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Week 18: The Saltwater Nitrogen Cycle

Last week we taught you how to cycle your saltwater aquarium when just setting it up. This week we are going teach you about the saltwater nitrogen cycle in more depth, as it helps you really understand what is happening in your aquarium. Having this deep understanding of your aquarium and the natural processes that make it possible, will help you take care of your aquarium better and to avoid buying things you don’t need!

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Table of Contents

This Week's Video:

What Is The Nitrogen Cycle?

The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere, terrestrial, and marine ecosystems. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems.  Thus, bacteria or some other process is usually needed to provide a biologically available form of nitrogen. 

As a whole, nitrogen is extremely important for all life. It is needed for  cells, amino acids, proteins and DNA. It is also needed to make chlorophyll, which is used in photosynthesis (how corals, macro algae, and many of our favorite marine creatures get their food)! While Nitrogen is abundant in the air, and often enters the ground or ocean through “nitrogen fixation”, due to our farming and sewage, a lot of nitrogen enters the ocean due to our waste waters! 

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How Does it Work?

  • There is the nitrogen cycle on land as well as in the ocean, and while they are similar, there are some differences in the details! 
  • N2 is nitrogen gas, and it makes up 78% of our atmosphere, but our bodies can’t use it in gas form.
  • N2 enters the ocean mainly through diffusion with the atmosphere and runoff from land.
  • Nitrogen fixation is the next step. This is the chemical processes by which atmospheric nitrogen is assimilated into organic compounds. Basically, it’s the conversion of N2, nitrogen gas, into NH4+, ammonium, which is an organic form. This process of converting N2 into NH4+ is called ammonification.
  • By the way, the N = Nitrogen, the H = Hydrogen, and the O = Oxygen.
  • In the marine ecosystem, nitrogen fixation happens primarily through cyanobacteria that convert N2 into NH4+.
  • NH4+, ammonium, is a building block for all other nitrogen based compounds, but it’s also toxic to fish! So when we are testing for an ammonia spike at the start of our saltwater cycle, we are making sure that bacteria is converting N2 into NH4+.
  • NH4+ forms the “amino” part of an amino acid, and amino acids make proteins that are involved in cell structure, enzyme reactions, and more.
  • NH4+ can be taken in by plants, fungi, & bacteria and incorporated into amino acids, thus building cell structure. So other organisms can eat those plants and absorb amino acids into their bodies
  • All organisms have nitrogen based waste. In humans, our nitrogen waste comes out in our urine. And we get our nitrogen from consuming other plants and animals.
  • For fish, the nitrogen based waste product is NH4+, and a lot of this ammonium is secreted through the gills. And again, fish get their nitrogen by eating other plants and animals.
  • NH4+ in high quantities suffocates fish and destroys gill tissue.
  • Alright, so we started with atmospheric nitrogen and through ammonification, converted it into NH4+. Now we’re moving onto the next step of converting NH4+ into NO2- or nitrite
  • This is a process called nitrification- which is the biological oxidation (break-down) of NH4+ to NO2- then NO3- (nitrite & nitrate)
  • This is accomplished by a genus of bacteria called nitrosomonas. Instead of eating proteins or carbohydrates like we do, they eat NH4+ ammonium.
  • The waste product of nitrosomonas is NO2- or nitrite.
  • NO2- is also toxic to fish.
  • So we’ve gone from N2 to NH4+ through ammonification, and then from NH4+ to NO2 with a process called nitrification, carried out by a genus of bacteria called nitrosomonas. The next step is converting NO2- into NO3- or nitrate.
  • In another process of nitrification, Nitrobacter, another genus of bacteria, oxidizes (breaks down) nitrite into NO3-, or nitrate.
  • We are almost done with the cycle, but how do we get rid of NO3- in our aquariums, and how does nitrate become N2, thus completing the nitrogen cycle.
  • NO3- can be removed in one of two ways. The first with water changes. The second is by adding live plants which use NO3- as fertilizer. Then when you remove the growth from the macroalgae, you are removing nitrate. But none of those two ways actually converts the NO3- into N2.
  • So we arrive at the last stage of the nitrogen cycle, Denitrification. Whereas nitrification converted NH4+ to NO2- and then to NO3-, denitrification is the opposite chemical process. 
  • Denitrification, or Nitrate reduction is the process involving anaerobic bacteria reducing NO3- (nitrate) to N2. Whereas the previous parts of the cycle relied on aerobic bacteria, this last step is carried out by anaerobic bacteria.
  • In an ideal world, your saltwater aquarium would become populated with these anaerobic bacteria, because instead of having to rely on water changes or macro algae to consume NO3-, the anaerobic bacteria would convert nitrate into nitrogen gas.
  • But as the name suggests, anaerobic bacteria require an extremely low oxygen environment to thrive.  And where do you find those in a fish tank?  Either at the bottom of a deep sand bed, or deep inside thick and porous rock.

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Glossary of Terms

N2 nitrogen gas (atmospheric nitrogen)

NH4+ Ammonium

NO2- Nitrite

NO3- Nitrate

Amino Acid– structural units that make up proteins

Ammonia & Ammonium

  • Ammonia and Ammonium are compounds that contain Nitrogen and Hydrogen. Ammonia contains one Nitrogen and three Hydrogen whereas Ammonium contains one Nitrogen and Four Hydrogen.
  • Ammonia is a weak base and is un-ionized. On the other hand, Ammonium is ionized.

Denitrification- Denitrification is a microbially facilitated process where nitrate is reduced and ultimately produces molecular nitrogen through a series of intermediate gaseous nitrogen oxide products

Nitrate Reduction– process involving anaerobic bacteria reducing NO3- (nitrate) to N2 (nitrogen gas.

Nitrification- Biological oxidation (break-down) of NH4+ to NO2- then NO3- (nitrite & nitrate)

Nitrogen Fixation– the chemical processes by which atmospheric nitrogen is assimilated into organic  compounds, especially by certain microorganisms as part of the nitrogen cycle.

Nitrobacter- genus of gram-negative bacteria that oxidizes NO2- into NO3-.

Nitrosomonas– genus of Gram-negative bacteria that consume NH4+ (ammonium) and produce NO2- (nitrite)

Organic- carbon based

Oxidation- Oxidation occurs when an atom, molecule, or ion loses one or more electrons in a chemical reaction. The opposite of oxidation is reduction.

Proteins- Proteins are large, complex molecules that play many critical roles in the body. They do most of the work in cells and are required for the structure, function, and regulation of the body’s tissues and organs.

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