Dimethyl sulfide (DMS) is a simple organosulfur compound with the chemical formula \(text{C}_2text{H}_6text{S}\), represented structurally as two methyl groups attached to a central sulfur atom (\(text{CH}_3text{SCH}_3\)). This volatile compound is recognizable by its strong, pungent odor, often described as the smell of the sea or cooked cabbage. DMS is the most abundant biological sulfur compound emitted from the ocean surface into the atmosphere, making it a major player in the global sulfur cycle.
The Molecular Architecture
The structure of dimethyl sulfide is defined by a central sulfur atom bonded to two methyl groups (\(text{CH}_3\)), classifying it as a thioether. The sulfur atom uses two of its six valence electrons to form single covalent bonds with the carbon atoms. The remaining four electrons exist as two non-bonding lone pairs situated around the central sulfur atom.
The molecule’s three-dimensional shape is determined by the arrangement of two bonded pairs and two lone pairs of electrons. Electron repulsion theory suggests these four electron domains arrange themselves in a tetrahedral geometry around the sulfur atom. However, the two lone pairs exert a greater repulsive force than the bonded pairs, compressing the bond angle between the two carbon atoms.
The resulting molecular geometry is a bent or V-shape, deviating from the symmetrical tetrahedral shape. The C-S-C bond angle is approximately \(98.6^circ\), which is smaller than the \(109.5^circ\) angle of a perfect tetrahedron. This asymmetrical geometry influences the compound’s physical properties.
DMS is classified as polar, possessing a net dipole moment. This polarity occurs because the sulfur atom is slightly more electronegative than the carbon atoms, pulling electrons in the carbon-sulfur bonds closer to the sulfur. The molecule’s bent shape prevents these individual bond dipoles from canceling out, resulting in an uneven charge distribution. This structural feature contributes to DMS’s volatility and its limited ability to dissolve in water compared to its high solubility in organic solvents.
How DMS is Created in Nature
The vast majority of dimethyl sulfide originates from biological processes in the marine environment. The precursor molecule is Dimethylsulfoniopropionate (DMSP), produced by marine phytoplankton and algae. These organisms synthesize DMSP to regulate their internal osmotic pressure, acting as an osmoprotectant to balance the salt concentration of the surrounding seawater.
When phytoplankton cells die, are grazed by zooplankton, or release DMSP, the compound becomes available to marine bacteria. These bacteria possess specialized enzymes called DMSP lyases, which initiate enzymatic cleavage. This reaction breaks the DMSP molecule into two products: dimethyl sulfide and acrylate.
While some DMSP is metabolized through an alternate pathway yielding methanethiol, the cleavage pathway is the direct source of volatile DMS. Although the ocean is the dominant source, DMS is also produced naturally on land through the decomposition of organic matter and by certain plant species, contributing to the odor of various vegetables like cabbage and corn. The oceans represent the most significant source, making DMS a purely biogenic compound linking marine ecosystems to the global atmosphere.
Link to Atmospheric Processes
The volatility of dimethyl sulfide allows it to transition readily from the ocean surface into the atmosphere. Once airborne, DMS is subjected to atmospheric oxidation, playing a significant role in atmospheric chemistry. The primary reaction involves the DMS molecule reacting with hydroxyl radicals (\(text{OH}\)) present in the air.
This oxidation process transforms the sulfur in DMS into various oxidized sulfur compounds. These intermediate products are further oxidized, ultimately forming sulfur dioxide (\(text{SO}_2\)) and methanesulfonic acid, which lead to the production of sulfuric acid. The resulting sulfuric acid molecules rapidly cluster together to create new, small particles.
These newly formed particles grow by collecting other atmospheric molecules until they function as Cloud Condensation Nuclei (CCN). CCN are microscopic particles that serve as surfaces upon which water vapor condenses, seeding the formation of cloud droplets. In remote marine regions, where other particle sources are scarce, sulfate aerosols derived from DMS oxidation are the main source of CCN.
The formation of CCN directly influences the properties of low-level clouds over the oceans. Increased CCN concentration leads to clouds with more, but smaller, droplets, making them brighter and more reflective of solar radiation. This creates a natural feedback loop: higher biological activity produces more DMS, which leads to more reflective clouds that influence regional climate by scattering sunlight back into space.
DMS and the Sense of Smell
Dimethyl sulfide is distinguished by its low odor threshold, meaning humans and many animals can detect it at minute concentrations. The ability to perceive DMS is hypothesized to have evolved as an important navigational and foraging cue for marine life. For seabirds, the release of DMS into the air acts as a reliable long-distance signal pointing toward productive foraging grounds.
When zooplankton graze on DMSP-producing algae, DMS is released, indicating a dense patch of primary production. Seabirds, such as albatrosses and petrels, use this atmospheric DMS gradient to efficiently locate feeding areas in the open ocean. Physiological studies show that certain Antarctic seabirds can detect DMS at the low, biogenic concentrations naturally encountered over the ocean.
This olfactory sensitivity is not limited to birds; harbor seals also detect DMS at concentrations associated with high-productivity zones. The ability of marine animals to perceive DMS provides a sensory link to the marine food web. They use the breakdown product of phytoplankton as an indicator for profitable hunting areas, making DMS act as a chemical beacon guiding them toward energy-rich parts of the ocean ecosystem.