Ugo Guignard, stagiaire Master 2 LCE collaborant avec le MIO, donnera un séminaire en salle de réunion du LCE à St Charles, le mardi 30 juin 2026 à 14h00 on the theme: Volatile Organic Compound Emissions and their Relationship with Phytoplankton Communities.
Videoconference link
Abstract
Species-resolved volatile organic compounds (VOC) emission inventories of marine phytoplankton remain incomplete, limiting the accuracy of marine biogenic gas flux parametrisations in atmospheric and climate models. In particular, the relative con-tributions of specific taxa to the budgets of key aerosol precursors – dimethylsulfide (DMS), methanethiol (MeSH) and isoprene – are poorly constrained because field measurements integrate signals from mixed communities. To address these gaps, this thesis combines two complementary approaches: the VACOA ship campaign aboard the Marion Dufresne II across the Southern and Indian Oceans (January–February 2026), providing large-scale in situ observations of VOCs, aerosol composition and phytoplankton communities across six oceanographic periods; and the SARNEIO lab-oratory incubation experiment, comparing VOC emissions from two ecologically con-trasting phytoplankton species – Thalassiosira pseudonana (diatom) and Phaeocystis globosa (haptophyte) under identical controlled conditions.
During VACOA, DMS concentration peaked at frontal zones as expected, but three findings departed from predictions. The campaign-wide DMS–sulfate corre-lation was negligible (ρ = −0.085), likely attributed to the multi-day atmospheric oxidation lifetime of DMS and to the variable marine boundary layer fraction, which reached zero percent boundary-layer-dominated days during the biologically produc-tive Polar Front period. MeSH concentration showed a more coherent positive relationship with sulfate (ρ = +0.224), consistent with its faster oxidation kinetics. DMS and MeSH concentrations were strongly negatively correlated in the most productive periods (ρ = −0.70 and −0.82), suggesting competition between the DMSP lyase and demethylation pathways. Isoprene persisted in oligotrophic subtropical regions despite minimal phytoplankton biomass, showed no consistent diurnal cycle and was decoupled from nanophytoplankton abundance, likely pointing to a significant abiotic photochemical source at the sea surface microlayer.
In SARNEIO, preliminary PTR-ToF-MS measurements revealed contrasting emission dynamics across all three compounds. P. globosa showed continuously el-evated DMS and MeSH concentration declining with cell senescence, and near-zero isoprene concentrations, consistent with a high-DMSP haptophyte channelling antiox-idant protection through the sulfur pathway. T. pseudonana displayed a delayed DMS concentration increase coupled to the bloom peak, no corresponding MeSH concentra-tion increase — suggesting lyase dominance over demethylation during active growth and a variable isoprene signal whose quantification awaits resolution of an elevated blank. These species contrastscontrast impliesy that haptophyte- and diatom-dominated blooms contribute to atmospheric VOCs with different fingerprints and at different phases of the growth cycle.
