A new type of symbiosis among marine planktonic microbes

A new type of symbiosis among marine planktonic microbes discovered

In the Ocean, planktonic photosynthesis allows atmospheric carbon fixation and is accomplished by microscopic algae which are either eukaryotic or prokaryotic. Some Bacteria and Archaea (Cyanobacteria) can fix atmospheric nitrogen, which confers to them a competitive advantage in the central areas of the oceans that are devoid of nutrients. The paper by Anne W. Thompson, Rachel A. Foster, Andreas Krupke, Brandon J. Carter, Niculina Musat, Daniel Vaulot, Marcel MM Kuypers, & Jonathan P. Zehr published in Science on September 21, 2012 describes a new type of symbiosis between a cyanobacterium that can fix nitrogen and a eukaryotic alga belonging to the prymnesiophytes, which cannot fix it. The cyanobacterium provides nitrogen to the alga, but as it is lacking a part of the photosynthetic apparatus and cannot fix carbon, it relies on the Eukaryote to get the carbon it needs for its growth. This type of planktonic symbiosis has never been observed before and none of the two partners have been isolated in the laboratory. This discovery underscores the potential importance of such associations among plankton. In contrast to coral symbioses, planktonic symbioses have been very little studied until now but could have a strong impact of mineral fluxes in central regions of the oceans.

This new type of symbiosis has been uncovered through an international collaboration between the University of California Santa Cruz, the Max Planck Institute of Bremen and the Biological Station of Roscoff (CNRS and UPMC-Paris 06). Several techniques have been associated to reach that result. Flow cytometry allowed separating the association of the two partners from other plankton organisms. Genetic and genomics approaches allowed the characterization of each of the two partners. Nano-SIMS, a new technique for oceanography, allowed to demonstrate exchanges of nitrogen and carbon between host and symbiont. The French team with support of Genoscope and of the European Program Micro B3 demonstrated that among samples, sorted by flow cytometry during the BIOSOPE cruise in 2004 in the South East Pacific Ocean, one could observe the metagenomic signature of the cyanobacterium and the genetic signature of the eukaryotic alga, confirming observations of the American team in the North Pacific. This illustrates the power of metagenomic approaches to analyze and characterize microorganisms in the marine environment.

Contact :

Daniel VAULOT

Directeur de Recherche CNRS, Groupe Plancton Océanique

Station Biologique, CNRS-INSU-UPMC, UMR 7144, Place G. Tessier, 29680 Roscoff, France

email: vaulot@sb-roscoff.fr

Photo: The BIOSOPE cruise in 2004 in the South east Pacific Ocean, during which one could detect the metagenomic signature between a cyanobacterial and a eukaryotic alga, (c) Daniel Vaulot, CNRS, Roscoff