The oceans of our planet are responsible for more than half of the global production of organic matter, thanks to the photosynthetic activity of marine phytoplankton. Indeed, these microorganisms suspended in water are involved in the carbon cycle by using sunlight to produce matter. As a driver of this oceanic photosynthesis, novel biodiversity has been revealed by researchers working at the marine research stations in Roscoff and Villefranche sur Mer (UPMC/CNRS).
These scientists have resolved an important oceanographic paradox: the omnipresence in seawater of a photosynthetic pigment(2)
(19'-hexanoyloxyfucoxanthin) borne by an unsuspected diversity of organisms. Thanks to novel genetic protocols, they have identified a multitude of microorganisms carrying this pigment: protists belonging to the Haptophyta lineage. Protists are unicellular, eukaryote (with a single nucleus) organisms that are dispersed throughout the biosphere. Neither bacterium, nor virus, nor plant nor animal as such, they display major anatomical and physiological plasticity. Haptophyta constitute one of the most ancient lineages of protists. Calculations suggest that a few liters of tropical water may contain more than 1000 genetically-different species of these haptophytes, of a size ranging from 2 to 8 microns (thousandths of a millimeter). A quantitative evaluation of the global importance of their pigment during the year 2000 suggested that the biomass of these organisms may be up to twice as important as that of cyanobacteria or diatoms, the two groups of phytoplankton classically considered as the champions of photosynthesis in open ocean waters. The success of these small haptophytes may reside in the fact that in addition to feeding from light via photosynthesis, they consume bacterial prey, organic matter or pico-protists (protists with dimensions of between 1 and 2 microns) and thus supplement their diet as they diversify.
Identifying the richness and functioning of ocean plankton biodiversity is essential to understanding how climate is regulated by living organisms. In this context, studies on marine protists are the subject of new research programs being coordinated by CNRS scientists in Roscoff, such as BioMarKs
(Biodiversity of Marine euKaryotes) along European coastlines, or Tara-Oceans, the round-the-world scientific expedition on board the yacht Tara.
© M.-J. Chretiennot-Dinet, CNRS, Banyuls sur Mer
Transmission electron microscopy image illustrating the shape of tiny haptophytes in marine plankton
(haptonema in A and the surface of organic scales in B).
(1) Haptophytes are unicellular organisms with a flagellum-like appendix called a haptonema.
(2) A photosynthetic pigment is a colored molecule that is capable of absorbing the light radiation necessary for photosynthesis.
Liu H., Probert I., Uitz J., Claustre H., Aris-Brosou S., Frada M., Not F., de Vargas C. 2009. Extreme diversity in noncalcifying haptophytes explains a major pigment paradox in open oceans. Proceedings of the National Academy of Sciences of the United States of America (PNAS published online before print July 21, 2009, doi:10.1073/pnas.0905841106).