​The ocean is our planet’s main carbon sink owing to two main mechanisms: the physical pump and the biological pump. The physical pump pulls surface water, rich in dissolved carbon dioxide, down to the deep ocean where it is isolated from the atmosphere. The biological pump fixes the carbon, either in the tissues of organisms through photosynthesis or in the calcareous shells of certain microorganisms. A part of the carbon thus fixed in the form of marine particles then sinks to the depths of the ocean (this is called carbon export), before reaching the ocean floor where it will be stored (this is called sequestration). The biological pump is thus one of the major biological processes that help to sequester carbon on geological timescales.

This process, which has been widely studied since the 80s, involves the ocean's plankton. These extraordinarily varied microscopic beings (plankton constitutes unicellular and multicellular eukaryotes, bacteria, viruses [1]) produce half of the Earth’s oxygen and are at the base of the oceanic food chain that feeds fish and marine mammals. Numerous studies have shown that the strength of the biological pump is directly correlated with the abundance of certain planktonic species. However, the structure of the communities involved in the carbon sink remains largely misunderstood.

By analysing samples collected during the Tara Oceans expedition (2009-2013), an interdisciplinary team comprising biologists, computer experts and oceanographers has shed light on these planktonic species, their interactions and the main functions associated with the biological pump in the oceanic regions that are particularly “poor” in nutrients. These regions occupy the most part of the oceans (more than 70%). Researchers, mainly from the CNRS, UPMC, University of Nantes, VIB, EMBL and CEA (refer to the list of laboratories below), referred to earlier articles published in Science on 22 May, 2015, especially the first mapping of the interactions between planktonic organisms [2]. Using computer analyses, they described the first “planktonic social network” associated with carbon export in the regions “poor” in nutrients. Numerous recorded elements like certain photosynthetic algae (particularly diatoms) or copepods (these are microscopic shrimps) were already known. However, the involvement of certain micro-organisms (unicellular parasites, cyanobacteria and virus) in the export of carbon was largely underestimated until then.

Moving further, the researchers then characterised a network of functions which was based on the analysis of the genes of bacteria and viruses. The Tara Oceans database thus helped to establish that the relative abundance of a small number of bacterial and viral genes predicts a significant proportion of the variability of carbon export to the depths of the ocean. A part of these genes is involved in photosynthesis and membrane transport, encouraging, among other things, the breakdown and sedimentation of organic matter. However, the role of a majority of these genes is still unknown.

Understanding the structure of these networks and the function of the genes involved in the carbon cycle provides numerous perspectives, mainly the possibility of modelling the biological processes involved in the oceanic carbon cycle. It should therefore be possible to test the strength of these networks under various climatic conditions and to better understand how the different planktonic species affect the carbon cycle and climate regulation. One of the future goals is to repeat this work for the nutrient-rich oceanic regions in order to complete the revealed planktonic networks and better understand their dynamics at the global level.