Dimethylsulfide. Does that word mean anything to you? "Why yes," you organic chemistry nerds may say, "It clearly is a atom of sulfur with two methyl groups attached." But it's so much more than that. Let me spoil it for you: it is both a chemical cue pervasive throughout the ocean's food web and an effector on the earth's climate. That's right: just a sulfur molecule with two methyl groups attached.
This past week, new research found that this little molecule may also influence the rate at which climate change progresses, potentially amplifying global warming. But before we get into that—what is DMS?
Dimethylsulfide is the largest natural source of sulfur gas into the atmosphere—larger than either volcanoes or vegetation—and 95% of it originates in the ocean. It is made within the cells of some species of phytoplankton, and is freed when their cell walls are broken, most often when algae-eaters take a bite.* All of this leaking DMS makes the ocean smell like, well, the ocean—a soft whiff of sulfur in the air.
Too much sulfur in the atmosphere can cause acid rain, but some is necessary because it is a major ingredient in cloud formation. And we need clouds to regulate our climate by reflecting sunlight (and thus heat) back into space, cooling the planet. In order for clouds to form, water has to transition from a gas to liquid—and to do that, it needs a small particle in the air to adhere onto, known as a cloud condensation nucleus. Sulfur aerosols, which are easily formed from DMS, do the trick.
DMS-as-climate-regulator rose to fame when it starred in one infamous Earth-as-organism idea—the Gaia hypothesis—just a few decades ago. In 1987, James Lovelock and
a handful of his lackeys a few of his colleagues hypothesized that phytoplankton moderate how much DMS they produce to keep the planet livable. (For example, they would produce more in warm, sunny times to reflect more sunlight and reduce global temperatures.) However, this idea requires that phytoplankton act altruistically, releasing DMS for the good of the planet—a concept that does not make much sense in light of natural selection.
The new research, published in Nature Climate Change, suggests that it does play a significant role in climate regulation, but not in the way Lovelock proposed, as it may amplify warming instead of stalling it. Beyond global warming, another major effect of excess carbon dioxide is ocean acidification. The slow rise in the ocean's acidity caused by carbon dioxide dissolving in seawater is known as "climate change's evil twin" because it goes largely unnoticed by us landlubbers, but may change ocean ecosystems significantly.
The researchers found that, under high acidity, phytoplankton generate less DMS than usual—and less DMS in the ocean means less DMS in the atmosphere. If atmospheric carbon dioxide concentrations double, which they will do before 2100 if we don't slow our emissions, the reduction in DMS could warm the planet 0.23 to 0.48°C, according to their model.
DMS doesn't just act on climate: it's also used by many different organisms, from microbes to whale sharks, to find food. On a microbial scale, herbivorous bacteria "smell" (aka follow the chemical trail) of DMS in the water to find phytoplankton. Carnivorous microbes that eat other microbes use the same smell to find their prey: bacteria eating phytoplankton or DMS itself.
This is not the end of the story, as DMSP is a prey indicator at higher trophic levels as well. Plankton-eating fish and whale sharks use DMS as a foraging cue, aggregating near hotspots where phytoplankton and their plankton predators have gathered en masse. And one more step up the food chain, seabirds and seals sniff out DMS to find patches of their own prey, fish and squid, which gather to feed upon the zooplankton.
So why do such a diverse group of animals follow their noses to DMS in particular? Any number of molecules released from a leaky phytoplankton cell could be used to indicate these clusters of consumption. However, DMSP has something special: smelly sulfur. We all know that smell, and perhaps it is this stinkiness that has allowed it to become such a pervasive indicator throughout the marine food web.
No matter the reason, this seemingly humdrum little molecule has found itself responsible for not just one but two positive feedback loops. As it is eaten, it draws in ever more eaters up the food web, from phytoplankton-eating zooplankton to fish and whale sharks to seabirds and seals. And as carbon dioxide warms the atmosphere and acidifies the ocean, it only makes the climate warmer.
* The phytoplankton itself actually makes a molecule called DMSP (dimethylsulfoniopropionate, if you must know). When its cell wall is broken, stores of DMSP (from an unknown location within the cell) and an enzyme (DMSP-lyase) are released into the surrounding water. This DMSP-lyase removes the propionate ion, leaving us with DMS.
Six K.D., Kloster S., Ilyina T., Archer S.D., Zhang K. & Maier-Reimer E. (2013). Global warming amplified by reduced sulphur fluxes as a result of ocean acidification, Nature Climate Change, DOI: 10.1038/nclimate1981