Sugars and Bacterial Growth Kill off Coral Reefs
By Pippa Wysong, Access Excellence
SAN DIEGO (10/08/06) - Bacterial growth, stimulated
by the presence of simple sugars in untreated sewage and agricultural runoff,
can now be added to the
list of things contributing to the demise of coral reefs.
A study by researchers from the Scripps Institution
of Oceanography, the Smithsonian Tropical Research Institute (STRI)
and San Diego
State University shows that elevated organic carbon levels
(present in simple sugars) lead
to a chain of events that adversely affects coral health. Findings were published
in studies in the May 22, 2005 and June 9, 2006 issues of the journal Marine
Ecology Progress Series.
reef death has been attributed to a number of factors including declining
water quality, pollution, global warming, over-fishing and reef-specific
disease. Now sugars can be added to the list – possibly even as a major
player in some parts of the world, said David Kline, PhD, a post-doctoral
researcher at STRI.
Indeed, the research shows that sugars, or dissolved
organic carbon (DOC),
trigger an overgrowth of normally coral-friendly bacteria that in turn overgrow
and kill off the coral. Studies have shown that bacteria are present in corals
and contribute to their survival. The role bacteria play is just barely starting
to be understood, Dr. Kline told Access Excellence in an interview. It was
only a few years ago that researchers discovered different species of coral
each have distinct communities of bacteria living in them, and that the bacteria
are constantly present.
"It looks like the bacteria are likely part of the symbiotic community
living within the coral tissue," he said. Most of the bacteria live
within a mucosal
layer in the coral, though some live in the tissue as well.
Studies have shown that some of the bacteria help fix nitrogen contributing
to the nutritional requirements of the coral, while others carry out photosynthesis
and help provide the coral with energy. It's possible some bacteria serve
to protect the coral from other, harmful bacteria, Dr. Kline said.
However, Dr. Kline's research shows trouble happens when the populations
of bacteria normally present in coral grow out of control. "If the system
becomes out of balance – let's say, by adding too much simple
sugars – and the bacteria start growing too quickly, then the balance
breaks down. The same bacteria that were likely beneficial grow so quickly
that they actually can end up killing the coral," he said.
A study was done in the Caribbean in which close to 400 small samples of
coral were harvested. Each was placed in a separate container in Dr. Kline's
laboratory. The containers were custom made specifically for this. Different
chemicals were added to each sample to see which ones had an adverse effect. "When
I tested a whole suite of chemicals that are commonly found in pollution,
the one that was having the most damaging effect on the corals were these
simple sugars," he said.
Follow-up experiments showed that in normal, healthy coral, the bacteria
grew at a controlled rate. "But when they're around all these
sugars they start to grow out of control to such high levels that they end
up dying," he said. The bacteria may be using up all the local oxygen
causing the corals to suffocated. Even worse, some of the bacteria
may be producing toxins, and when these bacteria are at abnormally high levels
the coral can't withstand the toxins and are poisoned.
Where do sugars come from? Mostly from untreated
sewage that runs into the
ocean, and fertilizers that run-off from the land and into the water, according
to Dr. Kline. "Close to 90% of the sewage produced in the Caribbean
receives no treatment whatsoever – it's just dumped into the
ocean. And a lot of reefs are in areas near small towns and villages where
there’s no sewage treatment at all. Untreated sewage has really high
levels of simple sugars," he said.
Algae also play a role in the killing off of coral. Pollutants containing
nitrates and phosphates (commonly found in fertilizers and sewage) flow
into the ocean and trigger algae populations to grow more rapidly. Algae gets
its energy from photosynthesis, the main product of which is glucose – a
simple sugar. So, simple sugars produced by algae add to the overgrowth of
bacteria, which in turn kills off coral.
"And as the corals die, there's more space for algae. Then there
is more algae, and more sugar, and the whole cycle gets really bad," Dr.
Kline said. In the past couple of decades, there has been a shift from coral
dominance to algae dominance on many Caribbean reefs which has mirrored human
population growth in coastal villages, along with an increase in fishing
The whole process shows just how dynamic and delicate the ecological balance
in reef life is. At this point, the key is to start monitoring DOC levels
in coral reefs, he said.