Plankton (credit: Christian Sardet/Tara Oceans/CNRS Phototheque)

The biological carbon pump is the process by which carbon dioxide (CO2) is transformed to organic carbon via photosynthesis, exported from the surface ocean as sinking particles and finally sequestered in the deep sea. While the intensity of the pump is directly correlated to the abundance of certain plankton species—free-floating micro-organisms—the underlying ecosystem structure driving the process has remained poorly understood.

By analyzing samples collected by the Tara Oceans expedition (2009–2013), an interdisciplinary team of biologists, computer scientists and oceanographers, led by Lionel Guidi, affiliated researcher of oceanography at the University of Hawaiʻi at Mānoa School of Ocean and Earth Science and Technology and CNRS researcher at Laboratoire d’Océanographie de Villefranche (France), has shed new light on these microbes, their interactions and the main functions associated with the biological pump in nutrient-poor ocean regions—areas which represent more than 70 percent of the surface ocean.

Plankton (credit: Christian Sardet/Tara Oceans/CNRS Phototheque)

The ‘planktonic social network’

Ocean microbes are extraordinarily varied, produce half the world's oxygen from photosynthesis, and form the base of the oceanic food chain that feeds fish and marine mammals.

In the recently published study, Guidi and co-authors (see complete list below) made use of articles previously published in Science, and especially the first-ever survey of interactions between planktonic organisms. They used computer analyses to describe the first ‘planktonic social network’ associated with carbon export in nutrient-poor regions. Many of the players involved, such as certain photosynthesizing algae (especially diatoms) and copepods (tiny shrimp-like organisms) were already known. However, the role played by certain microorganisms (single-celled parasites, cyanobacteria and viruses) in carbon export was previously grossly underestimated.

The genetics tell a deeper story

Going further, the researchers then characterized a network of functions, based this time on the analysis of the genes of bacteria and viruses. The Tara Oceans database enabled them to establish that the relative abundance of a small number of bacterial and viral genes can predict a significant proportion of variations in carbon export from the upper layers of the ocean to the deep ocean. Some of these genes are involved in photosynthesis and membrane transport, promoting among other things the formation of sediments and breakdown of organic aggregated material. However, the function of most of these genes is still unknown.

Taking a global view

One future objective for the team is to repeat this work for nutrient-rich oceanic regions, to determine whether the planktonic networks are different in various marine environments.

The ocean is the largest carbon sink on the planet. The recent findings will enable researchers to better understand the sensitivity of this network to a changing ocean and to better predict the effects that climate change will have on the functioning of the biological carbon pump, which is a key process for removing carbon from the atmosphere and sequestering it into the deep sea at global scale. This work highlights the important role played by plankton in the climate system.

Principal laboratories involved in the study

  • CEA–Genoscope, Institut de Génomique
  • Center for the Biology of Disease, VIB (Belgium)
  • Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology (USA)
  • Department of Microbiology, The Ohio State University (USA)
  • Department of Oceanography, University of Hawaiʻi School of Ocean and Earth Science and Technology (USA)
  • EMBL (European Molecular Biology Laboratory) (Germany)
  • Laboratoire d’Océanographie de Villefranche (CNRS/UPMC)
  • Laboratoire “Evolution Paris Seine” (CNRS/UPMC), part of the Institut de Biologie Paris-Seine
  • Laboratoire d’Informatique de Nantes Atlantique (CNRS/Université de Nantes/École des Mines de Nantes)
  • Institut de Biologie, Ecole Normale Supérieure (CNRS/ENS Paris/INSERM)
  • Institute for Chemical Research, Kyoto University (Japan)
  • Institute of Marine Sciences (Spain)
  • Laboratoire “Adaptation et Diversité en Milieu Marin” (CNRS/UPMC), Roscoff Biological Station
  • Laboratoire “Génomique Métabolique” (CNRS/CEA/Université Evry-Val-d’Essonne)
  • Laboratoire “Information Génomique et Structurale” (CNRS/AMU)
  • Laboratoire de Météorologie Dynamique (CNRS/UPMC/Ecole Polytechnique/ENS Paris), part of IPSL
  • Stazione Zoologica Anton Dohrn (Italy)
  • University of Bremen (Germany)
  • University of Maine (USA)