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.

Coral 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 and pollutants.

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.
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