When microbiologist Jared Leadbeater returned to his office for the first time in months after a work trip, he found something strange. A cream-colored manganese carbonate compound (MnCO3), which coated the glassware he had left in his sink, had gone dark. Something had stolen some of his electrons.
“I thought, ‘What is that?'” Said Leadbeater, a researcher at the California Institute of Technology (Caltech). The dark substance was a form of manganese oxide, a product that is formed when manganese ions lose electrons and undergo a reaction called oxidation.
But something had to be starting the reaction: an electron thief.
“I began to wonder if the much sought-after microbes could be responsible,” Leadbeater explained, “so we systematically tested to solve it.”
To verify if this was actually happening due to a biological process, Leadbeater and his team coated more jars with MnCO3 and sterilized them with searing steam (MnCO3 is known to be stable under these conditions).
The manganese compound in them did not darken (even a year later), but the jars that had not been sterilized did. Therefore, the electron thief had to be something that could be destroyed by hot vapor.
The researchers then cultivated what was in the jars. RNA analysis revealed 70 species of bacteria, but with further testing, the team managed to rule out some, until only two possible culprits remained.
They were Nitrospirae bacteria, which are generally crescent-shaped, and the rod-shaped betaproteobacterium. Relatives of both species of bacteria are known to live in groundwater.
“We isolate [the betaproteobacterium] of broken oxides as individual colonies … but this species does not oxidize MnCO3 alone. Either Nitropirae is solely responsible for Mn (II) oxidation or the activity is consortial, “the team writes in a new study.
The electron theft may have been a team effort, the team realized. But what was the reason? The investigators had their suspicions.
They used carbon-13-labeled manganese in some of their crops, and indeed the bacteria incorporated these carbon isotopes into their bodies.
This confirmed that the suspected bacteria were autotrophic: they can produce their own food using an energy source.
The bacteria were using the energy from the manganese electrons to convert CO2 into usable carbon, as plants use sunlight to convert CO2 and water to sugars and oxygen during photosynthesis.
This process is called chemosynthesis, and although it is known to occur using other metals, it is the first time that we have seen cells use manganese in this way.
“These are the first bacteria to use manganese as a fuel source,” Leadbetter explained, although such microbes were predicted to exist more than a century ago.
Manganese is an essential nutrient for us too. Our bodies use it to process fat, protein, and bone formation, and we get it from foods like nuts, teas, and leafy green vegetables.
While it is one of the most common elements on the surface of our planet, much about manganese and its cycle on Earth remains a mystery, including its strange tendency to clog water pipes.
“There is a complete set of environmental engineering literature on drinking water distribution systems that are clogged with manganese oxides,” said Leadbetter.
“But how and for what reason such material is generated has remained an enigma. Clearly, many scientists have considered that bacteria that use manganese as energy could be responsible, but the evidence supporting this idea was not available until now.”
Manganese oxide also mysteriously appears as nodules across much of the seafloor, and manganese is involved in many interconnected cycles of elements including carbon, nitrogen, iron, and oxygen.
So the existence of thieves stealing manganese electrons, like these newly discovered bacteria, could explain a lot.
The researchers say the bacterial cell doubling times and oxidation rates would create manganese oxides in amounts equivalent to the world’s reserves in just two years.
The close relatives of these species seem to be present in many places, so their potential to circulate this metal on Earth could be enormous.
“This discovery fills an important intellectual void in our understanding of Earth’s elemental cycles and adds to the many ways in which manganese, an abstruse but common transition metal, has shaped the evolution of life on our planet. “said Caltech geobiologist Woodward Fischer. who did not participate in the study.
This research was published in Nature.
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