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Temperature and Salinity changes as drivers of dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO) production in the sea ice brine habitat: an experimental approach

par Wittek, Boris
Président du jury Fripiat, François
Promoteur Gypens, Nathalie
Co-Promoteur Tison, Jean-Louis
Publication Non publié, 2019-12-13

Thèse de doctorat

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Résumé :

Sea ice, and especially its brine network, is one of the most extreme biomes on Earth. Furthermore,with its specific characteristics, sea ice drives major important biogeochemical cycles. One of them,the cycle of dimethylsulfide (DMS), a climate active gas, is poorly understood despite years of research.In sea ice, DMS has a biogenic origin through two microalgal metabolites: dimethylsulfoniopropionate(DMSP) and dimethylsulfoxide (DMSO).In this thesis, we propose a culture approach based on two emblematic polar microalgae, the diatomFragilariopsis cylindrus and the prymnesiophyceae Phaeocystis antarctica to measure the impact ofenvironmental drivers (temperature and salinity) on the algal DMSP and DMSO production. Theselaboratory experiments show that temperature and salinity variations are involved in the intracellularproduction of DMSP and DMSO, produced therefore as cryoprotectant and osmoregulator.Nevertheless, the algal production depends on the intensity of these environmental changes (the algalgrowth is strongly impacted for salinities above 100 and temperatures below -3.9°C) and on the speciesstudied (P.antarctica is a higher producer of dimethyled sulfur compounds than F.cylindrus but appearsto be more challenged by environmental changes). These laboratory studies, conducted on F.cylindrus,also allow a first explanation for the difference of natural sulfur isotopic composition in DMSP observedbetween sea ice and the marine environment. This difference could be due to various DMSPproduction pathways in microalgae subject to salinity and temperature variations.Field Antarctic data sets also support the impact of environmental factors and taxonomy on theseasonal DMSP and DMSO production in sea ice. The taxonomy appears to be the dominant factor toexplain the production of DMSP and DMSO while temperature and salinity limit the algal developmentin sea ice. Large DMSO concentrations have been also measured at the sea ice bottom duringmicroalgal blooms. We suggest that this DMSO is part of an antioxidant system induced by the highphotosynthetic activity producing dioxygen in the sea ice habitat.Further, we propose many attempts of estimation of the DMSP and DMSO production in sea ice usingempirical relations from our experimental approach on microalgae and taxonomy, brine salinity andchlorophyll-a measured in situ. We conclude that some further work must be provided in the futurefor a better understanding of the DMS cycle in sea ice.