Eaux de surface : du bateau au laboratoire

Thanks to the unique techniques invented for this expedition, scientists are able to collect large volumes of surface waters as the schooner sails between 2 coral-rich areas.  A long process follows, leading to dozens of different samples preserved in alcohol, liquid nitrogen or formaldehyde depending on the research to be carried out in the laboratory.

To understand coral and its ecosystem, Tara’s scientific team is also focusing on characterizing the environment in which corals live, including surface waters between reefs. Therefore, filtration is the key activity of the incessant ballet taking place every morning on Tara’s deck. “The goal is to sample the entire surface planktonic ecosystem, from zooplankton to viruses and bacteria”, says Fabien Lombard, one of the Tara Pacific scientific coordinators. “To do so, we have to separate sample groups according to their size.” The collected water passes through several filters, each of a well-defined mesh size varying from 300 microns (to retain the largest organisms) to 0,2 micron (for viruses). For instance, a 3-micron filter through which passes a water sample previously filtered to 20 microns will retain all organisms in this size range – in this case small phytoplankton (vegetal plankton).

Guillaume Bourdin, ingénieur de pont, replie une « galette » de plancton déposée sur un filtre.

Guillaume Bourdin, deck engineer, folds up the layer of plankton collected on a filter © Yann Chavance / Fondation Tara Expéditions

Sample separation into different size classes is not the only variation between tubes. The preservation method will also depend on the type of research to be carried out. From a single filtration, several samples will be collected and immersed in alcohol, formaldehyde or other preservatives, based on the approach subsequently conducted in laboratories. “Roughly speaking, we conduct 2 types of analysis: a genetic approach to see what organisms are present, and a quantitative approach with imaging to enumerate them”, summarizes Fabien Lombard.

In the end, the contents of certain tubes collected aboard Tara will be directly observed under the microscope, analyzed in complex analytical tools capable of counting the number of organisms present in the sample, or sequenced to determine genes expressed by the entire plankton community. Finally, some samples will be chemically analyzed to study the environment in which all these organisms live: concentration of nitrates, phosphates, iron, etc. The purpose is to thoroughly describe the surface planktonic ecosystem throughout Tara’s expedition.

Credits Yann Chavance - Tubes science-1

The scientific team deals with dozens of tubes holding the various collected samples © Yann Chavance / Fondation Tara Expéditions

But why focus on plankton during an expedition primarily dedicated to coral? In addition to the obvious interest to multiply the scientific contributions of this 2-year expedition, Fabien Lombard recalls that plankton and coral are much more linked: “Firstly, the plankton feeds coral; but above all, larval coral is part of the planktonic ecosystem when released into the marine currents and before it settles on a suitable substrate.” Therefore, studying plankton along the schooner’s route will provide key information to better understand Pacific coral communities.

Yann Chavance

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