The strong, pungent odor associated with seaweed and the seashore is often described as “fishy” or “of the sea.” This pervasive scent is not the smell of the ocean itself, but rather the airborne signature of specific chemical processes occurring within marine organisms. Understanding this smell requires looking closely at the biology of seaweed and the subsequent chemical reactions that happen once it is removed from the water. The characteristic scent is the end result of a specialized compound that is released and then converted into a highly volatile gas.
Dimethylsulfoniopropionate: The Odorless Precursor
The biological origin of this marine scent begins with a molecule called Dimethylsulfoniopropionate, or DMSP. This organosulfur compound is produced in large quantities by most types of marine algae, including seaweed (macroalgae) and microscopic phytoplankton. DMSP serves several protective roles for the living organism that synthesizes it.
One of the main functions of DMSP is as an osmolyte, helping the seaweed regulate its internal cellular pressure against changing salinity. It also acts as a natural antioxidant, protecting the plant’s cells from oxidative stress caused by intense sunlight. DMSP is stored internally within the seaweed’s cells and is an odorless molecule in its original form. Healthy, intact seaweed does not typically emit the strong smell associated with it.
Enzymatic Release of the Volatile Compound
The familiar aroma only appears when DMSP is chemically transformed into a different, highly volatile compound called Dimethyl Sulfide, or DMS. This conversion is triggered when the seaweed is damaged, dried out, or begins to decompose after being washed ashore. Cellular damage releases DMSP from its storage location, allowing it to mix with specialized enzymes.
These enzymes, known as DMSP lyases, are present in the seaweed and in the marine bacteria that colonize its surface. The lyases act as molecular scissors, rapidly cleaving the odorless DMSP molecule into two products: DMS and another compound, often acrylate. DMS is a gas that possesses the sharp, distinctive sulfurous odor identified as the “smell of the sea.” Because DMS has a low boiling point, it easily volatilizes and enters the atmosphere, making the odor readily noticeable.
The Connection to Fish and the Marine Cycle
The reason the seaweed smell is often described as “fishy” is that DMS is a pervasive chemical signature throughout the entire marine food web. The odor is a general indicator of sulfur cycling in the ocean, not unique to seaweed decomposition. Fish and other marine life are constantly exposed to DMSP and DMS by consuming DMSP-producing organisms or through the action of bacteria on their own tissues.
When fish or other sea creatures die, the organic compounds in their bodies, including DMSP, begin to degrade. Bacteria present on the dead organism convert these compounds into volatile sulfur gases, including DMS, which contributes to the smell of decomposition. Furthermore, DMSP and DMS serve as infochemicals, acting as a chemical signal that helps seabirds and other marine predators locate feeding areas.
The cycle of DMSP and DMS also has a significant ecological role beyond the immediate smell. Once DMS is released from the ocean into the atmosphere, it oxidizes to form sulfur-containing compounds. These compounds can act as cloud condensation nuclei, influencing the formation of clouds and affecting local climate patterns. Therefore, the familiar “fishy” scent is a sensory clue to a massive, climate-regulating biological process occurring across the world’s oceans.