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THE skeletons of stony corals are made of calcium carbonate, in the
form of aragonite secreted by the coral polyps. Though the process by which
the skeleton forms is unclear, the coral probably removes calcium directly
from sea water and combines it with inorganic carbon, consisting of carbon
dioxide from respiration and bicarbonate from sea water.

Different forms of coral have charac teristic skeletons – the very solid
brain corals and the more fragile branching staghorns, for example. The
type of skeleton is determined by the balance between the processes of calcification
and the extension of the animal’s tissues. The tissue of brain coral, for
example, extends very slowly while laying down calcium carbonate at a fairly
constant rate, so its skeleton grows very dense; the staghorns, on the other
hand, grow much faster, while depositing calcium carbonate at similar rates,
and so are much less dense.

The rate at which the coral skeleton is laid down is strongly influenced
by the amount of light available, because there is a direct link between
photosynthesis by the coral’s symbiotic algae and the rate of calcification.
The temperature of the water around the reef is also important; the optimum
temperature for the deposition of calcium carbonate, and so for the growth
of coral, is between 26 and 27 Degree C. If the temperature falls below
23 Degree C or rises above 29 Degree C, calcification slows dramatically.
Seasonal changes in temperature and light intensity produce recognisable
bands in the coral. Researchers have been able to chart such changes over
the past 750 years from the patterns of banding in some massive brain corals.

Other environmental factors that influence the growth of corals are
salinity, the amount of sediment in the water and the concentrations of
nutrients around the reef. Most corals are adapted to life in clear water
containing very low concentrations of inorganic nutrients, such as phosphate
and ammonium. An excess of phosphate alters the architecture of the coral
skeleton, producing a very fragile structure. The inorganic chemistry of
calcium carbonate precipitation suggests that phosphates may be acting as
‘crystal poisons’. Phosphates, particularly polyphosphates, fit almost perfectly
onto the growing calcium carbonate crys tal, effectively inhibiting further
growth of the crystal, or leading to the formation of ‘stressed’ growth
forms.

With the Great Barrier Reef as a major tourist attraction, tropical
Australia is experiencing a massive increase in coastal development as well
as a huge increase in activity on the reef itself. The influx of phosphate
from sewage, added to the runoff from agriculture and effluent from industry
on the mainland, will inevitably add nutrients to the water unless strict
controls are enforced.

Already the effects of nutrient pollution are visible on reefs surrounding
island resorts such as those around Green Island (off Cairns) and reefs
subject to heavy recreational use, such as John Brewer Reef. Here, the range
of colours typical of a living reef has largely been replaced by the monotone
green of algae that have overgrown dead coral. In other parts of the world,
almost all coral reefs close to centres of population have been des troyed.
The Great Barrier Reef must not suffer the same fate as, for example, the
reefs off Japan.

David Yellowlees is in the department of chemistry and biochemistry
at James Cook University of North Queensland.