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Impact of environmental drivers and phytoplankton diversity on the dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO) cell quotas: laboratory experiments and natural variability.

par Royer, Colin
Président du jury Fripiat, François
Promoteur Gypens, Nathalie ;Borges, Alberto V
Publication Non publié, 2021-02-24

Thèse de doctorat

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Résumé :

In the last two decades, particular interest has been given to the cycle of dimethylsulfide (DMS), a climate active gas, and its precursors the dimethylsulfoniopropionate (DMSP) and the dimethylsulfoxide (DMSO). DMS is involved in the Earth’s radiation budget while the DMS(P,O) are produced by a wide variety of micro- and macroalgae, corals, bacteria, or angiosperms in response to diverse environmental stresses. Several functions have been suggested for these sulfur compounds such as antioxidants, cryoprotectants, overflow mechanisms, osmolytes, zooplankton deterrents or signalling compounds. This research aims at improving the knowledge about the antioxidant role of DMS(P,O) within three major phytoplankton groups: diatom (i.e. Skeletonema costatum), Prymnesiophyceae (i.e. Phaeocystis globosa) and dinoflagellate (i.e. Heterocapsa triquetra). The experimental results demonstrate that cellular DMS(P,O) have the ability to lower cellular reactive oxygen species concentrations produced during high-light and chemically-induced oxidative stresses; thus supporting the antioxidant function. However, the initial DMS(P,O) concentrations of each species are not informative of their ability to tolerate a further oxidative stress, and their concentrations do not increase in high-light grown cells. The DMS(P,O) may then act as antioxidants without being part of the antioxidant response of the cell. We recommend analysing more constituents of the antioxidant system (i.e. enzymes, carotenoids, redox buffer) along with DMS(P,O) by-products and DMSP-lyase activity to better understand the cellular function of DMSP. Field measurements in the North Sea, including the Belgian Coastal Zone (BCZ) and the Northern North Sea (NNS), bring additional information on the DMS(P,O) variations. While abiotic parameters (nutrients, temperature, and incident light) influence the Chlorophyll-a (Chl-a) concentrations in the BCZ, this impact is not reflected in the DMS(P,O) concentrations. The latter depend on the succession of low- and high-DMSP producing species (i.e. diatoms and Phaeocystis, respectively). In the NNS in August, no distinct pattern can be drawn for the DMS(P,O) evolution regarding the phytoplankton diversity or abiotic parameters. Investigated by correlations between DMS(P,O), photoprotective pigments and incident light, the antioxidant function is not observed for this short-term period of sampling in a temperate sea. Based on Chl-a linear regressions, DMS(P,O) concentrations are successfully estimated with two distinct relationships for diatoms and Prymnesiophyceae in the BCZ. However, this estimation lacks accuracy in the NNS due to the mixed phytoplankton community observed. Further work will provide a better understanding about the antioxidant function – especially on the field – and its association with the phytoplankton diversity in temperate regions such as the North Sea.