SCIENCE sciencemag.org 21 AUGUST 2020 • VOL 369 ISSUE 6506 905plain just off the coast. (They didn’t find
them in a sandy area populated by worms
that stir up the sediments and disrupt the
cables.) Elsewhere, researchers have found
DNA evidence of cable bacteria in deep,
oxygen-poor ocean basins, hydrothermal
vent areas, and cold seeps, as well as man-
grove and tidal flats in both temperate and
subtropical regions.
Cable bacteria have also shown up in
freshwater environments. After reading
Nielsen’s papers in 2010 and 2012, a team
led by microbiologist Rainer Meckenstock
re-examined sediment cores drilled during
a study of groundwater pollution in Dus-
seldorf, Germany. “We found [cable bac-
teria] exactly where we thought we would
find them,” at depths where oxygen was de-
pleted, recalls Meckenstock, who works at
the University of Duisburg-Essen.
Nanowire bacteria are even more broadly
distributed. Researchers have found them
in soils, rice paddies, the deep subsurface,
and even sewage treatment plants, as well
as freshwater and marine sediments. They
may exist wherever biofilms form, and
the ubiquity of biofilms provides further
evidence of the big role these bacteria may
play in nature.
The broad range of electric mud bacteria
also suggest they are a major force in eco-systems. By preventing the buildup of hy-
drogen sulfide, for example, cable bacteria
are likely making mud more habitable for
other life forms. Meckenstock, Nielsen, and
others have found them on or near the roots
of seagrasses and other aquatic plants,
which bubble off oxygen that the bacte-
ria likely exploit to break down hydrogen
sulfide. That, in turn, protects the plants
from toxic gas. The partnership “seems to
be a very generic property of water plants,”
Meckenstock says.
Robert Aller, a marine biogeochemist at
Stony Brook University, thinks the bacteria
may also aid many undersea invertebrates,
including worms that build burrows that al-
low oxygenated water to flow into the mud.
He has discovered cable bacteria sticking
out the sides of worm tubes, likely so they
can tap that oxygen for electron storage. In
return, those worms are kept safe from the
toxic hydrogen sulfide. “The bacteria make
[the burrow] more livable,” says Aller, who
described these connections in a July 2019
paper in Science Advances.
The microbes also alter the properties
of mud, says Sairah Malkin, an ecologist at
the University of Maryland Center for En-
vironmental Science. “They are particularly
efficient ... ecosystem engineers.” Cable bac-
teria “grow like wildfire,” she says; on inter-tidal oyster reefs, she has found, a single
cubic centimeter of mud can contain 2859
meters of cables, which cements particles
in place, possibly making sediment more
stable for marine organisms.
The bacteria also alter the mud’s chemis-
try, making layers closer to the surface more
alkaline and deeper layers more acidic,
Malkin has found. Such pH gradients can
affect “numerous geochemical cycles,” she
says, including those involving arsenic,
manganese, and iron, creating opportuni-
ties for other microbes.
With vast swaths of the planet covered
by mud, cable and nanowire bacteria are
likely having an influence on global cli-
mate, researchers say. Nanowire bacteria,
for example, can strip electrons from or-
ganic materials, such as dead diatoms, then
shuttle them to other bacteria that produce
methane—a potent greenhouse gas. Under
different circumstances, cable bacteria can
reduce methane production.
In coming years, “We are going to see a
broad acceptance of the importance of these
microbes to the biosphere,” Malkin says.
Just over a decade after Nielsen noticed
the mysterious disappearance of hydrogen
sulfide from the Aarhus mud, he says, “It is
dizzying to think about what we’re dealing
with here.” jDerek Lovley (left) discovered mud bacteria that sprout electron-transporting nanowires, while Lars Peter Nielsen (right) described microbes that build conducting cables.PHOTOS: (LEFT TO RIGHT) VOLKER STEGER/SCIENCE SOURCE; LARS KRUSE/AU FOTO
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