NEWS | FEATURES | MUD
904 21 AUGUST 2020 • VOL 369 ISSUE 6506 sciencemag.org SCIENCE
but they are on par with conductors used in
solar panels and cellphone screens, as well
as the best organic semiconductors.
The researchers also dissected the cable
bacteria’s anatomy. Using chemical baths,
they isolated the cylindrical sheath, finding
it holds 17 to 60 parallel fibers, glued along
the inside. The sheath is the source of the
conductance, Meysman and colleagues re-
ported last year in Nature Communications.
Its exact composition is still unknown, but
could be protein-based.
“It’s a complicated organism,” says
Nielsen, who now heads a Center for
Electromicrobiology, established in 2017 by
the Danish government. Among the chal-
lenges the center is tackling is mass pro-
ducing the microbes in culture. “If we had
a pure culture, it would be a lot easier” to
test ideas about cell metabolism and envi-
ronmental influences on conductance, says
the center’s Andreas Schramm. Cultured
bacteria would also make it easier to isolate
the cable’s wires and test potential applica-
tions for bioremediation and biotechnology.EVEN AS RESEARCHERS puzzle over cable
bacteria, others have been studying another
big player in electric mud: nanowire bac-
teria, which instead of stacking cells intocables sprout protein wires spanning 20 to
50 nanometers from each cell.
As with cable bacteria, some puzzling
sediment chemistry led to the discovery
of nanowire microbes. In 1987, microbio-
logist Derek Lovley, now at the University
of Massachusetts, Amherst, was trying to
understand how phosphate from fertil-
izer runoff—a nutrient that promotes algal
blooms—is released from sediments be-
neath the Potomac River in Washington,
D.C. He suspected microbes were at work
and began to sieve them from the mud.
After growing one, now called Geobacter
metallireducens, he noticed (under an elec-
tron microscope) that the bacteria sprouted
connections to nearby iron minerals. He
suspected these wires were transporting
electrons, and eventually figured out that
Geobacter orchestrates chemical reactions
in mud by oxidizing organic compounds
and transferring the electrons to minerals.
Those reduced minerals then release their
hold on phosphorus and other elements.
Like Nielsen, Lovley faced skepticism
when he first described his electrical mi-
crobe. Today, however, he and others have
documented almost a dozen kinds of nano-
wire microbes, finding them in a variety of
environments besides mud. Many shuttle
electrons to and from particles in sediment.
But some rely on other microbes to obtain
or store electrons. Such biological partner-
ships allow both microbes to “do new types
of chemistry that neither organism can do
on their own,” says Victoria Orphan, a geo-
biologist at the California Institute of Tech-
nology. Whereas cable bacteria solve their
redox requirements by long-distance trans-
port to oxygenated mud, these microbes de-
pend on each other’s metabolisms to satisfy
their redox needs.
Some researchers are still debating how
the bacterial nanowires conduct electrons.
Lovley and his colleagues are convinced that
chains of proteins called pilins, which consist
of ring-shaped amino acids, are key. When
he and his colleagues reduced the number
of ringed amino acids in pilin, the nanowires
became poorer conductors. “That was really
surprising,” Lovley says, because proteins are
generally thought to be insulators. But others
think the issue is far from settled. Orphan, for
one, says that although “there is some com-
pelling evidence ... I still don’t think [nano-
wire conductance] is well understood.”WHAT IS CLEAR is that electrical bacteria are
everywhere. In 2014, for example, scientists
found cable bacteria in three very differ-
ent habitats in the North Sea: an inter-
tidal salt marsh, a seafloor basin where
oxygen levels drop to near zero at some
times of the year, and a submerged mudNext up: a phone powered by microbial wires?
T
he discoverers of electric microbes have been quick to think about how these
bacteria could be put to work. “Now that we have found out that evolution has
managed to make electrical wires, it would be a shame if we didn’t use them,” says
Lars Peter Nielsen, a microbiologist at the University of Aarhus.
One potential use is to detect and control pollutants. Cable microbes seem to
thrive in the presence of organic compounds, such as petroleum, and Nielsen and
his team are testing the possibility that an abundance of cable bacteria signals the
presence of undetected pollution in aquifers. The bacteria don’t degrade the oil
directly, but they may oxidize sulfide produced by other oil-eating bacteria. They might
also aid cleanup; sediments recover faster from crude oil contamination when they
are colonized by cable bacteria, a different research team reported in January in Water
Research. In Spain, a third team is exploring whether nanowire bacteria can speed the
cleanup of polluted wetlands. And even before nanowire bacteria were shown to be
electric, they showed promise for decontaminating nuclear waste sites and aquifers
contaminated with aromatic hydrocarbons such as benzene or naphthalene.
Fighting climate change is another target. Lab tests have demonstrated that cable
bacteria can reduce the amount of methane—a major contributor to global warming—
generated by rice cultivation by 93%, researchers reported on 20 April in Nature Com-
munications. They do this by helping break down substances that methane-producing
bacteria rely on.
Electric bacteria could also give rise to new technologies. They can be genetically
modified to alter their nanowires, which could then be sheared off to form the basis of
sensitive, wearable sensors, says Derek Lovley, a microbiologist the University of Mas-
sachusetts (UMass), Amherst. “We can design nanowires and tailor them to specifi-
cally bind compounds of interest.” For example, in the 11 May issue of Nano Research,
Lovely, UMass engineer Jun Yao, and their colleagues described a nanowire sensor that
detects ammonia at concentrations relevant for agricultural, industrial, environmental,
and biomedical applications.
Fashioned into a film, nanowires can generate electricity from the moisture in
the air. The film generates power, researchers believe, when a moisture gradient de-
velops between the film’s upper and lower edges. (The upper edge is more exposed
to moisture.) As the water’s hydrogen and oxygen atoms separate because of
the gradient, a charge develops and electrons flow. Yao and his team reported on
17 February in Nature that such a film can create enough power to light a light-emitting
diode, and 17 such devices connected together can power a cellphone. The approach
is “a revolutionary technology to get renewable, green, and cheap energy,” says Qu
Liangti, a materials scientist at Tsinghua University. (Others are more cautious,
noting that past attempts to wring energy from moisture, using graphene or poly-
mers, have not panned out.)
Ultimately, researchers hope to exploit the bacteria’s electrical talents without having to
deal with the finicky microbes themselves. Lovley, for example, has coaxed a common lab
and industrial bacterium, Escherichia coli, to make nanowires. That should make it easier
for researchers to mass produce the structures and explore practical applications. – E. P.Published by AAAS