Numerous fish species, such sea bass, live in the salty water, said 28-year-old Elias Dzul, from a village near the Mayan ruins of Tulum, one of a growing number of people concerned about the reef's future.

"Commercial fishing is not viable, because it damages the ecosystem," said Dzul, who works in a sports fishing camp where tourists catch fish for fun and throw them back in the sea.

The Mesoamerican Reef, the world's second longest after Australia's Great Barrier Reef, stretches down from the top of the Yucatan peninsula some 1,000 kilometers (600 miles) in waters off Mexico, Belize, Guatemala and Honduras.

It is a clear and delicate example of how global warming appears to be affecting marine habitats.

Dzul works on the Sian Ka'an reserve that spreads around the reef, some 150 kilometers (100 miles) south of Cancun, where international negotiators this week hope to advance on agreements to limit climate change worldwide.

"If the negotiators limit themselves to a stabilization agreement to try to keep global temperature rises to two degrees Celsius, it will be the end of the coral reef," said Roberto Iglesias, a researcher from the National Autonomous University of Mexico's marine institute in Puerto Morelos, near Cancun.

"When the temperature rises, the coral loses its main source of energy, which comes from a symbiosis with the algae, which are losing their photosynthetic function," Iglesias said, underlining that 2010 has seen the highest recorded sea temperatures for a second consecutive year.

The reef plays an important role in protecting the coast, forming a natural barrier that breaks the force of the waves and tones down erosion of the beaches, particularly in the face of regular tropical storms and hurricanes.

But commercial fishing and marine pollution from tourist areas have weakened the ecosystem, which is struggling to adapt to the effects of climate change.

Scientists from the Puerto Morelos laboratory collect samples from the sea on a daily basis to measure the threats to the reef and how it is changing, including the effects of the acidification of the ocean.

"Thirty percent of carbon dioxide emitted into the atmosphere is absorbed by the ocean and that changes the chemical make-up of the ecosystems," Iglesias said.

Known as the Maya Riviera, the Mexican coast south of Cancun is a fast-developing tourist area, where the population is also expanding rapidly.

Some hotel owners, like Paul Sanchez, aim to show that it is possible to keep developing, but in a sustainable way.

Sanchez manages a complex of hotels that have around 12 rooms, in a former coconut plantation in the Bay of Akumal, on a beach of white sand formed from coral skeletons.

A garden of banana trees and mangroves lies near a restaurant terrace, but it is not only part of the scenery.

"The plants grow on a swamp of residual water from the hotel. The trees filter and purify the dirty water before it enters the sea," Sanchez said.

"The proof that it works is that it doesn't even smell," he said, underlining that if the water is acid it prevents the coral from reproducing.

Guatemalan biologist Marie-Claire Paiz, from The Nature Conservancy non-governmental group, said it was important to show hotel developers that preserving the reef made good business sense.

"For example in Cancun, where construction has broken the natural dynamics of the sand dunes, hotels spend 20 million dollars every two or three years to return sand to the beaches," Paiz said.

"They could save money if they managed them in an ecological manner."

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