Mote Marine Laboratory

Slime Sleuth

By: Nadine Slimak

Researchers have been studying coral diseases for years, but the key to coral survival might lie in unlocking the bacterial makeup of the slime that corals produce.

Never mind all that wash-your-hands-germ-and-disease stuff. Bacteria are cool, to hear Dr. Kim Ritchie tell it.

For one thing, they’re old. Fossil records show that bacteria have been around for at least 3.5 billion years — on a 4.5 billion-year-old planet, that’s not too bad. And scientists say that life as we know it started with bacteria as building blocks.

For another thing, bacteria are a pretty weighty group — that is, if you tried to weigh all the bacteria estimated to be living on the planet, you’d end up with about a gigaton of material, about the weight of all the Earth’s plant material. Also not bad, considering that most bacteria can’t be seen without a microscope.

But one of the coolest things of all? They’re a mystery. “It’s estimated that less than one percent of bacteria are able to be cultivated in the laboratory,” says Ritchie, head of Mote’s Coral Microbiology Program. “That’s just a small percentage of what’s actually there. People underestimate bacteria.”

Not Ritchie.

She thinks that the key to understanding why some corals die and why others remain healthy lurks in the slime that corals produce — that is, in the bacteria living on the slime.

“Researchers have gone straight to diseased coral to try and figure out why it’s sick,” Ritchie says. “But, just like people, if you don’t understand the immune system, you won’t understand why a coral is sick.”

Ritchie’s theory is that the mucous that corals produce — slime, if you will — houses certain bacteria that help keep the corals healthy. When the corals’ bacterial makeup changes, they get sick.

“This is an entirely new area as far as coral reef research goes,” says Billy Causey, whose job as superintendent of the Florida Keys National Marine Sanctuary is to protect the world’s third-largest bank barrier reef. “Science is so fixed on (coral) diseases — they have workshops on what to name them. But as a manager, I want to get at the problems causing the disease so we can prevent the train wreck that’s coming.”

Understanding the animal

Though they may seem plant-like, corals really are animals like jellyfish or sea anemones. The coral animals, called polyps, secrete calcium carbonate that makes up the reef skeleton, the place they call home.

The polyps are only part of the equation. Another part is the zooxanthellae (zoh-zan-THEL-ee), which are tiny single-celled algae that live in the polyp tissues and give reefs their colors. The zooxanthellae produce oxygen and energy during their photosynthetic feeding that, in turn, helps feed the coral polyps. In this symbiotic relationship, zooxanthellae help feed coral, and coral give zooxanthellae a place to live.

But there may be a third part of the equation — call it the bacteria factor.

Ritchie’s initial work has shown that certain groups of bacteria — the Vibrios and the Pseudomonads — live on healthy coral reefs in just about equal parts at all times. But that balance changes under certain conditions.

To keep growing and to stay healthy, coral polyps and their zooxanthellae depend on the right water temperatures and the right amount of salt content. They even depend on how clear the water is, how clean the water is and how well the water is flowing. When any one of these things is thrown off kilter — a hurricane, for example, kicks up silt that makes the water murky, or temperature or water flow changes — it can affect how healthy the corals are.

Ritchie has found that when corals are bleached, meaning the zooxanthellae that give them their color leave, the balance of Vibrio-type and Pseudomonas-type bacteria change. “The Vibrios go up and the Pseudomonads go down,” Ritchie says. “Once there is this unhealthy shift, I believe that the corals are more susceptible to disease.”

That’s because most pathogens are opportunistic — that is, they wait for a weakness and exploit it. For humans, that may mean catching a cold when we’re overworked and harried. For stressed corals it could mean catching white pox or black band disease.

But understanding the disease isn’t the key to coral health, Ritchie says. “The fact that corals have diseases is not that interesting or surprising. How they stay healthy is.”

Since some bacteria can cause disease, how is it that corals — which have bacteria on them all the time — are able to survive at all? You could ask the same thing about humans, who have an average of 200 different kinds of bacteria living on them or in them all the time. They only make us sick when the proportions are out of balance.

The antibiotic factor

In corals, just talking about the bacteria living in the slime on a reef isn’t enough. You’ve got to look at the kinds of bacteria on the reef and how they make a living to understand coral reef immune systems. “You have to figure out what the bacteria do,” Ritchie says.

Some bacteria survive by being bigger and badder and muscling out the guys next to them. Others survive by making a substance that keeps the bigger badder guys in check.

“They produce compounds that most efficiently kill the competition,” Ritchie says.

That’s the antibiotic factor.

Initial tests have shown that the group of Pseudomonas-like bacteria living on the reef may produce more antibiotics than the Vibrios. So when the ratio of Vibrios and Pseudomonas bacteria change — that is, there are fewer bacteria on the coral producing their own antibiotics — that may be letting “bad” bacteria get out of hand and kill the coral.

“People have been grinding up coral and knocking chunks off coral to study them,” Ritchie says. “But the key may be in the microbes that make these antibiotics. There’s a high incidence of bacteria that produce antibiotics that may play a role in natural defense.”

Looking at coral’s natural defenses could help sanctuary officials manage the reef, Causey says. “We can detect temperature changes through satellites, but we can’t do anything to help us understand the changes at the smallest level. We need both scales.”

Slime sleuth

To study the bacteria, Ritchie doesn’t break off pieces of coral. She merely holds her breath, free-dives 15 to 20 feet to a coral colony and uses a needle-less syringe to suck a sample of slime from the coral.

Ritchie’s initial work in this study has focused on elkhorn corals, which federal government officials listed as threatened under the U.S. endangered species act in 2006 because their populations have been so devastated by bleaching and disease.

She takes multiple samples from multiple elkhorn colonies at each of three general sites: the upper, the middle and the lower Keys. “I don’t want to look at a coral that is too old or too young, because that puts too much variability into the study,” Ritchie says. “I try to look for corals that are roughly the same size, in the 5- to-10-year range.”

She’s looking at three sites because there are differences in the health of the corals along the reef tract, which runs from just south of Fort Lauderdale to beyond the Dry Tortugas.

The inshore reefs in the Keys tend to be healthier than the shallow offshore reefs, and the bacterial colonies living in upper Keys coral are not exactly the same as the colonies living on middle or lower reefs.

Ritchie also takes water samples at each site, to make sure she’s looking at bacteria on the coral and not just in the water column.

Once she collects her samples, Ritchie takes the slime back to the lab and puts it on a special marine agar to grow the bacteria. After that, she isolates different kinds of bacteria and looks at their different attributes to get a general idea of what kind of bacteria there are and what they can do. Then she sequences the bacterial genes used for identification and feeds that information into an international database.

“The general rule with bacteria is that if it’s 97 percent identical to something in the database or greater, it’s the same species,” Ritchie says. “If it’s less than 97 percent, it may be a new species.”

In her work on many different coral species, Ritchie has found more than 100 new strains of bacteria. Those numbers are only expected to increase as researchers focus more on the microbiology of the oceans.

In this study, if Ritchie finds “good” bacteria that can fend off “bad” bacteria, Causey thinks that might hold the key to helping sick coral. “If certain bacteria can fend off other invading bacteria, can we use it to inoculate coral?” Causey asks. “Could there be mass inoculations? Some things may work and some things may not, but that’s what scientific study is all about.”

One thing Causey is sure of, though, is that doing nothing is just not an option. “Forty percent of North America drains into the Gulf of Mexico,” he says. “We have global climate change. We are part of the evolutionary process. Can we sit back and not intervene when we have the tools to intervene? We have to be willing to intervene as managers if we determine man is the driving force behind the decline in coral health. But we have to have the support and backing and blessing of the scientific community.”

Learn more about: Coral Reef Research

Where's the reef? Without your help, this could become a too-familiar cry in years to come. So support our efforts to shore up reef ecosystems and continue to learn about the "good" bacteria that help them thrive. Mote lab studies and fieldwork both depend on your donations.