Metal-eating bacteria identified for the first time

Bacteria that gain their energy from eating metal have long been suspected to exist, but have never before been identified.

Manganese dioxide (illustrative) (photo credit: Wikimedia Commons)
Manganese dioxide (illustrative)
(photo credit: Wikimedia Commons)
Scientists have confirmed the existence of metal-eating bacteria, more than a hundred years after first suspecting they existed – and all because a researcher left some dishes in the sink.
Dr. Jared Leadbetter, professor of environmental microbiology at the California Institute of Technology (Caltech) in Pasadena, California, stumbled across the bacteria after leaving a glass jar lined with a light, chalky form of manganese in a sink to soak before going to work off campus for several months. When he returned, the jar was coated in a dark substance.
"I thought, 'What is that?' " Leadbetter explained. "I started to wonder if long-sought-after microbes might be responsible, so we systematically performed tests to figure that out."
The tests revealed that the black material was oxidized manganese generated by the novel bacteria, which was likely residing in the tap water itself.
"There is evidence that relatives of these creatures reside in groundwater, and a portion of Pasadena's drinking water is pumped from local aquifers," Leadbetter said.
Manganese is one of the most abundant elements on earth. An important transition metal, it is used in a range of industrial alloys, particularly stainless steels. In nature, it is found in minerals and ores, and was historically named for black minerals from the Magnesia region in Greece.
A required trace mineral for all living organisms, manganese had already been identified as a key element in certain biological processes such as photosynthesis, thanks to its ability to capture oxygen free radicals. The human body utilizes manganese as a coenzyme in a number of biological processes, including bone formation and free-radical defense systems, although it is toxic in higher amounts.
Manganese oxides, dark and lumpy in shape, are common in nature, but have also been found clogging up water distribution systems. Until now, it was never clear how they got there, although bacteria were considered as a culprit.
"There is a whole set of environmental engineering literature on drinking-water distribution systems getting clogged by manganese oxides," said Leadbetter. "But how and for what reason such material is generated there has remained an enigma. Clearly, many scientists have considered that bacteria using manganese for energy might be responsible, but evidence supporting this idea was not available – until now."
Leadbetter and his colleague, postdoctoral scholar Hang Yu, noted that the finding helps researchers understand the biological and chemical processes taking place in groundwater, as it helps complete a picture of the bio-remediation cycle previously observed. In that process, bacteria are known to break down pollutants in groundwater by using the oxygen found in manganese oxides within the pollutants. But scientists have wondered where the manganese oxide comes from in the first place.
"The bacteria we have discovered can produce it, thus they enjoy a lifestyle that also serves to supply the other microbes with what they need to perform reactions that we consider to be beneficial and desirable," the professor said.
It also helps to shed light on the presence of manganese nodules that cover much of the seafloor. Again, these metal balls, some the size of grapefruits, have been observed as far back as the 1870s. In recent years, mining companies have begun making plans to mine the nodules, thanks to the rare materials often found within them. But no one was sure how they got there.
"This underscores the need to better understand marine manganese nodules before they are decimated by mining," Yu said. He and Leadbetter wonder whether microbes similar to those found in freshwater could be responsible, and plan to investigate further.
But the finding is also significant in itself, given that it is the first time a bacterium was found to get its energy by eating metal.
"These are the first bacteria found to use manganese as their source of fuel," Leadbetter noted, adding: "A wonderful aspect of microbes in nature is that they can metabolize seemingly unlikely materials, like metals, yielding energy useful to the cell."
The findings of the study, "Bacterial chemolithoautotrophy via manganese oxidation," which was co-funded by NASA, were published by Leadbetter and Yu in the Nature journal this week.
Woodward Fischer, professor of geobiology at Caltech, who was not involved with the study, said: "This discovery from Jared and Hang fills a major intellectual gap in our understanding of Earth's elemental cycles, and adds to the diverse ways in which manganese, an abstruse but common transition metal, has shaped the evolution of life on our planet."