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What causes that "marshy" smell? - March 26, 2017

                  Ah, the smell of salt air as you drive by the marsh towards the ocean! Where does that unique scent come from? It originates from the tiniest members of the wetlands ecosystem. These one-celled living organisms, called bacteria, are too small to be seen without the help of a microscope. The billions of bacteria living in just one gram of marsh mud are critically important to the healthy functioning of wetlands. Many different species of bacteria exist and are very diverse in terms of their metabolic requirements.

                Humans need oxygen from the air and organic carbon from the foods we eat to generate useful energy for us to grow and function normally. We also give off carbon dioxide as a waste product (when we exhale). However, this is not the case for all bacteria. Some bacteria depend on organic carbon and oxygen, but many do not. These bacteria are more like the earliest bacteria that existed before the Earth’s atmosphere had oxygen.

                Bacteria species have evolved in ways so they can use different forms of nitrogen, sulfur, and phosphorus, as well as carbon, as sources of energy. Consequently, bacteria are critically important to the major biogeochemical cycles in wetlands. Bacteria produce sources of these chemicals in forms that plants, and the animals that eat plants, require for growth and function. For example, bacteria are essential to the nitrogen cycle. Plants need nitrogen primarily in the form of nitrates or ammonium. Certain species of bacteria can change or fix atmospheric nitrogen into ammonium in a process called nitrogen fixation. Other bacteria are decomposers, which means when something dies in the marsh, bacteria will slowly break down the cells of the dead organism and eventually generate ammonia or ammonium. This process is called ammonification. Additional species of bacteria use ammonia and ammonium to generate nitrates that plants need. This process is called nitrification.

                In addition to nitrogen fixation, ammonification, and nitrification, there is another important part of the nitrogen cycle. In order for plants to grow, farmers require nitrogen in the form of nitrates or ammonium. There is a limited amount of nitrogen in soil, so if a farmer keeps planting in the same place, eventually this natural source of nitrogen will be depleted. Farmers will then turn to fertilizer as a source of nitrogen nutrients. However, the more fertilizer that is added over time, the more likely water run-off from rain will carry the nitrates into streams and wetlands. High levels in the waterways will cause eutrophication; excess nutrient pollution. That is when bacteria come to the rescue! Some species of bacteria break down nitrates for energy and their waste product is nitrogen gas. These microorganisms convert nitrates to nitrogen, which is a safe component of air, therefore removing excess nitrates from the environment. This additional part of the nitrogen cycle is called denitrification. Nitrogen fixation, ammonification, nitrification, and denitrification, are all parts of the nitrogen cycle that depend on bacteria.

                A wide variety of bacteria are critically important for balancing different parts of the nitrogen cycle in the wetlands. Other species of bacteria affect the carbon, sulfur, and phosphorus biogeochemical cycles in similar ways. Note there are bacteria that use sulfates that create hydrogen sulfide gas as a waste product, which is definitely something you can smell!

                We tend to focus on the wetlands environment that we can see: grass, mud, crabs, mussels, water, and birds. But the next time you breathe in the marsh air, appreciate the microorganisms that are just as important to the ecosystem balance.

                Try this experiment! Take a deep breath of air before a rainstorm. Then take deep breath when the rain subsides. Does the air smell different? Rainwater causes nutrients to be released in the soil and dormant bacteria will actively start metabolizing, releasing by-products that scent the air.

Nancy Zeller is a volunteer with the Maryland Coastal Bays Program and retired professor from American University. 



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