Plantanimal 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 Symbi-odinium) 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 Sym-biodinium spp. are enormous, resulting in large photo-synthetic 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

Sunlight

Figure 7.7 Illustration of the relationship between Symbiodinium (transmission electron micrograph, scaled 1 pm) and the host endodermal cells of reef building corals. Sunlight illuminates the transparent host cells driving photosynthesis of Symbiodinium. Symbiodinium passes copious amounts of resulting photosynthetic products (labelled 'photosyn') to the host cell that in return allows Symbiodinium access to inorganic nutrients such as ammonium and phosphate arising from host catabolism (labelled 'Inorg N, P'). n, nucleus; s, starch cap; cl, chloroplast; p, pyrenoid; v, vacuole space between host vacuole membrane and out plasmalemma of the enclosed Symbiodinium cell. (TEM image: O. Hoegh-Guldberg.)

Figure 7.7 Illustration of the relationship between Symbiodinium (transmission electron micrograph, scaled 1 pm) and the host endodermal cells of reef building corals. Sunlight illuminates the transparent host cells driving photosynthesis of Symbiodinium. Symbiodinium passes copious amounts of resulting photosynthetic products (labelled 'photosyn') to the host cell that in return allows Symbiodinium access to inorganic nutrients such as ammonium and phosphate arising from host catabolism (labelled 'Inorg N, P'). n, nucleus; s, starch cap; cl, chloroplast; p, pyrenoid; v, vacuole space between host vacuole membrane and out plasmalemma of the enclosed Symbiodinium cell. (TEM image: O. Hoegh-Guldberg.)

upper layers of the Earth's crust. As pointed out elsewhere, the limestone structures generated by corals and other organisms generate the habitat for over a million species of plant, animal, fungi and bacteria worldwide.

Studies of the food webs on coral reefs have identified a major role for the mucus generated by reef-building corals. Mucus is considered to be relatively cheap to produce due to the abundant energy available for corals in shallow habitats (hence the concept of 'junk carbon'). It is primarily produced to prevent the surfaces of corals from being colonised by fouling organisms and may have a role in protecting corals from excessive light (PAR and Ultra-Violet Radiation, UVR). It tends to be sloughed of corals at the end of the day (after extensive photosynthesis has occurred). Corals on the intertidal reef flat at Heron Island exude up to 4.8 litres of mucus per square metre of reef area per day, and, of that, up to 80% dissolves in the reef water. While the dissolved component stimulates a burst of metabolic activity in the sediments where it is largely metabolised, the remaining particulate proportion is eaten by fish and other particle feeders on the reef crest. This transfer of energy is thought to represent a major trophic exchange of energy, and relative to other marine food webs is fairly unique.

In summary, this chapter explored the production and flow of energy through coral reefs, which are highly productive ecosystems that prosper in the nutrient poor waters of the tropics. While tropical oceans that surround most coral reefs, they have a primary productivity that is close to zero, the coral reefs that they bathe often have levels of primary productivity that are among the highest in the ocean. This productivity is a manifestation of the efficient photosynthetic processes and recycling that occurs within the warm and sunlit setting of coral reefs. We also examined one of the key nutrient cycles of coral reefs, that of nitrogen, observing that nitrogen is regenerated by nitrogen fixation and that it cycles between the different organisms within the food web of coral reefs along with the energy of organic carbon bonds. The 'wall of mouths' clouds of small fish and other particle feeders that forage at the interface of coral reefs and the open ocean play an important role in acquisition of energy. As well, the efficiencies of mutualistic symbioses like those seen between corals and symbiotic dinoflagellates have huge benefits to a wide range of organisms in the dilute nutrient conditions of tropical seas.

Probably no single factor can explain why coral reefs are so productive. The answer probably lies in combinations of characteristics and mechanisms that generate and recycle nutrients. There is one important take-home message that may not be obvious at first: it is a mistake to think that much of the energy generated can be harvested as a net product of the system. Tight recycling of nutrients and energy means that the majority of primary production is rapidly recycled back into the ecosystem by the many pathways elucidated in this chapter.

This has been likened to a 'beggar's banquet' where the table is set for a feast that looks at first glance to be generous and abundant yet very little can be eaten or taken away from the table. This situation appears to be fundamentally different than that seen in marine ecosystems such as kelp forests where the rate of primary production can be large seasonally and substantial amounts of energy and organic carbon are exported out of the kelp ecosystem. Another way of understanding this is to compare the measures of PG and PN on a community basis. The rate of gross photosynthesis of the community is large in both coral reefs and in kelp forests, but P., of coral reefs is far less than P.. of kelp

N community N community r forests. These differences strike at the heart of the unique nature of coral reefs and may drive other emergent features such as the sensitivity of coral reefs to small changes in environment that surrounds them.

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