They can be described as microscopic cable makers: all over the world, in muddy sediments, tiny bacteria feed and grow, building electrically conductive circuits in the mud. Now they’ve discovered how they do it – by weaving tiny plates of nickel and organic compounds into conductive fibres.

First in nature

The authors of the study, published as a preprint on bioRxivclaim to have found the first biological examplemetal-organic frame– material whose creators were awarded the Nobel Prize in chemistry last month. These bacterial conductors also conduct electricity much better than synthetic organic wires, so they could form the basis for flexible, biocompatible electronics with low metal content and low energy consumption.

“If confirmed, it will be a major step in our understanding of cable’s capabilities. This is what we have been chasing for years,” commented electromicrobiologist Lars Peter Nielsen from Aarhus University.

His team discovered cable bacteria in 2009 in Aarhus harbor. Since then, they have been found throughout the world in the sediments of lakes, rivers and oceans. These microbes feed on sulfur-rich compounds that are released when organic matter decomposes, such as hydrogen sulfide.

The bacteria steal electrons from the foul-smelling, colorless gas and eventually transfer them to oxygen, which is abundant in the upper layers of the sediment.When electrons move from a higher energy state in hydrogen sulfide to a lower energy state in oxygen, microbes can take some of the difference.

To carry out redox reactions, bacteria need to combine hydrogen sulfide in the depths with oxygen on the surface. Nature has found a universal solution: share and cooperate. As microbes multiply and extend the “wires” in the sludge, they forma single superorganism with a common cell membrane.

“This is a unique phenomenon in biology,” marvels microbiologist Derek Lovley of the University of Massachusetts Amherst.

Thousands of kilometers of live wires

So far, “wires” up to 5 centimeters long have been found, consisting of 25,000 cable bacteria that interact to form something like a breathing tube. It is estimated that one square meter of sediment may contain about 20,000 kilometers of such bacterial wires. These “little engineers” rearrange the chemical landscape of sediments, promoting the transformation of minerals, stimulating nutrient cycling and causing a flow of ions that acidifies deeper layers.

than ten years of research have not been able to determine the structure of these wires. Electron microscopy helped.

With its help, individual threads of bacteria were identified, and along their perimeter – dozens of ridges, each of which contains conductive fibers with a diameter of only 50 nanometers. These fibers, in turn, consist of even smaller bundles of woven nanoribbons.

According to X-ray spectroscopy, X-ray fluorescence and computer modeling, the cable bacteria extract trace amounts of nickel from sediment and water and create long, lamellar structures by linking metal atoms to sulfur-rich organic compounds. These plates are stacked to form nanoribbons, which are woven into bundles, similar to stranded copper wires in electronics. “It’s impressive how evolution has optimized this structure,” admits Nielsen.

This structure represents the first biologically derived metal-organic framework, a class of materials that has captivated chemists for years. The voids in these porous materials can be tuned to trap specific molecules, making them useful for storing gases such as hydrogen and methane, or extracting water vapor or carbon dioxide from the air.

What’s next

Chemists have developed their own methods for synthesizing nickel and organic compounds into conductive nanowires. But bacterial nanoribbons are 100 times better at conducting electrons than their modern synthetic counterparts.

According to Lovley, researchers are already working on adapting other bacterial conductors to create artificial neurons and new types of chemical sensors – and even to generate electricity directly from moist air. “Perhaps the cable bacteria will lead to something else,” he concluded.