INTERVIEW – Véronique Cornet is an expert of microphytoplankton and its role in the biological carbon pump. Here she explains what phytoplankton are, how to collect them at sea, and how to analyze their diversity.
THE INSTRUMENTS OF OCEANOGRAPHERS – Véronique Cornet is a research engineer at CNRS working at the Mediterranean Institute of Oceanography (MIO). During her career she has participated in 10 oceanographic campaigns from the Southern Ocean (Kerguelen Islands – Macquarie Island), Pacific (New Caledonia – Polynesia, Solomon Islands) Mediterranean Sea, and 1 ice camp in the Arctic. Specialist of microphytoplankton and its role in the biological carbon pump, in this interview she describes what phytoplankton are, how to collect them in the sea using the phytonet and Niskin bottles and how to analyze their diversity using the PlanktoScope, the optical microscope and the scanning electron microscope (SEM).
First of all, what is phytoplankton?
The name comes from the Greek words φυτόν, meaning “plant,” and πλαγκτός, meaning “wanderer” or “drifting.” Phytoplankton are the autotrophic component of the planktonic community and are a key component of ocean and freshwater ecosystems. They are responsible for more than half of the global primary production of the oceans and for the export of anthropocentric carbon to the deep ocean. Knowing and understanding how phytoplankton function is essential in the context of current climate change.
What will you study about phytoplankton during the cruise?
During the BioSWOT-Med cruise, we will be looking for information on the share of diatoms in the spring bloom and the possible contribution of nanoplankton-sized diatoms. During the campaign we will have the opportunity to carry out in parallel a certain number of cytometry and molecular biology samples to analyze them in microscopy. This will allow us to compare the data obtained by these different approaches.
How are phytoplankton samples collected and how are they preserved before analysis?
During the BioSWOT-Med campaign we will use two sampling techniques: The first one by using a plankton net or phytonet, with a mesh size of 20 microns: a vertical line is drawn between 200 meters from the bottom and the surface; a sub-sample of this net is observed immediately and 2 sub-samples are fixed; 1 in acid lugol and 1 in neutral formalin and kept in a cool, dark place.The second one by is sampling with Niskin bottles. The depths studied will be 200 m – the “DCM” (variable depth) – and the surface. These samples will be fixed with acid lugol and neutral formalin.
How do you analyze the samples and how do you study phytoplankton diversity?
A subsample of the phytonet will be observed live under the microscope in order to evaluate the community structure and to acquire good quality images to feed our image databases. This sample will also be analyzed using a PlanktoScope, a bench-top device that allows us to acquire semi-continuous images of the plankton present in our sample. Compared to the microscope, this device allows us to quickly acquire a large number of images representative of the diversity of our sample; we will then use an automatic image recognition software to sort and classify them by major taxonomic groups.
Another unfixed sub-sample will be filtered for observation with a scanning electron microscope (SEM) in the laboratory. This technique allows to magnify the image up to 175 000 times (a “classical” microscope magnifies up to 1000 times), which allows us a very fine taxonomic identification, especially for nano diatoms that are difficult to recognize with optical microscopy.
The fixed samples will be studied in the laboratory with an optical microscope: identification, counting and measurement of the biovolume of phytoplanktonic cells. Biovolume measurements allow us to evaluate the carbon biomass of the cells. By combining counting, identification and measurement we have access to the community structure not only in terms of abundance but also to the contribution of species or functional groups in terms of biomass.
Contact: Tosca Ballerini (news@swot-adac.org)