Finding ends a
century-long search for microbes that live on manganese
CalTech – July 15, 2020 -- Caltech
microbiologists have discovered bacteria that feed on manganese and use the
metal as their source of calories. Such microbes were predicted to exist over a
century ago, but none had been found or described until now.
"These are the first bacteria found
to use manganese as their source of fuel," says Jared Leadbetter,
professor of environmental microbiology at Caltech who, in collaboration with
postdoctoral scholar Hang Yu, describes the findings in the July 16 issue of
the journal Nature. "A wonderful aspect of microbes in nature
is that they can metabolize seemingly unlikely materials, like metals, yielding
energy useful to the cell."
The study also reveals that the bacteria
can use manganese to convert carbon dioxide into biomass, a process called
chemosynthesis. Previously, researchers knew of bacteria and fungi that could
oxidize manganese, or strip it of electrons, but they had only speculated that
yet-to-be-identified microbes might be able to harness the process to drive
growth.
Leadbetter found the bacteria
serendipitously after performing unrelated experiments using a light,
chalk-like form of manganese. He had left a glass jar soiled with the substance
to soak in tap water in his Caltech office sink before departing for several
months to work off campus. When he returned, the jar was coated with a dark
material.
"I thought, 'What is that?'"
he explains. "I started to wonder if long-sought-after microbes might be
responsible, so we systematically performed tests to figure that out."
The black coating was in fact oxidized
manganese generated by newfound bacteria that had likely come from 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," he says.
Manganese is one of the most abundant
elements on the surface of the earth. Manganese oxides take the form of a dark,
clumpy substance and are common in nature; they have been found in subsurface
deposits and can also form in water-distribution systems.
"There is a whole set of
environmental engineering literature on drinking-water-distribution systems
getting clogged by manganese oxides," says 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."
The finding helps researchers better
understand the geochemistry of groundwater. It is known that bacteria can
degrade pollutants in groundwater, a process called bioremediation. When doing
this, several key organisms will "reduce" manganese oxide, which means
they donate electrons to it, in a manner similar to how humans use oxygen in
the air. 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," says Leadbetter.
The research findings also have possible
relevance to understanding manganese nodules that dot much of the seafloor.
These round metallic balls, which can be as large as grapefruit, were known to
marine researchers as early as the cruises of the HMS Challenger in the 1870s.
Since then, such nodules have been found to line the bottom of many of Earth's
oceans. In recent years, mining companies have been making plans to harvest and
exploit these nodules, because rare metals are often found concentrated within
them.
But little is understood about how the
nodules form in the first place. Yu and Leadbetter now wonder if microbes similar
to what they have found in freshwater might play a role and they plan to
further investigate the mystery. "This underscores the need to better
understand marine manganese nodules before they are decimated by mining,"
says Yu.
"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,"
says Woodward Fischer, professor of geobiology at Caltech, who was not involved
with the study.
The study, titled, "Bacterial
chemolithoautotrophy via manganese oxidation," was funded by NASA and Caltech.
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