The bacteria form a long conducting nanowire cable to transport electrons and capture the oxygen at the surface for metabolic use

Necessity is the mother of natural selection. When conditions become threatening, maverick or mutant members of a group which can cope with the threat survive and multiply. The latest example is the discovery of a special type of bacteria in the ocean, which join together to form a long conducting nanowire cable to transport electrons and capture the oxygen at the surface for metabolic use. This wire is not made of metal, alloy or other usual material, but of living biological cells. The report by Dr. Christian Pfeiffer and others in the 8 November 2012 issue of Nature is a live example of the Panchatantra tale which teaches the value of cooperation between individuals to win over a problem.

All organisms gain energy for living through metabolism. The vital step in the process is the burning or oxidation of the food molecules. Chemists define oxidation as the loss of electrons and reduction as the gain of electrons. We burn our food by the breathing of oxygen in the air. When we oxidize our food and gain energy, the oxygen molecule is reduced by accepting or gaining electrons to make water, while the food molecule is oxidized by losing electrons; this is not much different from burning petrol for energy.

What if no oxygen?

What about organisms that live in places where there is no oxygen? They too metabolize their food through oxidation. But, rather than oxygen, they utilize whatever electron-acceptor molecules are available in the environment. One such group lives in marine sediments, below the surface, and it use the sulphates in the sediment as the electron-acceptors for ‘burning’ and gaining energy, an example of making do with available resources. In the process, however, the sulphate gains electrons and is reduced all the way to hydrogen sulphide (HS), a poisonous material. How then is this sulphide removed?

The problem

Look at the problem. If HS can be oxidized to sulphur, the situation turns safer. But in the process electrons are liberated and should be accepted by a partner. If only oxygen at the surface can be reached and the electrons transferred to it, we will have HS becoming S and the O reduced to H O. How does one transfer the electrons centimetres away? It is no longer a process within the cell where reactions happen within nanometres, and the oxidant and reductant molecules are in contact. What is needed is an efficient method — an electrical cable or wire for transporting the electrons from the sulphide to the oxygen above.

It is here that biology springs an unexpected surprise. In the sedimental layer beneath the marine surface lives a class of anaerobic bacteria called Desulfobulbaceae, which Pfeffer and colleagues find to densely populate the sediments. And these live not as individuals but in groups strung together as long, multicellular filaments or rods, some as long as 1.5 centimetres. And these filaments reach out from the sulphide-rich sedimental layer to the aerobic top layer a few centimetres above, which has dissolved oxygen (from the air). These filaments thus connect the anoxic layers to the oxic layer. And what do they do? They capture the electrons generated when the HS is oxidized to S at the bottom, and transport them all the way to the oxygen at the top, which accepts them and generates water or HO. In other words, the Desulfobulbaceae bacteria line up to make a live wire.

The researchers conducted a series of experiments to show how the filaments form and work. They layered the sedimental layer below in the lab and covered is with the overlying oxic sea water and studied the process. As the sulphide oxidation happened in the deeper anoxic layers, distinct change in the pH was noticed, confirming the process. And when they gently disturbed the layer, they found the 12-15 run long fibrous filaments entangled. Genetic analysis of the filaments showed their identity as Desulfobulbaceae. It appears that at least 40 million cells come together to assemble filaments of lengths as much as 1.5 cm, showing that the bacteria could span the length of the entire anoxic layer.

Liquid-filled layer

Electron microscopy showed that the cells were connected lengthwise, and each cell had a liquid-filled layer in the periplasmic space between the outer and inner cytoplasmic membranes. These liquid compartments formed ridges connecting the each cell to its neighbour, suggesting electron transport occurring through this fluid tubular structure covered with a continuous outer membrane along the filament acting as the insulator — the ancient precursor, if you will, of the electric cable of today. Hair-like appendages, called pili, of some bacteria are known to be electron transporters, but the whole cell acting so, and joining with others to make a conducting wire is novel, and reported for the first time.

Plenty of room

The physicist Richard Feynman famously remarked that there is plenty of room at the bottom. Bacterial filaments acting as electric nanowires is but one example. Some cyanobacteria called Anabena, which are able to ‘fix’ nitrogen, also form such continuous periplasmic filaments. And when a fluorescent protein was engineered into some its cells, the fluorescence was found to move along the filament from one cell to the other. Here is an example of material transfer, while with Desulfobulbaceae, it is electrons that are transported. Surely there is far more room at the bottom, and nanotechnologists can learn a lesson or two from such bacteria.

D. BALASUBRAMANIAN, dbala@lvpei.org

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