SDSU researchers turn the tide on coral decline
If you’ve ever seen a coral reef, you know it’s a spectacular sight.
Explosions of pink, blue, red and purple tint the shallow waters off the coast of tropical beaches where the water stays a comfortable 78 to 82 degrees Fahrenheit.
A healthy reef provides a habitat for fish.
Producing energy through photosynthesis and laying down a habitat of skeleton, the coral reefs support a web of species from small invertebrates to sharks.
Under favorable conditions, they are possibly immortal — one coral colony in Japan is estimated to be 5,000 years old. But we may never know for sure, since corals are dying across the globe.
“We think 30 percent are destroyed, and more than 50 percent are seriously threatened,” said Forest Rohwer, an SDSU biology professor specializing in phage bacteria and coral reefs. “It’s hard to get reliable numbers, but it’s shockingly bad. I’m not very old and I’ve seen it happen.”
Human factors
Though the threats to coral reefs are many — habitat destruction, global warming, pollution and overfishing, to name a few — they can all be traced to human activity.
Habitat destruction is a regional problem, and can constitute anything from throwing explosives into the water to hunt fish to harvesting coral for cement or building directly on top of it.
“People who live on coral atolls out in the middle of the ocean, where there are few building products, use coral colonies to build their houses,” said Liz Dinsdale, SDSU biology postdoctoral fellow. “Building marinas, harbors and airports on top of coral reefs is really common and destructive.”
Global warming
Other factors threaten coral on a larger scale.
As humans’ energy needs increase and more greenhouse gases are released, more heat gets trapped in the earth’s atmosphere, causing rising temperatures and warmer oceans.
Warmer water caused this coral to start bleaching.
A rise in ocean temperature — even of just two degrees — can immediately kill coral. But more commonly, coral die a slow death through a process called bleaching in which their own food source is turned against them.
Corals derive energy from eating microscopic organisms and from the bioproducts of photosynthesis, conducted by the symbiotic microalgae that live inside them.
In the photosynthesis process, the microalgae take carbon dioxide and water, and cut the molecules in half to create sugar. Sugar is a source of energy for both the algae and the coral. A certain number of destructive radicals, a type of oxygen, are released; but not enough to harm the coral.
However, warmer water causes the microalgae to perform photosynthesis at a much faster rate, thereby creating a harmful concentration of radicals. In a response meant to remove the buildup of destructive radicals, the coral expunges algae, its own source of nutrition.
At this point, the coral has lost its vibrant color; it is not dead, but close.
“When they kick out the algae, at the very least, they are no longer laying down skeletons,” said Rohwer. “That’s important on a bigger scale. A reef is a large structure with all these caves. That’s the habitat that everything else is living in. If it all falls apart, which it does pretty fast, then you just get this dead zone, and it’s pretty spectacularly dead.”
Rohwer’s team
In addition to known human impacts, mere proximity of human settlement and its inevitable conditions, are enough to threaten coral.
SDSU professor Forest Rohwer samples reef water.
“Corals die in places where people are trying to keep their reefs in good shape,” he said. “For example, if you are building a hotel, you want to keep the reefs healthy because they attract tourists. People will pay a lot to save them, they will take a lot of precautions and yet the reef still falls apart.”
Rohwer and his team have been looking at the relationship between human proximity and coral death, since it is considered far more treatable than global climate change or regional damage.
While many researchers have looked to the disease-inducing microbes for the cause, Rohwer’s team looks at what the microbes are using for energy: the organic carbon that humans generate.
“That seems to be the key,” said Rohwer. “You start with about 100,000 bacteria per milliliter. When people move in, you now have a million bacteria per milliliter. It’s like swimming in sewage.”
Humans cause an increase in organic carbon directly through pollution and indirectly through overfishing. Human waste — fertilizers, pesticides, sewage — puts organic carbon directly into the ocean, creating an overabundance of food for microbes, which cause disease in corals.
Without fish to eat algae, it attacks the coral.
The sugar released by the algae in the photosynthesis process not only fuels the algae and the coral, but the microbes, as well. Fish normally keep these algae at bay.
“When you’re on a reef, it’s noisy because the fish are chomping. It sounds like buffalo on the range, it’s really that loud,” Rohwer said.
When the fish population disappears, algae and the microbe population grow rampant. This increased bacteria results in visibly cloudy water and a higher level of coral disease.
In 2005, the group visited several islands in the Northern Line Island chain, about 1,000 miles south of Hawaii. The researchers had the opportunity to compare reefs off the uninhabited island of Kingman with those off Christmas Island, which has a population of 5,000.
The difference was dramatic.
“From Kingman to Christmas, the coral systematically was lost,” said Stuart Sandin, a postdoctoral marine ecologist at the Scripps Institution of Oceanography who organized the expedition.
The increase of algae on the reef was indicative of an environment that is not conducive to coral growth and survival.
Hope for the future
Though some of these culprits, like overfishing, have been at work for the last 1,000 years, researchers have seen a dramatic shift over the last 50 years, most likely due to an increase in the number of microbes in the water surrounding the coral colonies. Rapid global warming and increased human population will further stress coral communities.
According to Rohwer, corals could be gone by 2050, when water temperature, human population and ocean acidity factors are expected to peak. The situation is so far gone that researchers are only now starting to recognize that what they call a “healthy” coral isn’t.
“What we’re calling ‘healthy’ is so far from healthy. It’s just because we’re seeing what it looks like now, and one looks better than this one, so we’re calling it healthy. But it used to be amazingly different,” said Rohwer.
Corals like this could be gone by 2050.
However, there is some hope. Though curbing global warming and local practices that harm the reefs will be a difficult and slow process, Rohwer said that by working now to treat sewage and stop overfishing, people can help turn the tide on coral decline.
Related information
Credits
- Story by Lauren Coartney
- Photographs courtesy of Forest Rohwer
- Graphics by John Signer
- Edited by Happy Aston and Aaron Hoskins
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