sciencemag.org SCIENCEF
or Lars Peter Nielsen, it all began
with the mysterious disappearance
of hydrogen sulfide. The microbio-
logist had collected black, stinky
mud from the bottom of Aarhus
Harbor in Denmark, dropped it
into big glass beakers, and inserted
custom microsensors that detected
changes in the mud’s chemistry.
At the start of the experiment, the muck
was saturated with hydrogen sulfide—the
source of the sediment’s stink and color. But
30 days later, one band of mud had become
paler, suggesting some hydrogen sulfide
had gone missing. Eventually, the micro-
sensors indicated that all of the compound
had disappeared. Given what scientists
knew about the biogeochemistry of mud,
recalls Nielsen, who works at Aarhus Uni-
versity, “This didn’t make sense at all.”
The first explanation, he says, was that
the sensors were wrong. But the cause
turned out to be far stranger: bacteria that
join cells end to end to build electrical
cables able to carry current up to 5 centi-
meters through mud. The adaptation, never
seen before in a microbe, allows these so-
called cable bacteria to overcome a major
challenge facing many organisms that live
in mud: a lack of oxygen. Its absence would
normally keep bacteria from metabolizing
compounds, such as hydrogen sulfide, as
food. But the cables, by linking the microbes
to sediments richer in oxygen, allow them
to carry out the reaction long distance.
When Nielsen first described the discov-
ery in 2009, colleagues were skeptical. Filip
Meysman, a chemical engineer at the Uni-
versity of Antwerp, recalls thinking, “This is
complete nonsense.” Yes, researchers knew
bacteria could conduct electricity, but not
over the distances Nielsen was suggesting.
It was “as if our own metabolic processes
would have an effect 18 kilometers away,”
says microbiologist Andreas Teske of the
University of North Carolina, Chapel Hill.
But the more researchers have looked
for “electrified” mud, the more they have
found it, in both saltwater and fresh. They
have also identified a second kind of mud-
loving electric microbe: nanowire bacteria,
individual cells that grow protein structures
capable of moving electrons over shorter
distances (see graphic, p. 903). These nano-
wire microbes live seemingly everywhere—
including in the human mouth.
The discoveries are forcing researchers to
rewrite textbooks; rethink the role that mud
bacteria play in recycling key elements such as
carbon, nitrogen, and phosphorus; and recon-
sider how they influence aquatic ecosystems
and climate change. Scientists are also pur-
suing practical applications, exploring the
potential of cable and nanowire bacteria toBacteria that conduct electricity are
transforming how we see sediments
By Elizabeth PennisiThreads of electron-conducting
cable bacteria can stretch
up to 5 centimeters from deeper
mud, where oxygen is scarce and
hydrogen sulfide is common,
to surface layers richer in oxygen. PHOTO: LARS RIIS-DAMGAARD AND STEFFEN LARSEN902
THE MUD IS
ELECTRIC
Published by AAAS