INTERVIEW. Gérald Grégori, BioSWOT-Med’s Co-Chief Scientist, describes the sampling strategy of the campaign and how it will allow gathering data to investigate the drivers of plankton diversity and distribution in the Mediterranean Sea by studying finescale features such as filaments or small eddies. Even if these structures are weak and short-lived (fed days) they can indeed strongly influence the microbial community.
THE RESEARCH THEMES – Gérald Grégori is a researcher at CNRS (National center for scientific research – Centre National de la Recherche Scientifique), in the Mediterranean Institute of Oceanography located in Marseille (France). His research focuses on marine planktonic microbes, and their relationships with the ecosystem and biogeochemistry. To do so, he studies the factors that influence their abundance and distribution along the water column, in various locations such as coastal or open ocean. The originality of this research is to use single cell analysis to study them, both in situ and in the laboratory, with a particular focus on flow cytometry. In the BioSWOT-Med campaign he is the Chief Scientist for Biology.
What are the main research themes that will be addressed during the campaign?
The themes that will be addressed during the campaign aim at unveiling the drivers of plankton diversity and distribution in the Western Mediterranean. Why the Western Mediterranean Sea? Because this region combines high plankton diversity, low nutrient concentration, and weak ocean circulation. We suspect that finescale features such as filaments or small eddies, even if they are weak and short-lived (few days) can strongly influence the microbial community.
During the BIOSWOT-Med campaign we plan to follow the temporal evolution of these finescale structures over the western Mediterranean crossover generated by the tracks of the new satellite, SWOT, equipped with a new generation of instrument. The new Ka-band Radar Interferometer (KaRIn) will measure the sea level at an unprecedented resolution and will help us to better identify, define and describe the small scale features met in situ.
During the BioSWOT-Med campaign, we aim at characterizing the physical-biogeochemical coupling in these fine scale structures thanks to an adaptive Lagrangian sampling strategy. To perform this Lagrangian strategy we will use the software SPASSO (Software Package for an Adaptive Satellite-based Sampling for Oceanographic cruises) developed in our group and applied in several research cruises. The physical information collected thanks to a panel of instruments (from drifters and CTDs, Moving Vessel Profiler MVP, satellite, and numerical models) will be combined to a huge biological dataset collected by a multi-sensor characterization of the planktonic community. We will combine flow cytometry analyses, to imaging, and advanced molecular (meta-transcriptomics, metagenomics and meta-barcoding) techniques, with the use of autonomous and robotic platforms deployed in situ or in the laboratory. The goal is to have the more complete picture of the distribution and dynamics of the various compartments of the first levels of the trophic network, and define the relationships between them and with the hydrological (nutrients, temperature, salinity) and physical (horizontal currents, vertical velocities, …).
In the BioSWOT-Med campaing you are in charge of microbiology. Can you explain what microbiology is and why it is important to study it?
In a small spoonful of sea water, millions, even hundreds of millions of organisms are present: viruses, archaea, bacteria, eukaryotes. Are they dangerous? No! All these organisms are natural hosts of sea water. They are even essential to the plankton machinery. Without this machinery, we would not eat fish, water infected with organic waste would be unfit for swimming and, moreover, man would probably not exist. Phytoplankton organisms are microscopic, invisible, apparently insignificant. And yet. Plant plankton is capable of transforming the invisible (dissolved carbon dioxide or CO2), the mineral (nutrient salts – nitrates, phosphates, silicates) and the immaterial (light) into organic matter. Like terrestrial plants, it is at the origin of a miracle: creating life, organic matter, from inert mineral matter thanks to the light energy provided by the sun. This process is called photosynthesis. Organisms capable of making organic matter from mineral matter and energy are called autotrophs.
Phytoplankton is at the base of the marine food web. It is a very complex network that starts with phytoplankton, then passes through zooplankton (consumers of phytoplankton and small carnivores), and extends to top predators (such as fishes, dolphins, … and humans). It is estimated that it takes one ton of phytoplankton to obtain 100 g of fish flesh. Phytoplankton cells can also precipitate to the bottom of the sea after their death and be incorporated into the sediments, trapping the carbon they contain over geological time. Did you know that they are one of the sources of oil? Finally, they can also be ‘recycled’ in the water column by bacteria. Phytoplankton organisms are tiny and their biomass (mass of living matter) does not exceed 2% of the planet’s plant biomass (sea and land combined). But they are responsible for almost half of the primary production of our planet. Indeed, half of the oxygen we breathe comes from the phytoplankton. Without the presence of phytoplankton in the oceans, the atmospheric CO2 content would be of the order of 600 ppm (parts per million) instead of the current 400 ppm, which would be equivalent to an additional increase in the average temperature of our planet of 1°C.
Phytoplankton is not the only compartment of the plankton. Heterotrophic prokaryotes, more commonly known as bacteria and archaea, represent the largest source of biodiversity still unknown on the planet. These tiny cells do not have a nucleus, unlike the so-called eukaryotic cells. Most of these bacteria, and all archaea, are heterotrophs: they do not photosynthesize and (like us) need organic matter for food. It is often dead organic matter, dissolved or particulate, that they degrade and recycle. Bacteria and archaea are therefore the indispensable ‘garbage collectors’ of plankton.
Not all bacteria are heterotrophic. One group of photosynthetic bacteria plays a considerable role in phytoplankton: the cyanobacteria (or ‘blue-green algae’). Some cyanobacteria are able to fix molecular nitrogen (N2), the gas that makes up most of the air we breathe, thus compensating for the lack of nutrient salts. Others simply photosynthesize, such as Prochlorococcus and Synechococcus; they are very small (0.2 to 2 µm in diameter) but extraordinarily numerous: up to 100,000/milliliter.
The cytometers in the laboratory aboard the R/V L’Atalante.
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