Nitrogen Cycle (More than you wanted to know)

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rrkss

Biology is Fun
Dec 2, 2005
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I wrote this thesis a while back and thought I would share it with the community. Its a bit more on the complex side but I am sure you will be able to understand it.

The Nitrogen Cycle
By: Robert R Koorn
Every day, many people go about the task of starting a new aquarium. They go out and buy a tank, fill it with some water. Add dechlorinator to it. Buy some decorations and throw in fish. They are so happy that they just started their first aquarium and show it off to their friends and family. In a few days, the water turns cloudy, the fish hang out at the surface hyperventilating and eventually they die off one by one. The person gets upset and possibly gives up on a very rewarding hobby.

Little do they realize that they have just witnessed the beginning of a cycle that makes keeping aquariums possible and also the biggest killer of captive fish. This series of events are the beginnings of the Nitrogen Cycle. “The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature.” (Wikipedia 1) This Nitrogen cycle is responsible for converting toxic fish wastes such as Ammonia (NH3) into a less toxic waste product called Nitrate (NO3-1). Then the Nitrogen cycle takes its final step called Denitrification where Nitrates get converted into Nitrogen Gas or taken up by some autotrophic organism as a nutrient (Barr 9). In an aquarium environment, this cycle is driven by the interactions of various heterotrophic and autotrophic organisms.

The first step in the Nitrogen Cycle is the production of ammonia. In an aquarium, the primary sources of ammonia are waste excretion from fish (Ricketts 1) and excretion from heterotrophic bacteria and fungi who break down organic molecules in the tank (Barr 9). A secondary source of ammonia comes from tapwater as part of the chloramines disinfectant used as a replacement for chlorine (United Water 1). This ammonia is a very toxic compound when it is in its unionized form (Fishdoc 1). This compound is corrosive and causes Gill Hyperplasia. “Gill hyperplasia is a condition in which the secondary gill lamellae swell and thicken, restricting the water flow over the gill filaments. This can result in respiratory problems and stress, as well as creating conditions for opportunistic bacteria and parasites to proliferate. Elevated levels are a common precursor to bacterial gill disease.” (Fishdoc 1)

In the author’s own tank, detectable levels of ammonia are usually followed by fish hyperventilation and the fish tend to hang out at the surface. In addition the blood vessels in the fishes’ caudal fins tend to become dilated and visible. The author also notes that the water loses its nice healthy earthy smell and tends to become cloudy.

Ammonia poisoning is a serious problem but luckily dealing with it is not that difficult. The author usually does a 50% waterchange to dilute the ammonia and adds chemicals such as Seachem’s Prime or API Ammolock to convert the Ammonia from the unionized NH3 form to the ionized NH4+ form. The best and longest term solution is to get a biofilter developed. A biofilter works by converting toxic ammonia into nontoxic nitrate (Hovenec 1). In order for one to develop, a steady supply of accessable ammonia must be present and the fish provide this in an aquarium. The biofilter starts by oxidizing the ammonia into the second product of the Nitrogen Cycle called Nitrite.

This oxidation is done by various chemoautotrophic bacteria. The 4 primary bacteria responisible are Nitrospira tenius, Nitrosomonas europaea, Nitrosococcus mobilis, and Nitrosomonas marina (Hovenec 1). In an aquarium environment, the most important of these four species are Nitrosomnonas marina like species. (Hovenec 9). These bacteria use the following reaction to produce the energy they need to convert CO2 into glucose.

4 NH3 + 7O2 -> 4NO2 + 6H20 + energy

While these bacteria provide the fish with a favor by eliminating the toxic and caustic ammonia, it leaves another toxic waste product which tends to kill more fish that its precursor. (Hovenec 1) This toxic waste product is the Nitrite ion (NO2-1). Nitrite kills fish by oxidizing the hemoglobin into methohemoglobin. (Fishdoc 1) Unlike hemoglobin methemoglobin is unable to transport oxygen and is therefore useless to the fish. This oxidation kills the fish similar to the way Carbon Monoxide kills a human.

Fish exposed to Nitrite tend to become lethargic and their gills turn from bright red to a brownish tan color as less and less hemoglobin remains in their blood. Eventually this fish will die from chemical asphixiation. Treating Nitrite poisoning can be done by removing Nitrites via waterchanges and using a 0.01% saline solution to reduce Nitrite uptake via osmosis (Fishdoc 1). The best solution for Nitrites is to have a functioning colony of biofilter bacteria which remove Nitrites by oxidizing them into Nitrates.

This bacteria responsible for the oxidation of Nitrite into Nitrate in aquariums are from the genus Nitrospira (Hovenec 1). They convert Nitrite into Nitrate via the following chemical reaction:

2NO2 + O2 -> 2NO3 + Energy

Nitrate is the final waste product of the Nitrification part of the Nitrogen Cycle. It is relatively nontoxic to fish and tends to accumulate in aquariums unless eliminated via waterchanges or denitrification processes.

Denitrification is the final step in the Nitrogen Cycle. It is done in two ways. One way is via plant, algae and cyanobacteria uptake of Ammonia or Nitrates and the second method is via the reduction of Nitrate into Nitrogen gas or to a lesser extent Nitrous Oxide gas by anerobic bacteria from the genus Psuedomonas (Barr 9).

In an aquarium bacterial denitrification does not typically remove much of the Nitrates in most aquarium systems due to the need for an anaerobic environment, but is possible in some saltwater systems such as the FOWLR (Fish Only With Live Rock) method. This method uses live rocks which have deep pores with the appropriate anoxic conditions capable of providing the environment where denitrification via bacteria is possible (Fenner 1).

In most systems, the majority of the Nitrogen accumulates untouched until a waterchange removes them from the tank. In other systems such as a heavily planted system, plants would absorb a large amount of Nitrates from the water column for use to manufacture proteins, nucleic acids and many other important macromolecules such as chlorophyll (Barr 6).
Now that the Nitrogen Cycle has been explained in depth lets return to our example at the beginning of this paper and outline steps to turn that Nitrogen Disaster tank into a wonderful aquarium to enjoy for years to come, by using a simple strategy to reduce toxic Nitrogenous waste buildup while the bacteria in the biofilter establish themselves. This strategy involves starting off slowly and adding more fish as the tank becomes capable of handling it.

When a new tank is first set up, its environment is basically sterile with very little bacteria available to eliminate toxic nitrogenous wastes such as ammonia. To start the Nitrogen we add some fish. The author based on his experience with aquariums recommends that a person adds 1/3 of the total amount of fish planned for the aquarium be added in the beginning to start off the Nitrogen Cycle. These fish will produce Ammonia and provide a energy source for nitrification bacteria. Soon after, the ammonia will start to buildup and nitrification bacteria will start to grow a colony. During this time, the author recommends daily testing of the water and the performance of waterchanges to keep ammonia below 1.0 ppm on a standard hobby test kit. After about 10-14 days the ammonia oxidizer colony should become large enough to keep the ammonia levels undetectable.

At this stage, the danger is not over yet. This ammonia oxidizing colony is producing toxic Nitrite as their waste product. The author recommends daily testing of Nitrite followed by water changes to keep Nitrite below 0.5 ppm. After two to three weeks from the ammonia going below detectable levels, the Nitrites should become undetectable. Only at this time can more fish be added with careful monitoring of both ammonia and nitrite to make sure that the fish don’t get poisoned while the biofilter catches up to the increased ammonia addition from the new fish.

Following these simple methods, and the understanding of the Nitrogen cycle allows a new fish tank to be converted from a fish killing glass box into a wonderful aquarium that can be enjoyed for years. The nitrogen cycle is a relentless machine that can be employed to keep pet fish happy, healthy and a pleasure to have for many years. The nitrogen cycle is by far the most important thing any aquarist should be aware of.

Works Cited:
Hovenec, Timothy, Lance Taylor, Andrew Blakis, and Edward Delong. "Nitrospira-Like Bacteria Associated with Nitrite Oxidation." APPLIED AND ENVIRONMENTAL MICROBIOLOGY. 64(1998): 258-264.
Paul C. Burrell, Carol M. Phalen, and Timothy A. Hovanec Ph.D.. Identification of Bacteria Responsible for Ammonia Oxidation in Freshwater Aquaria. APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Dec. 2001, p. 5791-5800
Fenner, Bob. "Nitrates in Marine Aquarium Systems." 26 10 2006 <http://www.wetwebmedia.com/nitratesmar.htm>.
Water, United. "Surface Water Treatment Process." 26 10 2006 <http://unitedwater.com/uwnj/wtrspply.htm>.
FishDoc, "Nitrite and Fish Health." 26 10 2006 <http://www.fishdoc.co.uk/water/nitrite.htm>.
FishDoc, "Ammonia and Water Quality." 26 10 2006 <http://www.fishdoc.co.uk/water/ammonia.htm>.
Barr, Tom. "Nitrogen Cycling in Planted Aquariums." The Barr Report 1,6 June, 2005 1-15.
"The Nitrogen Cycle." Wikipedia. 2006. Wikipedia . 29 Oct 2006 <http://en.wikipedia.org/wiki/Nitrogen_Cycle>.
 
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