The dinoflagellates continue to photosynthesize, as long as they have light and nutrients. However, they do not keep the products of photosynthesis for themselves. They release almost all of it into the tissues of the coral. The coral uses this energy as food, and typically has enough energy to build a considerable amount of calcium carbonate in the form of a cup-shaped skeleton that supports the coral animal, and helps to build a strong coral "colony". Therefore, the coral colony can build a very strong structure that can withstand the force of waves in shallow waters, and many corals together can form a "reef". Coral reefs are very abundant in clear warm tropical waters of the world.
The corals metabolize the photosynthetic products, and of course produce wastes. They do not release those into sea-water until the dinoflagellates have extracted the nutrients from them, especially phosphate and nitrate. The nutrients are thus re-cycled between dinoflagellates and coral, so that the photosynthesis can keep going.
The photosynthesis results in the release of oxygen, which the corals use in respiration. The carbon dioxide that the corals produce in respiration is absorbed by the dinoflagellates in their photosynthesis.
Altogether it is a very efficient system. However, it has serious implications for the coral. They MUST arrange their anatomy so that the dinoflagellates have light. So they build branching or sheet-like colonies in the shallow water, always pointing upward. The coral MUST protect its dinoflagellates (which it does with its stinging cells).
The dinoflagellates are not necessarily happy. Certainly they have nutrients, light, and protection, but they pay an enormous price for it in terms of giving up almost all their photosynthetic production. The dinoflagellates would not join in the symbiosis unless they had no alternative.
The fact is that the coral reef symbiosis only occurs where there is practically no food in the seawater. If there were nutrients out there, the dinoflagellates would abandon the coral and make a living for themselves in the water. (In fact, the best way to destroy a reef is to build a tourist hotel next to it and pour the sewage directly into the sea -- and many tourist hotels do exactly that.)
So the conditions for a successful reef include: shallow, warm, CLEAR, nutrient-poor tropical water.
The coral has to keep its surface clear of sand and other sediment, to keep light shining on its dinoflagellates. The cost here is the cost of continuous mucus production, which catches the sediment and sweeps it off the coral surface. The mucus is polysaccharide, which is a good food source for small creatures: in fact, given the low nutrients in the area, it is the ONLY major food supply. So many small reef creatures eat mucus, and then they provide food for other creatures, and so on, so that in the end the coral reef COMMUNITY is a very rich, diverse, set of organisms, all dependent on the original coral symbiosis.
However, it is very vulnerable because the entire community depends on the symbiosis. It is amazing how long-lasting coral reef communities have been since they evolved in the Jurassic.
Of course, other organisms can and do evolve similar symbiotic relationships with dinoflagellates, as long as they can provide the same services to them. So today, giant clams and some floating foraminiferal protists have symbiotic dinoflagellates too.
In the fossil record, one can try to identify hosts of symbioses, even though all trace of the dinoflagellates themselves has gone. The hosts would likely be organisms that grew large shells, in shallow, warm, tropical waters, and had adaptations to place their soft tissues in the light.
So in the Permian, one can identify some brachiopods as likely hosts for dinoflagellates, even though Paleozoic corals clearly were not (they did not form reefs). And in the Cretaceous, corals were for a while outcompeted by peculiar clams in reef environments. These clams, called rudists, were shaped like ice-cream cones, with one shell formed into the cone and the other into the lid. The lids often had holes in them, designed so that light could pass down through them into the interior.
The rudists were caught up in the K/T extinction and the corals recovered the reef environment after that extinction. The brachiopods that had symbiosis became extinct in the Permian extinction.
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