The quantitative roles of DMSP and DMS in sulfur and carbon cycling in the ocean and the atmosphere: the grand numbers

Dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) are recognized as important components of the oceanic sulfur and carbon cycles. Because measurements reveal that many phytoplankters invest up to 10-20% of carbon fixation and up to 75% of sulfate reduction in synthesising DMSP, this compound must have fundamental effects on the physiology and ecology of the individuals producing it, and their microbial partners. Because observations show that DMSP fuels up to 10% and up to 100% of prokaryotic heterotrophic carbon and sulfur demands, respectively, and up to 30% of the sulfur demands of herbivores, this compound must be fundamental in plankton trophic interactions. We will provide grand estimates of the quantitative significance of DMSP synthesis as part of carbon fixation and sulfate reduction fluxes in the pelagic ocean, and of the proportion of dissolved organic sulfur that is recycled through DMSP degradation.

Stimulated by its suggested climatic role, four decades of seawater DMS measurements have rendered knowledge of its spatial and temporal distribution not even dreamed for other aerosol forming substances. Global monthly climatologies estimate that the global ocean DMS source is about half of current anthropogenic sulfur emissions and three times the volcanic emissions. As an organic compound, DMS occurs in the surface ocean at molar concentrations generally 100 times higher than most of the other aerosol-forming carbon-containing volatile compounds. Actually, the oceanic annual carbon emission in the form of DMS is, given the associated uncertainties, of the same order of magnitude as those estimated for the totality of primary and secondary aerosol-forming organic compounds.

Still, too many unknowns remain and will be outlined. In any case, the evolution of the biosphere has given to DMSP and DMS such a prominent quantitative role that there should be no surprise at their functional involvement in the Earth’s biogeochemical and climatic machinery.