In addition to the coral sampling protocol repeated around 40 islands, the Tara Pacific expedition also provides an opportunity to punctually investigate related topics. The schooner is currently sailing across the Tuamotu archipelago and takes advantage of this particular biodiversity to focus on organisms crucial to reefs: coralline algae.
After a busy stopover in Tahiti, scientists have resumed diving to patiently collect samples to be sorted on the rear deck. Upon close examination, these samples don’t resemble coral, but look like small rocks covered with a pinkish calcareous layer. They are coralline algae, plants of primary importance in reef building. "These algae form a calcareous skeleton that cements together all the reef elements”, explains Laetitia Hédouin (CNRS/CRIOBE), co-leader with Maggy Nugues (EPHE/CRIOBE) of this mission in the Tuamotu Islands. “But the importance of coralline algae goes even further: some species are expected to attract coral larvae, promoting their recruitment."
Laetitia Hédouin (CNRS/CRIOBE) observes a sample under the microscope to identify the smallest corals. © Yann Chavance / Tara Expeditions Foundation
Coral is not restricted to a colony of polyps motionless in their stony envelope: in the early stages of its life, this organism is a simple larva drifting with currents, until it finds a suitable substrate to settle on. The larva – then referred to as “recruit” – divides to form a juvenile coral, beginning of the future large colony. According to recent studies, coral larvae wouldn’t settle anywhere on the substrate and might prefer particular coralline algae. “One of the mission objectives is to investigate these specific associations between coralline algae and coral. This is the first time such a study is conducted in situ in Polynesia”, says Maggy Nugues. “The idea is to study coralline algae, their microbiome and all associated microorganisms, and more generally to understand how these algae induce the settlement of coral larvae.” If the triggering mechanisms were to be discovered, the resulting advances on coral reef restoration could be crucial.
To detect the smallest corals, scientists use a fluorescent lamp, thus revealing coral in green. © David Hannan / Tara Expeditions Foundation
To explore this uncharted research, the scientific team aboard Tara organizes 3 dives around each island visited in the Tuamotu archipelago, repeating the same protocol every time. While 2 divers assess coralline algae abundance and diversity by identifying species on a 10-meter long segment, 2 other researchers are responsible for sampling all juvenile corals in a predefined area of 50 cm by 50 cm. “We collect all colonies smaller than 2 cm”, describes Laetitia Hédouin. “For recruits of a few millimeters in diameter, we use a fluorescent lamp to identify them more easily.” Back on deck, each sample will be patiently studied to determine the coral type, the coralline alga species, and of course, if the coral larvae have settled on it or in its vicinity. Finally, some samples will be sent ashore for genetic analysis to determine the chemical and biological substances they secrete. A potential key to decipher the secret links between corals and coralline algae.