Scientists have discovered a bacterium that can transform waste from copper mining into a more useful form of pure copper, providing a potentially inexpensive and environmentally friendly way to synthesize the valuable metal and clean up polluted environments.
The bacterium produced single-atom, or monoatomic copper, which is particularly useful for energy production and other applications. The microbe is the first reported to produce a single-atom metal, but the researchers, whose findings were published April 23 in Science Advances, suspect many more await discovery.
"They can actually remove something toxic and convert it into something good for us and clean the environment ... at the same time," said Debora Rodrigues, a professor of civil and environmental engineering at the University of Houston and last author of the study.
Copper is a vital component of electronics, solar cells and antimicrobial coatings. It's typically recovered from ores such as chalcopyrite.
"The ores are a mixture of copper with carbonate, sulfate, phosphate and oxide minerals," Rodrigues said.
The process of copper mining releases ionic, or charged, copper as a byproduct that contaminates the surrounding environment. When ingested by people, it can cause symptoms ranging from headaches and vomiting to liver and kidney failure and, when consumed in high enough concentrations, even death.
A whole host of microorganisms such as bacteria and fungi can produce useful metals such as silver, gold and even copper in the form of little clusters called nanoparticles, which for the latter range from 10 to 40 nanometers in size. However, scientists weren't previously aware of any microbes that could produce the single-atom form of a metal.
The so-called monoatomic metallic form of copper is 170 to 179 picometers in size. Producing this kind of copper in the lab is laborious, costly and requires toxic chemicals.
"Copper in general is a good conductor, but the [monoatomic copper] that these microorganisms are producing is very versatile because it is the pure form," Rodrigues said.
She and her colleagues found their enterprising bacterium while exploring the microbial community around a copper mine in Brazil. The rod-shaped bacterium belongs to the Bacillus genus, which includes many harmless soil- and water-dwelling members as well as the bacterium that causes anthrax.
"It's hard to recover this ionic copper because it gets dissolved in the water," Rodrigues said.
When the researchers added the microbe to flasks containing greenish-tinted copper sulfate — which dissociates into ionic copper and sulfate in water — they witnessed a striking transformation. After two days, the contents of the flask had turned orange.
"That was the first indication that something was going on," Rodrigues said. "We were like, 'Where is the copper; where did it go?'"
To find out what had happened, the researchers examined the bacteria under the microscope. When they couldn't detect any copper nanoparticles, they turned to a powerful transmission electron microscope to search for the isolated atoms.
The researchers were able to measure about 13,000 copper atoms in the bacterial cells. When the team examined the structure of these atoms, they were found to be the valuable nonionic form of copper.
To understand how the bacterium, referred to as Bacillus sp. strain 105, was making the single-atom copper, the researchers searched for proteins that the bacteria only produced when grown in the presence of the ore copper sulfate. In particular, they identified a protein called ferritin that stores iron in humans and other organisms. Previous laboratory experiments have suggested that it could also convert toxic ionic copper into its less harmful, uncharged form, but it hadn't been observed performing this function in living cells before.
"The ionic copper is very toxic to a lot of living organisms," Rodrigues said, "so they are trying to change it in a way that would be less toxic for them."
It's likely that this chemical process is energetically costly.
"They detoxify but there's a price for them, and at the same time they try to get from other places in the environment ... the energy to be able to make this conversion," Rodrigues said. "The sulfate is most likely used for energy for them."
Rodrigues suspects that there are more microbes waiting to be discovered with the ability to produce single-atom copper, as well as other metals such as gold and silver.
"This study is the tip of the iceberg," she said. "I don't think this is a unique case in nature; I'm pretty sure there are a bunch of microbes that are doing that, but we haven't actually seen it yet."
Researchers will still have to figure out how to use the microbes to mass produce single-atom copper. Nonetheless, Rodrigues and her colleagues wrote, "This study opens a new field of research of environmental micro-organisms that potentially are able to synthesize other monoatomic metals for applications in science, technology, engineering and medicine."
The study, "Copper mining bacteria: Converting toxic copper ions into a stable single-atom copper," published April 23 in Science Advances, was authored by Louise Hase Gracioso, Bruno Karolski, Bruna Bacaro Borrego and Claudio Augusto Oller do Nascimento, University of São Paulo; Elen Aquino Perpetuo, University of São Paulo and Federal University of São Paulo; Maria Aparecida Juliano, Federal University of São Paulo; Hiroki Hashiguchi, JEOL Ltd; and Janire Peña-Bahamonde, Francisco C. Robles Hernandez and Debora Frigi Rodrigues, University of Houston.