Bacteria dance the electric slide, officially named electrokinesis by
the USC geobiologists who discovered the phenomenon.
Their study, published online in the Proceedings of the National
Academy of Sciences (Early Edition), describes what appears to be
an entirely new bacterial behavior.
The metal-metabolizing Shewanella oneidensis microbe does not just
cling to metal in its environment, as previously thought. Instead, it
harvests electrochemical energy obtained upon contact with the metal and
swims furiously for a few minutes before landing again.
Electrokinesis is more than a curiosity. Laboratory director and
co-author Kenneth Nealson, the Wrigley Professor of Geobiology at USC
and discoverer of Shewanella, hopes to boost the power of microbe-based
fuel cells enough to produce usable energy.
The discovery of electrokinesis does not achieve that goal directly,
but it should help researchers to better tune the complex living engines
of microbial fuel cells.
"To optimize the bacteria is far more complicated than to optimize
the fuel cell," Nealson said.
Electrokinesis was discovered in 2007 by Nealson’s graduate student
Howard Harris, an undergraduate at the time.
Nealson had given Harris what seemed an ideal assignment for a double
major in cinema and biophysics.
"I had asked him if he would just take some movies of these bacteria
doing what they do," Nealson said.
Filming through a microscope is hardly simple, but with the help of
co-author and biophysics expert Moh El-Naggar, assistant professor of
physics and astronomy at USC, Harris was able to make a computer
analysis of a time-lapse sequence of bacteria near metal oxide
particles.
"Every time the bacteria were around these particles … there was a
great deal of swimming activity," Nealson recalled.
Harris then discovered that bacteria displayed the same behavior
around the electrode of a battery. The swimming stopped when the
electrode turned off, suggesting that the activity was electrical in
origin.
As is often true with discoveries, this one raises more questions
than it answers. Two in particular intrigue the researchers:
- Why do the bacteria expend valuable energy swimming around?
- How do the bacteria find the metal and return to it? Do they
sense it through an electric field or the behavior of other bacteria?
Nealson and his team so far have only educated guesses.
For the first question, Nealson believes that the bacteria may swim
away from the metal because they have too many competitors.
Bacteria get energy in two steps: by absorbing dissolved nutrients
and then by converting those nutrients into biologically useful forms of
energy through respiration, or the loss of electrons to an electron
acceptor such as iron or manganese (humans also respire through the loss
of electrons to oxygen, one of the most powerful electron acceptors).
"If electrons don’t flow, it doesn’t matter how much food you have,"
Nealson said.
However, he added, "in some environments, the food is much more
precious than the electron acceptors."
If a metal surface became too crowded for bacteria to absorb
nutrients easily, they might want to swim away and come back.
For the second question, Harris and co-author Mandy Ward, assistant
professor of research in earth sciences at USC, are planning other
experiments to understand exactly how Shewanella find electron
acceptors.
They expect the experiments to keep Harris busy through his doctoral
thesis.
The other co-authors on the PNAS paper were Orianna Bretschger of the
J. Craig Venter Institute in San Diego, Margaret Romine of Pacific
Northwest National Laboratory, and Anna Obraztsova, staff scientist in
the Nealson laboratory at USC.
Source : University
of Southern California
