As much as we like to marvel at the power and majesty of the ocean, we have to admit that as beautiful as it is, it also stinks. It stinks because it’s shot through with sulfur. And that sulfur has seen the insides of two different creatues before it got to your nose.
Our story starts with a substance called DMSP, which is a necessary abreviation of dimethylsulfoniopropionate. This substance consists of a long string of carbon, hydrogen, and oxygen, with two sulfur atoms thrown in. It’s made by phytoplankton, which use it as an internal workhorse. DMSP regulates osmosis inside the phytoplankton, shifting around liquids from place to place. Scientists think it’s also used as a kind of sunscreen, protecting the plankton from UV light.
Phytoplankton don’t last long. If they don’t get beached or eaten, they die of old age in a few days. Even before they die, they leak DMSP into the environment. After they’re dead, their bodies split open and split the chemisty of their guts everywhere.
That’s when the bacteria go to work. Bacteria chew their way through the dead plankton, and when they get to DMSP, they tuck in. Ripping off several of the strings of carbon, oxygen, and hydrogen from the complicated molecule, they excrete a comparatively simple (CH3)2S. This is dimethyl sulfide, and because it retains one sulfur atom it is pungent. It’s pungent enough to serve as a signal to predatory birds that there are a large concentration of plankton, and therefore a large concentration of fish, in the area. And it’s pungent enough to assault your nose every single time you go to the sea side. The molecule that started off inside of phytoplankton and wound its way through the (metaphorical) guts of bacteria, is now lodged in your nostrils.
[Source: Applied and Environmental Microbiology, Why Does the Sea Smell Like the Sea?]