Bio-batteries in the seabed

Case

Bio-batteries in the seabed

Bacteria that make electricity: the holy grail of materials science

There are bacteria in the seabed that produce electricity and conduct it over long distances. It is a spectacular discovery. How do they do this and under which conditions? The research team at the Royal Netherlands Institute for Sea Research (NIOZ) lead by researcher Filip Meysman has received a grant from the European Research Council to find the answers to these questions. There is a good chance that this research will lead to new applications for bioelectrical systems.

Danish researchers discovered something in 2010 that was so extraordinary that no one believed them. While conducting a laboratory experiment with samples taken from the seabed, they found bacteria that generate electricity and conduct it over long distances. 'I didn’t believe the Danes either,' Filip Meysman admitted. 'Until we detected the process in our own laboratory too, in experiments actually set up for a different purpose. After that, we specifically went looking for them in the North Sea and the Delta region and we discovered that there really are bacteria on the seabed that make electricity.'

It is a spectacular discovery that may offer opportunities for developing entirely new biomaterials. Meysman: 'Efficiently conducting electricity in organic materials is the holy grail of materials science. A great deal of research is being done, for example, into making light, flexible solar cells. And now it transpires that a bacterium has already devised that trick.'

What these bacteria do is unique for all life
- Filip Meysman

Unique life form

'What these bacteria do is unique for all life,' Meysman continues. 'All living cells need energy and their energy provision takes place in a similar way whether they are in bacteria or elephants. A cell needs a substance from which it can collect electrons and a substance to which it can give electrons. A muscle cell in our body, for example, gets electrons from sugars and gives electrons to oxygen. In chemistry, a reaction of this kind with an electron donor and an electron acceptor is called a redox reaction. All living cells get their energy from redox reactions.

The newly discovered bacteria are special because they split a redox reaction in two and specialised cells carry out just one part of the reaction. It is just like having two brothers where one only breathes in and the other only breathes out. Cells at one end of the bacterium carry out one part of the redox reaction: electrons are removed from the electron donor. These electrons are subsequently transmitted via a conducting cable to a fellow cell at the other end of the bacterium. This takes over the electrons and delivers these in the other half of the reaction to the electron acceptor. The bacterium benefits from this sophisticated form of collaboration between cells.

Battery

Splitting the redox reaction over two different places is exactly what happens in batteries. 'Volta invented the electrical battery in 1800. Now it would appear that bacteria discovered this millions of years ago and have used it to increase their chances of survival on the seabed.'

While human muscle cells use sugars and oxygen for their redox reaction, these bacteria on the seabed take their energy from a redox reaction between oxygen and sulphide. The oxygen is present in a very thin layer on the surface of the seabed. The sulphide lies deeper in the seabed. 'The bacteria form sorts of spaghetti strands in the seabed comprising 20,000 to 50,000 bacteria that pass these electrons to each other. The tail is at a depth of a few centimetres in the seabed; it collects electrons from sulphides there and passes them along the strand. In the cells at the top end of the long bacterium the electrons are released in a reaction with the oxygen.

'The flow goes in one direction through the strand of bacteria, from the bottom up. To compensate for the difference in electrical load, a flow of positive ions also goes upwards. This is also what happens in a battery, where the transport of ions takes place by means of a salt bridge. So what is in the seabed is a completely biological battery.'

Key question

Of course the crucial question is: how do these electricity-producing bacteria do that? To conduct research into this question, Meysman received a grant of 1.5 million euros from the European Research Council in 2012, for five years’ research. Another question is how important these new bacteria are for natural coastal ecosystems. And of course: what are the application possibilities? Meysman: 'Up until now, bacteria only assist as a catalyst in experimental biofuel cells, but apart from that, the fuel cells work like traditional batteries. Things are different in our bacteria in the seabed: they themselves form the entire battery. Once we have a better understanding of how that works, we will undoubtedly find applications for bioelectrical systems. There is already a little lamp in the NIOZ lab that is lit entirely by electricity produced by the bacteria.'

Cultivation

Before molecular research can take place into how the bacteria produce and conduct electricity, the right cultivation method will have to be found. 'Up until now, we have only been able to cultivate the bacteria in the lab in natural samples from the seabed. However, these samples also contain thousands of other bacteria. We will need to cultivate them much more purely in order to conduct molecular research into their internal structure. That is one of the big challenges.'

Evolution

There is also a fundamental line of research. 'The bacteria are a wonderful example of how ingeniously evolution works: oxygen is above and sulphide is below. The bacteria therefore have to try to form a bridge between the two. There are bacteria that have found a different solution: a sort of refuelling system. They fill up their tanks with nitrate on the surface of the seabed as electron acceptors and then take their full tank into the seabed for a redox reaction with the sulphide. When the tank is empty, they go up again. Evolution has therefore provided several ways of doing this and they appear together in the same places. But when is one method the best and when is another better, and why? That's what we want to understand.'

'Incidentally, the fundamental research has an important applied aspect too. We are conducting the research in Lake Grevelingen. There are problems with the water quality there in the summer due to a lack of oxygen. The bacteria we are studying ensure that sulphide is removed deep in the seabed so there is a reduced chance of this sulphide from the seabed diffusing up to the water above. Sulphide is extremely toxic for life on the seabed and it harms the oyster cultures in Lake Grevelingen. It also causes the sulphur smell, which is very objectionable in an area with a lot of water tourism. That is another reason why we need to develop a better understanding of these bacteria.'

SedimentSediment from the seabed examined under an optical microscope. The tangle of long white threads is bacteria that are capable of generating and conducting electricity. (Copyright: Sairah Malkin, NIOZ).

ElectrodesThe newly discovered bacteria offer a perspective for the development of new bioelectric systems. When electrodes are placed in the sediment, a red LED light flashes. This implies that the sediment is generating electricity and works as a microbial fuel cell. (Copyright: Sarah Engelhard, NIOZ)