This microscopic image shows cable bacteria reaching out for oxygen. The milky line is the deformed oxygen front, which contains smaller bacteria drawn to the interface with the lower oxygen-free layer. (Image Credit: Stefano Scilipoti)
A decade ago, Aarhus University researchers discovered cable bacteria that survive by conducting electricity over centimeter-long distances in marine and freshwater sediments. Now, the team has reported that some cells function with high oxygen consumption. Meanwhile, the remaining cells process food and grow without oxygen. Cable bacteria access oxygen at the bottom of a lake and sea's surface, whereas food is found a few centimeters beneath.
"While other organisms try to solve the problem by moving oxygen and food up and down, cable bacteria have developed electric wires. When consuming food, they produce electrons and send them through the 'biowires' to the surface for reduction of oxygen from the overlying water," says Lars Peter Nielsen, head of Center for Electromicrobiology, Aarhus University, Denmark.
A cable bacterium contains multiple cells, which are capable of growing centimeters in length. The cells are encased in a common coat where the wires extend. The team put the cable bacteria inside a tiny, clear chamber with access to oxygen-free mud filled with food. Oxygen dispersed in from the edges. Several unicellular bacteria produced a front where the oxygen was depleted, fighting for food and oxygen from both sides at the same time.
"In the microscope, I watched how single cable bacteria placed themselves across the front with one end into the zone with oxygen," explains Stefano Scilipoti, a Ph.D. student at Center for Electromicrobiology, Aarhus University and the primary discoverer.
An interesting observation was made when a cable bacterium distorted the front created by the unicellular bacteria. The cable bacterium took in so much oxygen that the unicellular bacteria moved closer to the chamber's edge to preserve the oxygen supply needed to breathe. Some of the cable bacterium's cells could be immersed in oxygen, allowing them to calculate how much oxygen was being used based on the distortion's magnitude.
"Less than 10% of the cells of the cable bacterium consumed oxygen, but they did it with a rate matching the highest rates known in biology. That only works because the cable bacterium runs an electric current between the cells being in contact with oxygen and the cells processing the food. The cells consuming oxygen can thus focus on this task only, while the other cells digest food and generate new cells," says Scilipoti.
The cable bacteria's predecessors, called anaerobic bacteria, survived without consuming oxygen. This is because oxygen is hazardous to them, and prolonged exposure could lead to their death. Now that electrical connection to oxygen has evolved, cable bacteria can explore the breathing's strength with oxygen without having their cells exposed to oxygen stress. As a result, this provides more energy while avoiding damage to the cells.
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