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Word Wednesday: Denitrification

Wednesday, April 20th, 2016

You may have heard that matter is neither created nor destroyed, but what does this really mean? This week’s Word Wednesday focuses on the process of denitrification and the nitrogen cycle.

ACER scientists are investigating whether the ecosystem services provided by our northern Gulf of Mexico coastal environments were impacted by the oil spill. As we discussed earlier, ecosystem services are processes or services provided by normally functioning intact habitats that benefit humans. One of the ecosystem services ACER scientists are measuring is denitrification. Denitrification is the conversion of one form of nitrogen, typically nitrate (or nitrite), to nitrogen gas. This process is carried out by bacteria living in anaerobic environments (those that do not contain oxygen). The salt marsh sediments and muddy benthic environments being studied by ACER scientists are areas where rates of denitrification can be very high.

sediment cores
Sediment cores taken in the field and brought back to the laboratory for denitrification measurements. Photo credit: A. Kleinhuizen

You might ask why do we care about nitrogen at all? Nitrogen is considered to be a macronutrient – that is something organisms need to grow and is needed in relatively high amounts. In fact, you may add a nitrogen fertilizer to your garden to help your vegetables or flowers grow faster and bigger. But why do living organisms need nitrogen? Well, think back to what you learned in high school biology about proteins, or even macromolecules. The element nitrogen, when combined with hydrogen molecules makes an amine functional group. An amine group added to a carbon skeleton makes an amino acid and a bunch of amino acids joined together make a protein. And surely, you’ve heard of proteins… Without proteins, life as we know it would not exist.

Since matter is neither created nor destroyed, all of the nitrogen that is currently on Earth was here when the Earth was formed. However, nitrogen (as well as other elements) changes in form by geological, chemical and biological processes. Some forms are only found in living organisms and some forms are found in non-living (abiotic) structures. Some elements, including nitrogen, occur in multiple forms in living organisms, whereas some elements occur only in one form. A description of these forms, the transformations that occur among forms and the processes causing the transformations is known as an element’s biogeochemical cycle. More simply, an element’s biogeochemical cycle is the pathway the element takes as it moves between living (biotic) and non-living (abiotic) reservoirs on Earth.

Nitrogen Cycle
Diagram of complete nitrogen cycle. Taken from Hopkinson, C., and A. Giblin. “Nitrogen dynamics in salt marsh ecosystems.” Nitrogen in the marine environment. Academic Press (2008).

Without nitrogen, things would not grow or reproduce. And it stands to reason that the more nitrogen you have, the more things grow or reproduce. However, there is a point at which too much of a good thing becomes a bad thing. You may have heard of eutrophication – the overfertilization of water that results in algal blooms and poorly oxygenated waters. Eutrophication is due to the oversupply of nitrogen to aquatic ecosystems. Denitrification transforms one specific form of nitrogen, nitrate (NO3-), to another, dinitrogen (N2) and in doing so, removes it from the biotic portion of the cycle. Thus, denitrification removes excess nitrogen and is therefore considered to be an important ecosystem service in coastal environments. ACER scientists are investigating how rates of denitrification changed in ecosystems impacted by the Deepwater Horizon oil spill. Tune in next week for our Tool Talk series when we’ll talk about how scientists measure denitrification….