The ocean is the Earth's main carbon sink due to two principal mechanisms: the physical pump, which pulls surface waters rich in dissolved carbon dioxide down to deeper layers, where it becomes cut off from the atmosphere; and the biological pump, which fixes carbon, either in the tissue of organisms via photosynthesis, or in the calcareous shells of certain microorganisms. Part of this fixed carbon in the form of marine particles then sinks to the deep ocean (a process called carbon export), finally reaching the ocean floor where it is stored. The biological pump is thus one of the major biological processes that can sequester carbon on geological timescales. This process has been widely studied since the 1980s with a particular focus on the role of plankton. A large number of studies have shown that the strength of the biological pump is directly correlated to the abundance of certain plankton species. However, the structure of the communities involved in carbon export has remained poorly understood.
The Tara Oceans expedition (2009–2013) is the largest DNA sequencing effort ever done for the ocean revealing around 40 million genes, the vast majority of which are new to science, thus hinting towards a much broader biodiversity of plankton (from viruses to eukaryotes) than previously known. Thanks to novel computer models, these data also allowed to predict how these diverse planktonic organisms interact. These resources provided a unique opportunity to look at the biological pump integrating its entire biological complexity, describing the first 'planktonic social network' associated with carbon export in the oligoptrophic ocean.
In this keynote conference, Guidi will present the main scientific breakthroughs allowed by the Tara Oceans data collection. In particular how genomics data have led to the description of the planktonic social network that is linked to the biological carbon pump in the oligotrophic ocean. This new approach identified already known players such as diatoms and copepods. However the role of radiolarian, or cyanobacteria and viruses in carbon export was previously grossly underestimated. Going further, a network of functions related to the biological pump was also highlighted, based on the analysis of the genes of bacteria and viruses. The Tara Oceans database allowed to establish that the relative abundance of a small number of bacterial and viral genes could predict a significant proportion of variations in carbon export from the upper layers of the ocean to the deep ocean. However, the function of most of these genes is still unknown. Understanding the structure of these networks and the function of the genes linked to carbon export opens up a wide range of possibilities, especially for modelling the biological processes associated with the oceanic carbon cycle.
View the talk on our YouTube-channel.
Lionel Guidi, Sorbonne Universités, UPMC Université Paris ,