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Back when the natural world could be neatly divided into plants and animals, the corals were a source of puzzlement. Like the related sea anemones, these animals look more like plants. What are they? In the 1700s, the great Swedish botanist and founder of modern taxonomy, Carl Linnaeus, dodged the question by classifying them in a group called Zoophyta (animal plants). Truth is, they are both.

By that I mean that the coral animal hosts algae that live inside its cells. (The technical term for this sort of mutual arrangement is endosymbiosis.) These microscopic, single-celled algae gave up their free-living lifestyle in exchange for accommodations provided by the coral. Algae aren't exactly plants, but like plants they contain chloroplasts and carry out photosynthesis. Being microscopic, they fall into that catch-all category-microbes. They are the zooxanthellae, their name coming from the Latin for animal-yellow-small.

They are the coral's internal farm. You might say that the coral exploits them. More than 90% of the sugars they produce via photosynthesis is appropriated by the coral. The coral limits their growth by restricting their supply of nitrogen. Without nitrogen, the algae can't make more proteins and DNA, and thus can't make more algae. But they can keep on photosynthesizing, which they do far more rapidly than their relatives that live free in the water column. The coral sees to that by providing them with the raw materials they need: carbon dioxide, inorganic nutrients, and sunlight.

The corals need those sugars, in abundance. The sugars provide both carbon and energy. Although corals can capture even fast-swimming protists with their harpoon-armed tentacles, and although they can snag other small prey with their mucus nets, the clear waters of tropical coral reefs simply don't harbor enough potential prey. Besides, the corals need a lot of food in order to build their skeletons faster than the waves and the bioeroders wear them away. Also, a third of those sugars is used to produce the coral's protective surface mucus.

How do corals get their zooxanthellae? In some species, the females package zooxanthellae in the eggs-a guaranteed supply for the coral larva. In most species, the larvae have to get them for themselves, by eating them. The larvae digest most algae, but the right kinds for their farm are instead taken into their cells and put to work.

All the zooxanthellae look pretty much alike under the microscope, so it was thought that they were all one species. Recently, comparison of their DNA sequences revealed that they are a diverse lot, some found only with particular species of corals or in certain geographical regions, others being generalists. They have their preferences with respect to sunlight, too. Thus different kinds are found in corals at different depths, or even in different regions of the same coral head.

For a long time, the spectacular success of the corals was viewed as the work of this two-member team. But there are more microbial partners, and they will be the subject of a future post.

Merry Youle is a collaborator with Small Things Considered, a microbe blog that showcases an appreciation for the width and depth of microbiology. She spends her time reading, writing, researching, freelance editing, and constructing her owner-built house in a lava field on Mauna Loa, Hawaii.

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