The Great Barrier Reef
70
yet another important part of the puzzle for how coral
reefs maintain themselves in the nutrient-poor (oligotrophic) conditions of tropical seas.
Plant-animal symbiosis
One of the hallmarks of coral reefs is the high number of
mutualistic symbiotic relationships across a large range
of organisms. These relationships number in the thousands and involve all sorts of interactions, from those
between gobies and burrowing shrimp to the cellular
symbioses between sponges and bacteria. One of the
central hypotheses surrounding coral reefs is that the
large proportion of mutualistic symbioses have arisen
due to the low nutrient conditions that dictate the advantages of a close association of primary producer and
consumer. The ultimate outcome of these close associations is that the inorganic nutrients required by the primary producer are obtained directly from the animal
consumer. This avoids the dilution that would otherwise
happen if the nutrients and organic matter were to enter
the water column. There is no better example of the ultimate close association than that of reef-building corals.
Reef-building corals form a mutualistic symbiosis
with single-celled dinoflagellate protists (genus Symbiodinium) that live inside the gastrodermal cells of corals
where they photosynthesise, passing large amounts
of captured energy to the coral host (see Fig. 7.7 and
Box 7.1). In return for the energy contributed to the
coral host, the symbiotic dinoflagellates receive access
to inorganic nutrients arising from animal metabolism.
The advantages of the close coupling of coral and Symbiodinium spp. are enormous, resulting in large photosynthetic rates that power the metabolically expensive
process of calcification. Significantly, only animals that
are symbiotic with Symbiodinium calcify at rates that are
significant enough to contribute significant amounts of
energy to reef accretion. The close relationship between
corals and symbiotic dinoflagellates has been in existence for at least 220 million years and is largely responsible for the huge reserves of limestone found in the
BOX 7.1 THE MUTUALISTIC ENDOSYMBIOSIS OF CORALS AND
DINOFLAGELLATES
Reef-building corals and invertebrates from at least five invertebrate phyla form close
associations with dinoflagellates from the genus Symbiodinium. Often referred to as
zooxanthellae (a loose, non-taxonomic term), these single-celled plant-like organisms
live within the endodermal cells (gastroderm) of reef-building corals. Here, they photosynthesise like other phototrophs, but instead of retaining the organic carbon that they
make, Symbiodinium releases up to 95% to the host. This energy is used by the coral to
grow, reproduce and produce copious amounts of calcium carbonate, which forms the
framework of coral reefs. Only corals that have a symbiosis with Symbiodinium are able
to calcify at the high rates that are typical of reef-building corals.
In return for this copious energy, Symbiodinium receives inorganic nutrients from the
waste metabolism of the animal host (Fig. 7.7). Given the shortage of inorganic nutrients
such as ammonium and phosphate ions in tropical and subtropical water columns, the
provision of these nutrients is critical to the high rates of photosynthesis and energy
production of Symbiodinium. Because there are benefits for both partners in this symbiosis and one cell lives inside the cells of another, this symbiosis is referred to as a ‘mutualistic endosymbiosis’. The tight recycling of energy and nutrients between the primary
producer and consumer avoids the problem of the low concentrations of these materials
in typical tropical seas. This is thought to be one of the key reasons why coral reefs are
able to prosper in the otherwise nutrient ‘deserts’ of tropical seas.
70
yet another important part of the puzzle for how coral
reefs maintain themselves in the nutrient-poor (oligotrophic) conditions of tropical seas.
Plant-animal symbiosis
One of the hallmarks of coral reefs is the high number of
mutualistic symbiotic relationships across a large range
of organisms. These relationships number in the thousands and involve all sorts of interactions, from those
between gobies and burrowing shrimp to the cellular
symbioses between sponges and bacteria. One of the
central hypotheses surrounding coral reefs is that the
large proportion of mutualistic symbioses have arisen
due to the low nutrient conditions that dictate the advantages of a close association of primary producer and
consumer. The ultimate outcome of these close associations is that the inorganic nutrients required by the primary producer are obtained directly from the animal
consumer. This avoids the dilution that would otherwise
happen if the nutrients and organic matter were to enter
the water column. There is no better example of the ultimate close association than that of reef-building corals.
Reef-building corals form a mutualistic symbiosis
with single-celled dinoflagellate protists (genus Symbiodinium) that live inside the gastrodermal cells of corals
where they photosynthesise, passing large amounts
of captured energy to the coral host (see Fig. 7.7 and
Box 7.1). In return for the energy contributed to the
coral host, the symbiotic dinoflagellates receive access
to inorganic nutrients arising from animal metabolism.
The advantages of the close coupling of coral and Symbiodinium spp. are enormous, resulting in large photosynthetic rates that power the metabolically expensive
process of calcification. Significantly, only animals that
are symbiotic with Symbiodinium calcify at rates that are
significant enough to contribute significant amounts of
energy to reef accretion. The close relationship between
corals and symbiotic dinoflagellates has been in existence for at least 220 million years and is largely responsible for the huge reserves of limestone found in the
BOX 7.1 THE MUTUALISTIC ENDOSYMBIOSIS OF CORALS AND
DINOFLAGELLATES
Reef-building corals and invertebrates from at least five invertebrate phyla form close
associations with dinoflagellates from the genus Symbiodinium. Often referred to as
zooxanthellae (a loose, non-taxonomic term), these single-celled plant-like organisms
live within the endodermal cells (gastroderm) of reef-building corals. Here, they photosynthesise like other phototrophs, but instead of retaining the organic carbon that they
make, Symbiodinium releases up to 95% to the host. This energy is used by the coral to
grow, reproduce and produce copious amounts of calcium carbonate, which forms the
framework of coral reefs. Only corals that have a symbiosis with Symbiodinium are able
to calcify at the high rates that are typical of reef-building corals.
In return for this copious energy, Symbiodinium receives inorganic nutrients from the
waste metabolism of the animal host (Fig. 7.7). Given the shortage of inorganic nutrients
such as ammonium and phosphate ions in tropical and subtropical water columns, the
provision of these nutrients is critical to the high rates of photosynthesis and energy
production of Symbiodinium. Because there are benefits for both partners in this symbiosis and one cell lives inside the cells of another, this symbiosis is referred to as a ‘mutualistic endosymbiosis’. The tight recycling of energy and nutrients between the primary
producer and consumer avoids the problem of the low concentrations of these materials
in typical tropical seas. This is thought to be one of the key reasons why coral reefs are
able to prosper in the otherwise nutrient ‘deserts’ of tropical seas.