Bacteria growing on particles of magnetic crystal may form natural rechargeable batteries, according to new research. Scientists at the University of Tübingen in Germany have found that colonies of two types of bacteria can grow on a mineral called magnetite. They found one species of bacteria could dump electrons onto the crystals while another harvested the electrons from them depending on the amount of sunlight they received. Scientists have found that colonies of two types of bacteria can grow on a mineral called magnetite. Magnetite is an iron (III) oxide mineral that forms smooth black crystals, like those shown above, in rock. One species of bacteria dumped electrons while another harvested  from them depending on levels of sunlight . The scientists said that in the wild this may allow the magnetite crystals to act like natural rechargeable batteries for the bacteria. The findings has raised hopes that these 'batteries' could be used to help develop new sources of energy, although the charge they currently hold is minute. Life has been discovered nearly a mile and half underground, raising hopes bacteria might be able to survive deep beneath the surface of other planets. Researchers found the single-celled organisms living inside coal beds more than 8,000ft (2,440 metres) below the seabed off the coast of Japan. It is the deepest life has yet been found beneath the ocean. At those depths the spherical microbes have to survive huge pressures that would crush most organisms while having no access to light or oxygen often needed for life. Instead, the bacteria scavenge the chemicals they need to survive by breaking down the hydrocarbon compounds in the coal around them. The findings have raised the prospect that life may still be clinging on deep underground on planets such as Mars, where there was once thought to be flowing water on the surface. Dr James Byrne, who led the work at the University of Tübingen, said bacteria could be genetically engineered to increase this activity. 'In our study we only looked at iron metabolising bacteria, but we speculate that it might be possible for other non-iron metabolising organisms to use magnetite as a battery as well - or if they can be made to use it, through genetic engineering,' he said. 'Could engineering the bacteria increase the energy production or is it limited by the magnetite itself? 'The magnetite itself will only hold a maximum number of electrons, which is one of the limiting factors. 'However, the bacteria access these electrons through specialized enzymes known as cyctochromes. 'In principle, if the bacteria could be engineered with more of these electron carriers, then it could increase the speed at which they use the magnetite as a battery. 'It is probably not something that could be used for an application in the immediate future though.' The researchers, whose work is published in the journal Science, incubated a species of purple bacteria called Rhodopseudomonas palustris, which is found in soil, and another bacterium called Geobacter sulfurreducens. This graph shows the percentage change in charge on the magnetite caused during daylight and nighttime conditions. Rhodopseudomonas palustris removed electrons from the magnetite, causing the charge to fall while Geobacter sulfurreducens added electrons to increase charge on the mineral during the night . These were grown on nano-sized crystals of magnetite - an iron oxide based mineral found in rock formations that was used to make lodestones. The research team then varied the light levels to simulate the effect of night and day. They found that during the 'day time' Rhodopseudomonas bacteria harvested electrons from the magnetite, reducing its electric charge. During the night, the Geobacter took over and dumped electrons back onto the magnetite crystals, recharging it. Geobacter adds electrons to the iron in the crystals while metabolising other organic material. The researchers grew the bacteria on nano-sized magnetic particles of magnetite (left), which can be seen gathering on the inside of the bottle beside the magnet on the right in a solution of purple bacteria . Dr Byrne said the findings suggest a way in which bacteria could survive in extreme environments deep under ground. He said: 'The geochemistry is interesting in itself, but there are also potentially useful implications which may derive form this work. 'The flow of electrons is critical to the existence of all life and the fact that magnetite can be considered to be redox active opens up the possibility of bacteria being able to exist or survive in environments where other redox active compounds are in short supply in comparison to magnetite.' The researchers also believe the findings could also be used to help clear up toxic metal pollution, using bacteria coated magnetite to convert the metal into safer forms. There are a number of species of bacteria that have been found to eat electrons by harvesting them from metallic particles and others that spit them out. This diagram shows the conditions that the scientists believe leads to the bacteria (marked in pink) adding or removing electrons to the magnetite (the black circle) according to water levels and sunlight available . These are generally found on seabeds and river beds, using energy in its purest form to power the reactions inside their cells. Most living species use a complex cascade of biochemical reactions to release the energy they need from their food. For example, most vertebrate cells break down glucose sugar to release packets of energy in the form of molecules known as ATP. These bacteria, however, appear to harvest energy directly from the electrons freed as they grow on the surface of iron minerals. Rhodopseudomonas palustris (above) harvests electrons to power the metabolic processes inside its cells. Most living species use a complex cascade of biochemical reactions to release the energy they need from their food but the studied bacteria appear to harvest energy directly from the electrons as they grow . Professor Andreas Kappler, another of the authors of the study and secretary of the European Association of Geochemistry, said: 'This may have some interesting geochemical applications. 'There has been considerable recent work on using magnetite to clean up toxic metals. 'For example, magnetite can reduce the toxic form of chromium, chromium VI, to the less toxic chromium (III), which can then be incorporated into a magnetite crystal. 'The fact that this magnetite may then be exposed to these reducing bacteria could potentially enhance its remediation capacity. 'But we are still at an early stage of understanding the bioengineering implications of this discovery.'
Scientists at University of Tübingen, Germany, grew bacteria on magnetite . One species of bacteria harvested electrons from the mineral and another  dumped electrons back on iron in the mineral at night . The amount of energy stored is too low to be used as a new power source . But other species may create stronger charges for natural batteries .