Beautiful spies in the sea

Like tree rings, the structure of coral records layers of history. A new technique for reading it could help in the fight against marine pollution.
The bleached, bone-white skeletons of coral washed up on the beach and sold in tourist shops could be marine diaries, a history of the sea stretching back for thousands of years, a tale of pollution, climate changes, volcanic eruptions, even the El Nino effect. Like trees, corals lay down annual rings in light and dark bands which can be read as if they are the pages of a book. Now Dr Henrietta Lidiard, from the University of Bradford, and her colleagues are pioneering a new technique which may enable them to read daily detail as well as the yearly entries in this journal.

Corals are mostly dead. Living tissue is found only on the tips of what is in fact a skeleton. The skeleton is composed of calcium carbonate, but coral also incorporates other chemicals into this structure so that each layer of "bone" is actually a signature of the sea at that particular moment.

Corals are extremely sensitive to any changes in light, temperature and saltiness. Just as cold weather causes trees to lay down narrow annual rings, corals, if they are stressed in any way, will slow down their growth (a maximum of 2cm a year) and may take years to recover.

Because of their sensitivity, and the way corals integrate chemicals into their skeletons, even more precise climate changes can be recorded. Dr Peter Isdale, of the Australian Institute of Marine Science in Queensland, was able to estimate the amount of rainfall that occurs by examining coral skeletons. During times of heavy rain, more soil is washed into the rivers and then the sea. One of the components of soil, fulvic acid, is incorporated in coral, and under an ultra-violet light shows up in fluorescent bands.

Man-made pollution is also recorded by coral. In Panama, where the oil industry is active, one of the chemicals in oil, vanadium, is picked up by corals, researchers from Newcastle University discovered. The greater the spillage, the more vanadium found in the indigenous coral.

The corals Dr Lidiard studies are reef-building massive corals which have lived for as long as 50 years. Fossil corals can provide an even more impressive record of climate change stretching back for thousands of years.

Up to now, coral analysis could only provide an estimate of when events happened. Looking at annual rings is like taking the average figure for pollution or climate change in a year. "If you want bulk measurement, it's OK," says Dr Lidiard. The technique she is developing in collaboration with the British Geological Survey involves pinpointing the exact amount a coral grew in a day by examining the skeleton under a microscope and vaporising a tiny sample with an ultra-violet laser. The particles of coral in the resulting aerosol are then analysed to see what kind of chemicals have been used to help build the coral's skeleton on that particular day.

This new technique may also help prove - or disprove - a new theory on how coral grows. Dr David Barnes, from the Australian Institute of Marine Science, has shown that a coral skeleton is a lattice made up of horizontal buttresses and vertical rods and that these vertical rods may be growing both up and out at the same time. Dr Lidiard describes a rod as if it were a pencil. "As the tip of the pencil extends, it also thickens all the way down." So cutting these rods lengthways will allow the team to analyse a complete set of the daily growth rings for the coral.

Dr Lidiard's techniques could one day be used to enable coral to act as spies in the sea to provide evidence of pollution, for example, when chemical companies evade the law.