Aarhus is a beautiful city sitting on the east coast of the Jutland Peninsula, Denmark. Founded in the 8th century when the Vikings were sailing and trading around Europe, Aarhus city still maintains its historic look while blending with modern society. The port of Aarhus Bay is one of the most important economic hotspots of the country holding about half of Denmark’s container traffic. Lying in the sediments underneath the bay seawater is a secret that Lars Peter Nielsen fortunately unearthed in his groundbreaking Nature paper in 2010. Nielsen and his colleagues found that there are bacteria lining up together, being wrapped around with wires, and transferring electrons from the bottom to the top of the sediment. These organisms, named “cable bacteria,” act as a conducting wire that can be up to 5 cm long. This finding is astonishing for a number of reasons. First, the distance of several centimeters long was hard for scientists to believe at first. It was previously known that some bacteria can form protein nanowires from their membrane to grab iron, but these nanowires limit in a tiny space. This spatial extension of electron transfer somewhat resembles the ability of neuronal axons, which can transmit signal up to even meters in length. Second, these biological power cords actually are chains of thousands of individual bacteria lining up together. Such communal behavior is unique and intriguing, raising more questions about their adaptability and evolution. After a decade of extensive research, the community now has developed a body of continuously expanding knowledge on these bizarre cable bacteria. Nielsen’s discovery was serendipitous, like many astonishing paradigm shifts in the history of science. But what made it serendipity? How did Nielsen find it? In this article, I will look into the detail of Nielsen’s story to avail the anatomy of a serendipitous discovery.
The story began with an unexpected observation that Nielsen noticed in the remnants of an experiment he did a while ago. A month before, he was taking the mud from the Aarhus Bay’s sediment for a geochemistry measurement. The mud was black, stinky, just like the mud in anywhere else. Nielsen put it in a big glass beaker and measured its chemistry with microsensors. The mud was then sitting there unnoticed. A few weeks later, Nielsen saw that on one band of mud, the dark color now became paler while there was a rusty hue at the top of the surface. Usually, in sediments, the presence of hydrogen sulfide makes the mud black and stinky. This pale, light-colored band might probably show up because this compound somehow disappeared from the mud. Indeed, Nielsen’s microsensors detected no hydrogen sulfide at all. This was unexpected. How did it get away from the beaker? Nielsen did not let this observation slide. It was a daunting question for him to tackle. This is the first common feature among people who make serendipitous discoveries: an inquisitive mind that catches the slightest weirdness deteriorating from their main experiments.
Hydrogen sulfide is created in the mud as a byproduct when bacteria decay plant debris and organic materials. In the sediment, bacteria do not have access to oxygen to further breaking down hydrogen sulfide, so this compound builds up and gives the mud its signature color. The rusty color on the surface of the mud, on the other hand, typically comes from the formation of iron oxide. What is the connection between these two observations? A student of a general chemistry course will recognize that they are two parts of an oxidation-reduction process or a “redox” reaction. But the mud in Nielsen’s big glass beaker was quite thick, up to several centimeters in height. How could such a redox reaction happen from a long-distance?
Nielsen is a prominent microbiologist. He has spent nearly his entire career to study microbial organisms, so microbes are all he has. From every step he walks to every ounce of air he breaths, Nielsen constantly thinks about microbes. One night, a peculiar idea popped up in his head and woke him up from his sleep. Could the redox reaction in the mud be completed by bacteria in the mud, he wondered? Could those bacteria use hydrogen sulfide in the bottom of the sediment as an electron donor while iron on the surface as an electron acceptor? Could somehow those bacteria transfer electrons between two layers? It turned out that, through a series of experiments, Nielsen and his lab finally isolated bacterial filaments from the mud. These bacterial filaments were long, thin chains of up to 2000 individual bacteria wrapped around by ridged membrane. This massive stack of encased cells looks exactly like an electrical conducting wire. To Nielsen, the identification of cable bacteria was a holy grail that had solved the mud riddle. This is the second characteristic of the classic serendipitous discoverers: the deep background knowledge in the fields of their interests combined with the creative thinking that dares to go beyond the normalcy.
Figure 1: Cable bacteria. From Gemma Reguera’s news and views paper.
Like other discoveries, science is built brick by brick. After Nielsen published his seminal work in Nature, some scientists were skeptic but others saw it as an opportunity. Among them was Filip Meysman at the University of Antwerp. Meysman and his team went out and looked for cable bacteria in the mud of their local bay, and they actually found them. However, studying these cable bacteria was tough. Their wire was fragile from degradation, and the tools to measure the electric current were not optimal. As a chemical engineer, Meysman and his team started to develop the right tools and techniques to get more insights into these mythical creatures. Through collaboration with Nielsen and other labs, Meysman successfully measured the conductivity and partially dissected the molecular structure of these cable bacteria. Their work was published in Nature Communications in 2019.
After a decade, cable bacteria have engaged and formed an expanding community with researchers in several institutions around the world. In 2017, the Danish government established the Center for Electromicrobiology under the leadership of Lars Peter Nielsen at the University of Aarhus. It is still quite early to know how or whether these cable bacteria can benefit human life. However, Nielsen’s discovery has certainly opened up a horizon with ample opportunities for more discoveries to come.
Meysman, Filip JR, et al. "A highly conductive fiber network enables centimeter-scale electron transport in multicellular cable bacteria." Nature Communications 10.1 (2019): 1-8.
Nielsen, Lars Peter, et al. "Electric currents couple spatially separated biogeochemical processes in marine sediment." Nature 463.7284 (2010): 1071-1074.
Pennisi, Elizabeth. ‘Electric mud’ teems with new, mysterious bacteria. Science (2020): News.
Pfeffer, Christian, et al. "Filamentous bacteria transport electrons over centimeter distances." Nature 491.7423 (2012): 218-221.
Reguera, Gemma. "Bacterial power cords." Nature 491.7423 (2012): 201-202.