The Scientist November 2019

THE PAPER

R.C. Aller et al., “Worm tubes as conduits

for the electrogenic microbial grid in marine

sediments,” Sci Adv, 5:eaaw3651, 2019.

Cable bacteria (family Desulfobulbaceae)

are known for forming long lines of cells that

stretch through underwater sediments and

usher a stream of electrons through their

ranks. It was thought that these living elec-

trical wires couldn’t grow in environments

shared by animals such as brittle stars or

mantis shrimp, which violently stir up the

mud and might cut the microbial cables to

bits. But it turns out that cable bacteria can

withstand this upheaval—if they team up

with parchment worms.

Cells at the sediment-embedded ends

of these bacterial filaments collect electrons

from sulfides and other donor molecules

in the sediment and pass the particles up

the cable to react with oxygen in the top-

most layers of mud. The process releases

energy for all the cells within the filament.

Researchers had thought that cable bacteria

must typically build filaments straight down

through centimeters’ worth of sediment to

have access to both the electron-donating and

electron-accepting environments, a feat that

couldn’t be achieved in agitated areas of the

seafloor. But Stony Brook University marine

biogeochemist Robert Aller wondered if

they might still be found lurking somewhere

in perturbed sediments.

Aller and his colleagues went searching

for the microbes in the mixed-up mud of the

Great Peconic Bay and Long Island Sound

in New York. The team collected sediment

cores, then used radiochemical analyses,

light microscopy, and other techniques to

learn how solutes and bacteria were distrib-

uted in each sample. The analyses revealed

that cable bacteria aggregate in stable zones

near parchment worms (Chaetopterus vari-

opedatus), with filaments growing up to 650

μm in length in these areas.

Parchment worms construct sturdy

U-shaped tubes from carbohydrates and

amino acids and continually collect dissolved

oxygen from water that they pump through

the fibrous structure. A tube can stay in place

for months or more while its resident worm

hunkers inside, providing bacteria with a

shortcut into the mud. The results of Aller’s

study suggested that cable bacteria, which

require both a reliable oxygen source and

physically stable surroundings, cozy up to

these worm tubes to survive.

Instead of growing straight down into

the mud, the microbes radiate in all direc-

tions from the parchment worm tube “just

the way people crowd around a luggage

carousel at the airport,” Aller says. Belinda

Martin of the University of Western

Australia and colleagues previously found

that cable bacteria form similar relation-

ships with seagrasses, huddling around their

roots for stability (ISME J, 13:707–19, 2019).

Because the sediment samples were only

gathered from a few locations, it’s not clear

how widespread the phenomenon of the

cable bacteria–worm partnership is, says

Sairah Malkin, a biogeochemist and ecologist

at the University of Maryland who was not

involved in the work. Nonetheless, the find-

ings suggest that cable bacteria may occupy

a larger variety of environments than pre-

viously thought, she adds—including sedi-

ments that get disturbed by other organisms.

Beyond parchment worms and sea-

grasses, questions remain about what other

species cable bacteria associate with, per-

haps even as partners with which they

exchange electrons. “A big exciting question

could be, can other microbes plug into the

cable bacteria?” says Malkin.

—Nicoletta Lanese © KELLY FINAN/CAT WILSON

48 THE SCIENTIST | the-scientist.com

The Literature

EDITOR’S CHOICE PAPERS

MICROBIOLOGY

Living Cables

PLUGGED IN: Parchment worms (Chaetopterus variopedatus) construct tubes in underwater sediment and

constantly pump water through the structures to collect oxygen. Cable bacteria (Desulfobulbaceae) act like

electrical wires, connecting to the tube lining and providing electrons that react with the oxygen, generating energy

for the microbes. The bacteria extend radially in the mud to gather these electrons from sulfi de—compounds often

produced by nearby bacteria.

Bacterial fi lament

Sulfate-reducing bacteria Worm

Electron transport

SO 4 2- H 2 S + 4 x H 2 O

SO 4 2- + 8 x + 10 x

O 2 + 4 x + 4 x

2 x H 2 O