One of the things I love most about salt marshes is their smell, especially at low tide – that’s when you can really smell the mud. “Stinky” is how my natural history books describe salt marshes, comparing the smell to rotten eggs. But to me, that smell says “life” – with an undertone of death.
That’s because salt marshes are an alchemist’s chamber, where death is turned into life, and that life – dying in its turn - creates more life. You can think of salt marshes as vast recycling machines, where the stuff of life – carbon, oxygen, nitrogen, sulfur and more – is endlessly transformed, one creature becoming another. Marsh grass is eaten by snails, snails are eaten by crabs, and snails and crabs are eaten by birds. Alive, the snails, crabs and birds excrete nutrients, which fertilize the growth of marsh grass. Dead, the nutrients stored in their bodies are released back into the marsh, fertiizing it further.
Carbon, oxygen, nitrogen – those all sound familiar from high school biology classes. As humans, we are carbon-based lifeforms. We need oxygen to breathe. We need nitrogen to make protein (and it’s an essential component of the DNA and RNA that transmit our genetic codes). But sulfur – what’s that about?
Twice a day, rising tides bring fresh seawater into the marsh. Along with water and salt come many other chemical compounds, including sulfates. Sulfates are usually toxic to plants, but the commonest salt marsh plant – Spartina, a type of grass – has found a way to turn the sulfates from a problem into a benefit. Specialized sulfur-eating bacteria living inside Spartina roots can transform those sulfates into energy; the plant then uses that energy to create its own fertilizer from nitrogen in the air. In the process of producing energy from seawater, the bacteria produce hydrogen sulfide, which gives marshes that rotten egg smell. So, the process that makes salt marshes smell awful is actually the process that makes marsh grasses grow!
Toxic sulfates are not the only problem faced by marsh organisms. As Spartina grows and dies over and over again through thousands of years, it builds up thick layers of peat. Over time the peat becomes anoxic: without oxygen. Under these conditions, the normal processes of decomposition stop. While this helps preserve the peat, it also halts the cycling of nutrients from dead plant cells into marsh soup and then into the growth of new marsh grass. Fortunately, other types of sulfur-eating bacteria can take over the job of decomposing the peat and continuing the nutrient cycle.
Salt marshes are not the only places where sulfur-eating bacteria are found. They are the ultimate survivors in extreme environments, such as the hot springs in Yellowstone National Park and hydrothermal vents in the deep oceans. Scientists believe that these bacteria evolved 3.4 billion years ago, when the oceans were boiling hot and there was no oxygen in the atmosphere. They may represent the first forms of life on Earth. So next time you’re in a salt marsh, take a deep breath, and believe that life – in some form – will endure whatever we humans do to the planet.