Abstract :
[en] Coccolithophores are unicellular microscopic algae (Haptophyta) surrounded by calcium
carbonate plates that are produced during their life cycle. These species, whose
contemporary contributor is Emiliania huxleyi, are mainly found in the sub-polar and
temperate oceans, where they produce huge blooms visible from space. Coccolithophores
are sensitive to ocean acidification that results from the ongoing accumulation of
anthropogenic carbon dioxide (CO2) in the atmosphere. The response of these organisms
to global change appears to be related to the reduction of their ability to produce calcium
carbonate at the cellular level. At the community levels, one anticipates changes in the
carbon fluxes associated to their blooms as calcification is reduced. However, the
consequences of such environmental changes on this species are speculative and require
improvements in the description of the mechanisms controlling the organic and inorganic
carbon production and export.
The first aspect of this work was to study the response of these organisms to artificially
modified CO2 concentrations representative of the conditions occurring in the past
(glacial) and those expected by the end of the century (2100). Two different levels were
examined: the continuous monospecific cultures (chemostats) allowed us to work at the
cellular level while the mesocosms gave light to the mechanisms taking place in an
isolated fraction of the natural community. The second aspect of this work consisted of
field studies carried out during four cruises (2002, 2003, 2004 and 2006) in the northern
Bay of Biscay, where the occurrence of E. huxleyi blooms were observed in late spring. I
describe the vertical profiles of biogeochemical variables (nutrients, chlorophyll-a,
dissolved inorganic carbon chemistry, particulate carbon, transparent exopolymer
particles (TEP)) and study processes such as primary production, calcification and
bacterial production. The properties of these blooms are compared with those reported in
the literature and enriched with original measurements such as the abundance and
concentration of TEP that could play an important role in carbon export to the deep
ocean, modifying the properties of the settling ballasted aggregates.
