On the following subject:
"Study of the degradation of ice algae and open-water phytoplankton in the Arctic: impact of the state of stress of the bacteria associated with this material on its preservation and its contribution to sediments".
Director:JEAN-FRANCOIS RONTANI, RESEARCH DIRECTOR
Co-director: Ms PATRICIA BONIN, RESEARCH DIRECTOR
Summary:
The Arctic Ocean and the disruption to its functioning caused by global warming are of major concern to the world's scientific community. The Arctic Ocean is underpinned by primary production by sympagic organisms known as ice algae. In some Arctic regions, ice algae can account for up to 60% of total primary production. Given the relatively low level of degradation of this material by bacterioplankton in these regions (between 25 and 80% of bacterioplankton being dead or inactive) and the ability of sympagic material to sink rapidly to the sediments, ice algae make a major contribution to the export of atmospheric carbon to the seabed. The low degradation activity of bacterioplankton can be attributed to: (i) osmotic stress in hypersaline brine channels in the ice, (ii) chemical stress linked to the production of bactericidal compounds by ice algae or (iii) photooxidative stress.
The aim of this thesis is therefore to gain a better understanding of the impact of these different stresses on the physiological state of bacteria associated with sympagic material and on the preservation of the latter as it falls towards the sediments. The effect of haline and chemical stresses has been demonstrated through the analysis of numerous samples of particles in suspension, sedimentation or sediments collected during various sampling campaigns in the Canadian Arctic. These analyses have demonstrated the importance of haline stress on bacteria associated with sympagic algae, particularly at the start of the melt cycle, which results in reduced biodegradation activity by the associated bacteria. The importance of stress linked to the production of free fatty acids by ice algae, particularly in the event of strong irradiation of this material, was also highlighted. Indeed, the quantities of free palmitoleic acid measured in our samples (up to 4.8 mg.L-1) seem to be the cause of the high mortality (up to 75%) of associated bacteria observed in the ice and in the particles. The importance of photochemical stress on sympagic bacteria and the increased resistance of pigmented bacteria to this stress were also demonstrated. These results may partly explain the high occurrence of pigmented bacteria in bacterioplankton communities at the end of phytoplankton blooms.
Key words: Ice algae, Associated bacteria, Photo-oxidative stress, Haline stress,