Written by Rachel G. Jordan
Pungent, fishy, and memorable. When you stand along the shore of a coral reef and breathe deeply, you inhale a peculiar aroma. This “smell of the sea” is more than oceanic perfume; it is the byproduct of coral chemistry that connects to global atmospheric cycling.
Chemistry is Connected
The chemistry of our world is cyclical. In the same way that oxygen is created by plants, breathed by humans, and then exhaled back into the atmosphere as carbon dioxide (and eventually converted back into oxygen by plants, thus completing the loop), other chemical elements of our world function within cycles, several of which connect directly to coral reefs.
The astute reader might know that one such cycle is that of calcium, which is an important building block in the formation of coral skeletons and contributes to the reef-building compound calcium carbonate. However, there is another less talked-of chemical cycle that corals deal with on a regular basis: sulfur.
The Stinky Cycle
Sulfur might make you think of the way rotten eggs smell, and for a good reason. Rotten eggs release hydrogen sulfide, a gas that contains the smelly sulfur element. Sulfur deposits are often found in rock formations near volcanoes and hot springs, which are other places you might experience this unique eggy scent. While it may seem odd to think of coral reefs in relation to sulfur, they are an essential part of the global sulfur cycle.
The coral holobiont, which includes the coral animal as well as its symbiotic algae and microorganisms, has long been recognized as the largest natural producer of the compound dimethylsulfoniopropionate (or DMSP for short). As part of their regular metabolism, corals produce DMSP and, shortly after, break down the molecule into smaller pieces through a process called cleavage. The two subsequent pieces of this cleavage are dimethylsulfide (DMS) and acylate.
The chemical structure of DMSP and its two product molecules after cleavage, DMS and acrylate.
DMS is a volatile gas, meaning that it moves through sea water and evaporates into the atmosphere relatively quickly. This is what we recognize as the “smell of the sea.” When you take a deep breath of ocean-scented air beside a coral reef, you are inhaling DMS.
Perhaps most amazingly of all, the DMS molecules that collect in the atmosphere over coral reefs contribute to cloud formation. In other words, if you see a collection of clouds over a coral reef, they are likely the result of DMSP cycling.
Corals contribute to the global sulfur cycle by breaking down DMSP, which results in cloud formation. (Illustration credit: Felicity Kuek)
An Unsolved Mystery
Besides DMS, the other product of DMSP cleavage by corals is acrylate. Organic chemists refer to this hard, relatively stable compound as zwitterionic, meaning that the compound simultaneously contains one positive and one negative charge. Corals might process acrylate further by adding a proton to it to create acrylic acid, which accumulates in coral tissues.
Several species of corals, such as branching Acropora spp., are known for containing substantially higher levels of acrylate than other kinds of corals. This is peculiar because we have yet to discover the purpose of acylate within coral tissues, beyond being a product of DMSP cleavage. Perhaps acrylate serves as an antioxidant, a predator deterrent, or a foraging cue. Whatever the reason, these species of corals make for fascinating study when it comes to further unraveling the chemical mechanisms and metabolic processes underlying DMSP cycling.
Smell of the Sea
Corals connect to the rest of our world in unexpected ways, namely through the organic chemistry of the global sulfur cycle. Next time you stand at the shore of a tropical sea, take a deep breath of smelly air and think of the corals. Gaze at the wispy strands of clouds overhead and ponder the chemicals that created them. And if you want to impress a fellow nature lover, drop the term dimethylsulfoniopropionate in casual conversation to prove your point of just how cool corals are.
References:
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