Plankton Attached to Coral

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30 April 2010

Already 8 months since Tara left Lorient. Faithful readers, now that you’re completely ‘fluent’ in plankton, we can move on to the fine points of a local, tropical dialect: coral.

You’re already familiar with Dinoflagellates… those single-celled organisms capable of photosynthesis, but also feeding on particles, and sometimes doing both at the same time.

Let’s observe one of these dinoflagellates fighting against the ocean current using its two flagella. It floats around in the open sea until it reaches a coral reef. From the outside, this reef looks very beautiful: calcareous spirals, solid bushes, and potato-shaped lumps with a labyrinth of ridges. Closer up, it’s even more beautiful: an envelope of calcium carbonate created by little animals called coral. Take away their protective covering and the corals themselves resemble a colony of tiny sea anemones.

Our dinoflagellate comes to rest on this providential structure. It sheds its two little tails, and attaches itself inside the coral. How exactly? This is what Roxane Boonstra and other researchers at the University of Miami are trying to find out: “These dinoflegellates, called zooxanthellae, live in symbiosis with the coral.” During the day, zooxanthellae create matter by synthesizing sunlight. The coral takes over at night. They extend their tiny tentacles to grab or filter small particles in the ocean. In this exchange of methods, each of the two organisms benefits from the other.

Today’s harvest was good for Francesca Benzoni, responsible for coral studies aboard Tara. She spreads out on a table the samples of coral taken underwater. “I try to collect 3 examples of each. The first will stay at Mauritius Oceanographic Institute on Mauritius Island, the two others will be sent to University of Milan Bicocca to be analyzed.” The Tara coral team includes specialists in morphology like Francesca, and molecular biologists who are interested in the DNA of coral. “It’s rather easy to determine the genus of a coral with the naked eye, but identifying its species is much more complicated. Often we have to look at its DNA to be sure.” This combined approach has only been possible for the last ten years, since the development of tools that allow us to delve inside the genome. “Very often, these tools make us question all the old classifications of coral!” adds Francesca.

Each sample is carefully identified and labelled. Francesca and Roxane cut the coral into little pieces and put them into test tubes for the DNA analysis. Add some liquid fixative, and they’re put away in a cool place in the hold, Tara’s treasure chest. Big pieces of coral are cleaned with bleach to conserve only the skeleton of the animal, then carefully wrapped in newspaper to be stored away.

Francesca’s objective is to identify the different species in the Indian Ocean. “The zone has already been studied, but we’re interested in the places rarely sampled: Djibouti, Mayotte and Saint Brandon.” David Obura, another specialist on our coral team, confirms this: “Saint Brandon is a special place, very isolated. Perhaps there are fewer species than in other regions of the Indian Ocean, but for us it’s interesting because there’s very little impact from human activities here. We can observe how coral recovers after a rise in temperature, for example.”

A few degrees higher, and the entire harmony of a reef can be destroyed. Finished, the beautiful symbiosis that unites coral and zooxanthellae. The dinoflagellates go back to the open sea where they can continue a new existence, until they find new coral where they can attach themselves.

Sacha Bollet