OCEAN-ATMOSPHERE INTERACTIONS IN THE GLOBAL BIOGEOCHEMICAL SULFUR CYCLE

Sulfate is taken up by algae and plants and then reduced and incorporated into organosulfur compounds. Marine algae produce dimethylsulfonium propionate (DMSP), which has an osmoregulating function but may also be enzymatically cleaved to yield the volatile dimethylsulfide (DMS). Attempts to identify the variables which control the oceanic production of DMS have shown that there are no simple relationships with algal biomass or primary productivity, but suggest that the concentration of DMS in the ocean is regulated by a complicated interplay of algal speciation and trophic interactions. Part of the biogenically produced DMS diffuses into the atmosphere, where it is oxidized, mostly to aerosol sulfate. The ability of these aerosol particles to nucleate cloud droplets, and thereby influence the reflectivity and stability of clouds, forms the basis of a proposed geophysiological feedback loop involving phytoplankton, atmospheric sulfur, and climate. Carbonylsulfide (COS) is produced photochemically from dissolved organic matter in seawater. The mechanism of this reaction is still unknown. Diffusion of COS from the ocean to the atmosphere is a globally significant source of this gas, which participates in the stratospheric ozone cycle. Hydrogen sulfide and carbon disulfide are produced in the surface ocean by still unidentified processes, which appear to be related to biogenic activity. For these gases, the oceans are a minor source to the troposphere. © 1990.