Human activities have contributed to the increase in atmospheric greenhouse gases such as carbon dioxide (CO2). This anthropogenic gas emission has led to a rise in the average Earth temperature. Moreover, the ocean constitutes the major sink for anthropogenic CO2 and its dissolution in surface waters has already resulted in an increase of seawater acidity since the beginning of the industrial revolution. This is commonly called ocean acidification. The increase in water temperature could induce modifications of the physical and chemical characteristics of the ocean. Also, the structure and the functioning of marine ecosystems may be altered as a result of ocean acidification. <p>Phytoplankton productivity is one of the primary controls in regulating our climate, for instance via impact on atmospheric CO2 levels. Coccolithophores, of which Emiliania huxleyi is the most abundant species, are considered to be the most important pelagic calcifying organisms on Earth. Coccolithophores are characterized by calcium carbonate platelets (coccoliths) covering the exterior of the cells. They form massive blooms in temperate and sub-polar oceans and in particular along continental margin and in shelf seas. The intrinsic coupling of organic matter production and calcification in coccolithophores underlines their biogeochemical importance in the marine carbon cycle. Both processes are susceptible to change with ocean acidification and warming. Coccolithophores are further known to produce transparent exopolymer particles (TEP) that promote particle aggregation and related processes such as marine snow formation and sinking. Thus, the impact of ocean warming and acidification on coccolithophores needs to be studied and this can be carried out through a transdisciplinary approach.<p>The first part of this thesis consisted of laboratory experiments on E. huxleyi under controlled conditions. The aim was to estimate the effect of increasing water temperature and acidity on E. huxleyi and especially on the calcification. Cultures were conducted at different partial pressures of CO2 (pCO2); the values considered were 180, 380 and 750 ppm corresponding to past, present and future (year 2100) atmospheric pCO2. These experiments were conducted at 13°C and 18°C. The cellular calcite concentration decreases with increasing pCO2. In addition, it decreases by 34 % at 380 ppm and by 7 % at 750 ppm with an increase in temperature of 5°C. Changes in calcite production at future pCO2 values are reflected in deteriorated coccolith morphology, while temperature does not affect coccolith morphology. Our findings suggest that the sole future increase of pCO2 may have a larger negative impact on calcification than its interacting effect with temperature or the increase in temperature alone. The evolution of culture experiments allows a better comprehension of the development of a bloom in natural environments. Indeed, in order to predict the future evolution of calcifying organisms, it is required to better understand the present-day biogeochemistry and ecology of pelagic calcifying communities under field conditions.<p>The second part of this dissertation was dedicated to results obtained during field investigations in the northern Bay of Biscay, where frequent and recurrent coccolithophorid blooms were observed. Cruises, assisted by remote sensing, were carried out along the continental margin in 2006 (29 May – 10 June), 2007 (7 May – 24 May) and 2008 (5 May – 23 May). Relevant biogeochemical parameters were measured in the water column (temperature, salinity, dissolved oxygen, Chlorophyll-a and nutrient concentrations) in order to determine the status of the bloom at the time of the different campaigns. Calcification has been shown to be extremely important in the study area. In addition, TEP production was significant at some stations, suggesting that the northern Bay of Biscay could constitute an area of important carbon export. Mortality factors for coccolithophores were studied and the first results of lysis rates measured in this region were presented. <p>Results obtained during culture experiments and comparison with data reported in the literature help to better understand and to predict the future of coccolithophores in a context of climate change. Data obtained during either culture experiments or field investigations allowed a better understanding of the TEP dynamics. Finally, the high lysis rates obtained demonstrate the importance of this process in bloom decline. Nevertheless, it is clear that we only begin to understand the effects of global change on marine biogeochemistry, carbon cycling and potential feedbacks on increasing atmospheric CO2. Thus, further research with a combination of laboratory experiments, field measurements and modelling are encouraged.<p><p> / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

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.<p><p>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 and early summer. We describe the vertical profiles of biogeochemical variables (nutrients, chlorophyll-a, dissolved inorganic 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.<p> / Doctorat en Sciences / info:eu-repo/semantics/nonPublished