. ‘Currently, to produce the power provided by a standard USB output you would need a microbial fuel cell the size of a room,’ says Ieropoulos. ‘We are aiming to produce the same output with fuel cells that would be the size of a small microwave oven, but to do this we need to miniaturise the cells and stack them together in a way that maximises their power. We are aiming to stack 1000 units and at the moment we have achieved 100.’
Lovley, on the other hand, believes that efforts to improve microbial fuel cells should not just focus on engineering better fuel cell architecture and/or materials, but should also look at increasing the current-production capacity of the microorganisms.
This could be done through genetic modification. One experiment in 2014, for example, found that over-expressing a gene involved in the electron shuttle pathway of Pseudomonas aeruginosa increased the output of an MFC to 166.68 μW/cm2 – four times that of the original strain.8
Jasmin Fox-Skelly is a freelance science writer based in Cardiff, UK