For more than a century, microbiologists have hunted for bacteria that power their metabolism by oxidizing manganese, which is abundant in the Earth’s crust. Now, scientists report that they have found such microbes lurking in the water system of Pasadena, California.
The discovery broadens the known repertoire of microbes that harness energy from inorganic chemicals and use it to turn carbon dioxide into carbohydrates the cell needs for growth (Nature 2020, DOI: 10.1038/s41586-020-2468-5).
Microbiologists’ certainty that manganese-munching bacteria must exist has been driven by basic physics. “If there is a thermodynamically favorable energy source, there will be an organism that can use that energy source for growth,” says Bradley Tebo, a marine microbiologist at Oregon Health & Science University who was not involved in the study. Scientists have described bacteria whose energy cycles rely on oxidizing inorganic chemicals such as sulfur, nitrogen, and iron. Over the years they’ve found many hints that manganese can also do the job. But although some bacteria are known to grow using oxidized manganese, none have ever been caught in the act of actually oxidizing it themselves.
The home turf of these elusive bacteria turned out to be surprisingly mundane. “There are all these amazing and wonderful environments out there to study,” says lead investigator Jared Leadbetter, an environmental microbiologist at the California Institute of Technology. Others have found metabolically unusual bacteria in the hot springs of Yellowstone National Park or hydrothermal vents deep in the ocean. “But this was literally from the tap water from my office sink,” he says.
Leadbetter left some dirty glassware in his office sink before leaving on a two-month teaching stint last summer. Before his trip, he was working on a set of unrelated studies that had resulted in a chalky, whitish-pink precipitate of manganese carbonate. The carbonate was caked onto the side of a glass jar in the sink. Leadbetter had filled the jar with water, but forgot to wash it out. When he returned, the substance had turned dark brown.
Leadbetter knew that manganese-oxidizing microbes had been predicted. “I thought, ‘Well, if these bacteria existed, this is what it might look like, so maybe before I pour this down the drain I should see if that’s the case,” he says.
Leadbetter and his postdoc Hang Yu, who coauthored the study, serially diluted the tap water, which contained about 70 bacterial species, in a manganese(II) carbonate medium. They homed in on two bacterial species, which they provisionally named Manganitrophus noduliformans and Ramlibacter lithotrophicus, that together generated manganese oxide nodules. The duo found that the growth of these bacteria depends on manganese oxidation—and that when the rate of oxidation goes up, so does their growth rate.
Manganitrophus noduliformans is genetically dissimilar from any previously identified bacterial species. Most of its closest relatives are found in groundwater, soil sediments, or marine environments several meters below the surface. Leadbetter hopes to sample some of these types of environments to find other manganese oxidizers. “My guess is that now that we know how to look for them, they will turn up lickety-split in all sorts of places,” he says.
Traditionally, Tebo says, microbiologists have focused on microbes that can grow in the lab, but molecular approaches like the kinds Leadbetter and Yu used make it possible to find and study bacteria in conditions closer to how they normally grow. Leadbetter now plans to explore the biochemical and physiological quirks of such organisms, as well as how their manganese-based energy cycle might interface with the energy cycles of bacteria that oxidize other inorganic chemicals.
This story was updated on July 16, 2020, to correct typos in two chemical names. Bacteria had never been observed oxidizing manganese, not magnesium. And the medium the researchers used was manganese(II) carbonate, not manganese(I) carbonate.