Chitin, the Earth's second most abundant biological
material, is a major component in the flurry of skeletal
debris discarded daily by crustacean creatures in the
world's oceans. If left undisturbed, this tough insoluble
material, a cousin to cellulose, would pile up on the
ocean's floor and wreak havoc with marine ecosystems.
Fortunately, armies of bacteria act as chitin's cleanup
crew, and two Johns Hopkins biologists have made a key
discovery about how and when these microscopic soldiers
launch their search-and-devour missions.
Writing in the Online Early Edition of Proceedings
of the National Academy of Sciences for the week of
Dec. 29, 2003, Xibing Li and Saul Roseman reported that
they had found a genetic master switch that reacts to the
presence of nearby chitin and sets off a biological chain
reaction, causing the bacterial feast to begin.
Understanding this process is important because 100 billion
tons of chitin (pronounced KITE-in) are dumped annually in
the oceans, largely by tiny sea animals called copepods,
which shed their shells as they grow.
"If nothing happened to this debris, we'd be up to our
eyeballs in chitin, and the carbon and nitrogen cycle upon
which marine life depends would be gone within 50 to 75
years," said Roseman, a professor of
biology in the
Krieger School of Arts and Sciences.
Researchers were puzzled about the disappearance of
chitin because little of the material turned up in sediment
on the ocean floors. Where did all of the chitin go? Then,
about 70 years ago, two microbiologists determined that
bacteria were quickly consuming the sinking shells and
preserving the ecological balance. Since then, however,
several mysteries have remained. How do the bacteria find
these undersea meals? How do these microorganisms attach
themselves to the chitin? How do they degrade the tough
material and turn it into food?
During the past decade, Roseman and his colleagues
have made several advances in answering these questions. In
the new PNAS paper, Li and Roseman reported that they had
identified and isolated the bacterial master switch that
controls at least 50 and perhaps up to 300 other genes
involved in the chitin sensing and consumption process. The
biologists made their discovery by studying mutated
versions of Vibrios, the ocean's most common bacteria,
which can cause illnesses such as cholera. The scientists
separated and tested the mutant strains according to their
ability to detect and break down chitin; then they analyzed
the bacteria's genetic structure to pinpoint the master
switch.
"We believe," Roseman said, "that when the Vibrios are
not in their feeding mode, this master switch remains in
the 'minus' or 'off' position, locked in place by a binding
protein. This keeps the cells from wasting energy by
manufacturing proteins that won't do them any good at that
time."
Roseman added, "When the bacteria are starving,
however, they secrete an enzyme called chitinase into the
water. When chitinase touches the discarded shell material,
it begins breaking down chitin, releasing a partially
degraded soluble form into the water. These molecules are
the signals to the bacteria that chitin is nearby.
Diffusing through the ocean near the bacteria, these
dissolved fragments of degraded chitin bind to the binding
protein and remove the 'lock,' allowing the master switch
to move into a 'plus' or 'on' position."
When the switch is on, the bacteria's genes get to
work, moving the organisms along the trail of partially
degraded chitin back to its source material, like a hungry
traveler following the aroma of hot food to a roadside
restaurant. In the ocean, the bacteria follow a gradient
stream of higher and higher concentrations of dissolved
degraded chitin until they reach the solid shell material.
The bacteria then latch on and begin their feast.
"The master switch gene appears to be the key to this
complex feeding process," said Li, an associate research
scientist in the Department of Biology and lead author of
the new paper. "This gives us a better understanding of the
microscopic processes that keep our oceans from being
overwhelmed by biological debris from sea creatures."
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