Ugh, what’s that smell??

Every day I cross the Dumbarton Bridge from my home in Fremont to MSI in Redwood City, and almost always I’m greeted by the strong odor of the mudflats along the eastern shore of the Bay. It’s a pretty gross smell, and so strong that even with the windows up the car starts to smell like rotten eggs. But it doesn’t bother me too much, because I know the stench is evidence of essential organisms hard at work.

Marshes and mudflats are super-productive biomes, filled with plants and animals both aquatic and terrestrial. Eventually, all those living things must die. This is when the decomposers come in to play.

Decomposition doesn’t “just happen.” Take your Halloween decorations, for instance: in order to get disgusting and rotten, your jack-o-lantern needs a lot of help. You can try this at home if you want to! Leave your jack-o-lantern outside after Halloween for at least a week, and see what happens.  As you watch it change, you’re actually seeing a whole community of microorganisms work together to dismantle that pumpkin at a molecular level. Fruit flies, mold and bacteria all want a piece of that sweet, rotting pie.

Essentially, the same thing is happening in a mudflat, but with some important differences. Tons of organic matter ends up buried in the mud, and the microorganisms go to town on it. But unlike your front porch, mud is a low-oxygen, or hypoxic, environment. The mold and bugs that take over your pumpkin can’t live in this habitat. But certain bacteria can!

These bacteria are called sulfate-reducing bacteria. Unlike most living things, they don’t use airborne oxygen (O2) for respiration. Instead, they use sulfate (SO4), which is a molecule made of sulfur bonded with oxygen. This makes them able to live where most bacteria can’t – in super low-oxygen environments like a mudflat – and break down matter in the absence of oxygen. Just like other organisms, these bacteria produce a waste product when they “breathe,” but it’s not carbon dioxide (CO2) – it’s hydrogen sulfide (H2S).  This produces that funky rotten egg smell.

So the next time you find yourself wrinkling your nose in disgust at the smell of a mudflat, remember that you’re smelling the evidence of decomposers hard at work! Thanks, decomposers!


2 Responses to “Ugh, what’s that smell??”

  1. Luis Says:

    Is it possible to be exposed to high concentration of H2S while stepping in the mud that may cause severe health issues or even death? One day while wade fishing I step in mud and bubbles started submerging around me, then the smell of H2s hit me. I panic a little but as soon as it started I step into a hard bottom shell layer and it went away.

  2. Nikolaos Peterson Says:

    Oh, I am so sorry! I had an accident of the runny shits!

    Actually some friends and I had driven over the Dumbarton Bridge (SR-84) and my friend who was driving had the nerve to ask me whether I had actually shit my pants in his car! Well playing along with him, I said:’ Yes! Sorry! I have the runs!; the others in the back seat were laughing! (Actually I have produced uh… ‘human products’ that had been equally offensive! Oh my poor olfactory system!

    Okay, now to brass tacks; the odour is caused by Dunaliella Salina, a microalgae, that also gives these salt ponds their characteristic colours especially visible from arriving aircraft on the crosswind leg for SFO.

    Interesting that this stinky algae is also an excellent source for beta carotine used as a nutritional supplement, as well as glycerol. This algae itself has been also marketed as a nutritional supplement proper (with the sulphuric odour removed of course).

    Next time you should drive across Dumbarton Bridge, think of me! Yeah, I know!

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