Friends for high noon

No one doubts that human-induced climate change has been killing corals across the globe. The question is whether humans can help save them before the devastation is complete.

For decades, rising sea surface temperatures have been driving out and killing the algae, called zooxanthellae, that give reefs their often-spectacular colour.

That has left behind the lifeless, bleached skeletons built by clustered colonies of thousands of corals. Meanwhile, the oceans' growing acidity, caused by rising levels of carbon dioxide in the water, impedes the biological processes that allow corals to create their limestone structures.

Those changes have devastating effects on the intricate collaboration necessary to build a coral head or reef or fan.

That process is the product of a symbiotic marriage between the tiny marine creatures that are corals and even tinier single-cell algae that take up residence in corals and provide them with nutrients as the algae photosynthesise.

A 2004 study estimated that global warming had destroyed 20 per cent of the world's reefs since the 1950s.

esearchers have found that in seven tropical regions where most coral reefs grow, waters had warmed by 1.3 to 3 degrees over the past century. A temperature rise of 1.8 to 3.6 degrees above the average high temperature for the summer can trigger bleaching in many reefs.

Since then, however, scientists have learnt that some corals seem to resist warming temperatures better than others. Andrew Baker, a University of Miami marine biologist, is about to embark on an experiment to learn whether scientists can help corals adapt by providing them with symbiotic partners better prepared to cope with waters that are growing warmer largely because of the buildup of greenhouse gases from burning fossil fuels.

Self -help and the best

Some corals have evolved to do this on their own, over a long period of time: Now, researchers want to see if they can speed up the process.

“It's really important we make the effort to make sure there are as many corals as possible left to save,'' Baker said.
Steve Palumbi, a Stanford University marine biology professor, directs a mapping project at his laboratory to determine where heat-resistant zooxanthellae reside and where they may appear in the years to come. He called Baker's work “one of the first clinical trials for a response to climate change and coral bleaching''.

Researchers used to believe there was just one species of zooxanthellae in corals. But since the start of the decade, they have begun to discover that there are more than a dozen and that some are better at adapting to rising temperatures than others.

“Not all zooxanthellae are created equal,'' said Mark Erdmann, a senior adviser to Conservation International's Indonesia marine programme. He has found some algae species thriving in waters from 66.2 to 95Þ Fahrenheit, depending on the place and time of year. (In Saudi Arabia's waters, corals flourish in nearly 97Þ Fahrenheit temperatures.)

The algae that can tolerate the hottest conditions, Erdmann said, “will very likely be crucial to the future survival of corals''.
The task of mapping reef areas that are both most resistant and most vulnerable has taken on urgency among scientists.

Palumbi and his colleagues are preparing to publish their map of both present and future locations of heat-tolerant algae worldwide, and Wildlife Conservation Society senior zoologist Tim McClanahan has mapped the vulnerability of Indian Ocean reefs to climate change.

A group of researchers led by Benjamin S. Halpern at the National Centre for Ecological Analysis and Synthesis in Santa Barbara, California, has published a synthesis of 17 global data sets that concluded that nearly half of all coral reefs are experiencing “medium high to very high impact'' from human pressures, including temperature increases, pollution and overfishing.

Meanwhile, Baker — who first broached the idea that corals might switch algae partners on their own in a 2001 article in the journal Nature — is hoping to reverse the trend.

He keeps 12,000 coral tissue samples from 20 countries in his laboratory at Miami's Rosenstiel School of Marine and Atmospheric Science, freezing them at 176Þ Fahrenheit below zero, and has spent several years culturing them and studying their properties.

The thermally resistant species of zooxanthellae Baker first identified goes under the name “clade D''; he calls the algae “sort of the weeds that do well when others don't''.

The Pew Institute for Ocean Science has awarded Baker a three-year, $150,000 grant to help identify the specific genetic and physiological factors that allow some corals to cope better with the warming.

Initially, Baker and his team will artificially bleach corals and then add cultured algae to the water to see if other zooxanthellae varieties can help the corals adapt to the temperature shift.

In another set of experiments, the scientists plan to inject thermally resistant algae into the polyps that allow corals to reproduce. If these two trials succeed, they will try injecting these zooxanthellae into the oldest and largest coral colonies that produce the most larvae.