May 17, 2011
As we pump more and more carbon dioxide into the atmosphere, the ocean absorbs some of it. And as CO2 dissolves, it makes the oceans’ water more and more acidic. This acidification creates plenty of potential problems for life in the oceans, but corals might have it the worst. If the ocean becomes too acidic they won’t be able to create their calcified skeletons; the chemical reaction they rely on slows down under lower pH levels . But scientists in Australia say that the situation is more dire than expected. In their study, published in Ecology Letters, they show that higher CO2 levels may be give seaweed an advantage in a competition with coral.
Corals compete with seaweeds for space on the reef. When corals are healthy, the coral–seaweed competition reaches a balance. But if the corals aren’t doing so well because of something like eutrophication, then seaweed can take over.
In this new study, the researchers studied the coral-seaweed battle in miniature, setting up bits of each (Acropora intermedia, the most common hard coral in the Great Barrier Reef, and Lobophora papenfussii, an abundant reef seaweed) in tanks in the lab. Each tank had one of four CO2 levels in the air above it, resulting in four different pH levels: 300 parts per million (equivalent to pre-industrial CO2 and pH levels), 400 ppm (present-day), 560 ppm (mid-21st-century estimate) and 1140 ppm (late-21st-century estimate).
When there was no seaweed, the corals survived. But with its competitor present, the corals declined under each scenario. However, the decline was worse under higher CO2 levels, to the point where under the late-21st-century scenario, there was no living coral left after a mere three weeks.
“Our results suggest that coral (Acropora) reefs may become increasingly susceptible to seaweed proliferation under ocean acidification,” the researchers write. This area of research is still in the early stages and this experiment was a simplification of the coral–seaweed dynamic (there were only two species tested, for example, and plant-eating fish were left out of the equation), but it may provide even more reason to worry about the future of the coral reefs.
November 19, 2009
My colleague Megan Gambino visited the Smithsonian Tropical Research Institute earlier this year to watch coral spawn. A report appears in the December issue of the magazine, and she also blogged about the experience over at Around the Mall. We asked her if anything interesting got left out of her previous reports. Yes, lots, she replied, and wrote this:
This past September, I joined marine scientist Nancy Knowlton, of the National Museum of Natural History; her colleague Don Levitan, of Florida State University; and a crew of research divers on their annual coral spawning trip. Just days after the September full moon, a mass coral spawning happens at their study site, a 260-foot arc of reef about 20 minutes by boat from the Smithsonian Tropical Research Institute’s field station in Bocas del Toro, Panama, and each year, since 2000, they have been there to collect data.
Knowlton, a renowned coral reef biologist, has been called Dr. Doom for the grim, but realistic, picture she paints of reefs suffering worldwide. (Her husband Jeremy Jackson, also a prominent marine scientist, is Dr. Gloom.) But she has also been billed as a savior. Vanity Fair, in its May 2007 “Green Issue,” called her a “mind aquatic” that our future, and our lives, may depend on. Along with other marine scientists, Knowlton has been trying to help reefs survive by better understanding coral reproduction.
Early in Knowlton’s career, the assumption was that most coral colonies picked up sperm and brooded embryos internally—and some do. But in 1984, Science published the first description of a dramatic mass-spawning event witnessed on Australia’s Great Barrier Reef. Around that time, research biologists were observing the phenomenon in the Caribbean as well. From this, scientists deduced that the majority of corals—called “broadcast spawners”—actually reproduce in this way. Many are hermaphrodites, meaning they release gamete bundles containing both eggs and sperm. But, unable to self-fertilize, they synchronize their spawning with neighboring corals. The more scientists study the annual orgies, the better they have become at predicting when they will happen. The corals appear to use three cues: the full moon and sunset, which they can sense through photoreceptors; and, most likely, a chemical that allows them to smell each other spawning.
Knowlton’s team has been monitoring three closely related coral species—all dominant reef builders in the Caribbean—called the Montastraea annularis complex. What they have found is that M. franksi, one of the species, spawns on average 100 minutes after sunset and M. annularis and M. faveolata, the other two, follow about 100 minutes later, typically five and six days after the September full moon. Over the nine years of the project, the researchers have spotted, flagged, mapped and genetically identified over 400 spawning coral colonies.
As with any long-term study, the scientists’ questions have evolved. At first, they wondered how the three species, spawning at or close to the same time, didn’t hybridize. Their lab tests show that of the three, the early spawner and one of the later spawners are reproductively compatible. But they have found that the hour and a half or so between the species’ peak spawning times is enough time for the gametes to disperse, dilute, age and effectively be rendered unviable. In fact, their data indicates that if corals spawn just 15 minutes out of sync with the majority, their chance at reproductive success is greatly reduced. The looming question now is, what will happen to fertilization rates as coral colonies become few and far between?
By the third of four nights of diving (and no spawning), the suspense was building. The divers playfully suggested playing Barry White as mood music and gorging, pre-dive, on aphrodisiacs like oysters and strawberries.
Around 7:25 PM, just as everyone was slinking into their wetsuits, sea worms called palolo worms began spawning around the boat. The worms break in half and the tail section swims to the surface and releases eggs or sperm in a cloud of bioluminescence.
“This is it,” said Knowlton. “Everybody’s in the mood for sex.”
The water got buggy and electric, and like clockwork, the coral colonies started spawning around 8:20, one triggering another triggering another. The tapioca-like gamete bundles, about two millimeters in diameter and containing about 100 eggs and one million sperm, lifted in unison, slowly drifting to the surface.
The dive team observed 162 different coral colonies set or spawn, and the next night, they saw another 189. Knowlton surfaced that final night, exhilarated. What did you think? She asked each of the divers. Amazing, huh? She didn’t want to get out of the water and grabbed hold of the side of the boat, arching her back, her eyes cast toward the sky. Even the stars looked like gametes.
May 15, 2009
The ocean is getting warmer, higher and more acidic due to climate change. How well will coral reefs respond to such stresses?
To find out, a team of researchers led by the University of Texas at Austin is looking to corals’ genes. Sequencing a genome can take years, but a new method developed by the UT researchers reduced that time frame to one month. They focused on the nearly 11,000 genes that the coral actually uses, instead of the unused genes and DNA bits that make up most of the organism’s genome.
The scientists tested their method on the Pacific coral, Acropora millepora, and hope to see an explosion in research about coral adaption and evolution as a result.
– by Joseph Caputo