11 MAR 2019

A team of researchers from Sorbonne University, the CNRS, the National Museum of Natural History and the CEA have exploited the planktonic data from the Tara Oceans 1 scientific expedition, collected by the Tara schooner between 2009 and 2013, over a distance of 150,000 km. traveling throughout the world's oceans to study the biogeography of marine microorganisms.

Le plancton mixotrophe, acteur méconnu de la photosynthèse
Mixotrophic plankton, unknown player in photosynthesis - © Tara Expéditions

The results of their study, published in The ISME Journal, open new perspectives for a finer modeling of the carbon cycle in the oceans and raise questions for scientists in their ideas about the environmental changes.
 
The production of organic plant matter, resulting from photosynthesis2—called primary production—in the oceans, is essential to life on Earth. Nearly half of global photosynthesis is done by marine plankton. This production is traditionally thought of as being carried out by phytoplankton (plant organisms) and then consumed by zooplankton (animal organisms). Today, however, it is recognized that a large part of the marine plankton does not follow this traditional "plant/animal" dichotomy but is actually capable of simultaneously carrying out photosynthesis and phagocytosis: these are called mixotrophic organisms.
 
Using the DNA sequencing data collected during the Tara Oceans expedition, the researchers have identified 133 so-called mixotrophic plankton lines for the first time, some of which have been detected in all the oceans of the planet. Qualified as "plastics" because of their adaptability, these species have the particularity of being able to feed like animals or, conversely, as plants depending on the environment in which they live. Although it is known that 45% of carbon is fixed by plankton in the process of photosynthesis (55% by terrestrial plants), the percentage fixed by mixotrophic plankton species remains unknown at present is an open question.

The discovery of the amazing properties of these marine microorganisms, present in all corners of the globe, is a notable advance in our knowledge of the oceans as well as our understanding of their role in the carbon cycle. This discovery paves the way for new lines of research to better understand the functioning of marine ecosystems.

 

1 The Tara schooner has created more than 150 stations to sample plankton and study coral reefs. These planktonic samples are analyzed to understand the functioning and diversity of marine life; and predict the response of marine ecosystems to climate change. More information on the Tara Expeditions website.
 
2 Photosynthesis is the essential process by which plants use sunlight to convert atmospheric carbon dioxide (CO2) into the carbohydrates they need to grow.  
 
 
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Mixotrophic protists display contrasted biogeographies in the global ocean, Emile Faure, Fabrice Not, Anne-Sophie Benoiston, Karine Labadie, Lucia Bittner & Sakina-Dorothea Ayata.