Nitrogen is a critical nutrient used by all living things. Everything from the largest animals down to the smallest microbes require nitrogen in some form or another, using it to build proteins, amino acids, chlorophyll and other chemicals that are essential to life. As it circulates its way through ecosystems, nitrogen makes its way from the atmosphere to soil and water, and back again, in a complex set of processes known as the nitrogen cycle. For aquarium owners, understanding how the nitrogen cycle operates in nature is helpful for maintaining a healthy, successful tank.
What is the Nitrogen Cycle?
Defined simply, the nitrogen cycle is a natural process that converts nitrogen into food and then breaks down wastes in a repeating cycle. Through a complex set of biochemical reactions, nitrogen gas from the atmosphere is transformed into compounds that can be used by all forms of life—including microbes, plants, and animals.
The nitrogen cycle in nature
The nitrogen cycle is a circular process, meaning that there is no true beginning or end. There are, however, major steps in the process, as outlined below:
Nitrogen Fixation
Most of the nitrogen on Earth exists in our atmosphere, with free nitrogen gas making up nearly 80% of the air we breathe. Despite this abundance, plants and animals cannot use nitrogen in its gas form. This is where nitrogen fixation comes in. A critical biological process, nitrogen fixation converts free nitrogen gas into compounds that can be absorbed
by plants, such as ammonia (NH3) or its ion form, ammonium (NH4+). This is done mostly by nitrogen-fixing microbes that live in water and soil, including several species of free-living cyanobacteria and blue-green algae, as well as symbiotic soil bacteria, which invade the root hairs of certain types of plants, like beans and clover.
Nitrogen-fixing microbes play a foundational role in the food chain by converting nitrogen gas into compounds that can be taken up by plants to fuel their growth.
Ammonification
Plants and animals take in nitrogenous compounds and use them to synthesize proteins and other organic compounds. As organisms shed old tissues, die, or excrete wastes, they leave behind organic matter that is rich in nitrogen. Microbes such as bacteria, fungi, and algae, feed on these organic wastes, breaking them down into simpler compounds. Part of this degradation process is ammonification, which is a process where microbes break down complex organic compounds into ammonia (NH3) and ammonium (NH4+).
Nitrification
Both nitrogen fixation and ammonification are processes that add ammonia to water and soil. While plants take up a portion of this ammonia directly, much of it is still left in the environment. Nature deals with this excess ammonia through nitrification, a two-step process where ammonia is converted to nitrite, then nitrate. As we’ll see later, nitrification is particularly important in aquariums.
Nitrification is made possible by several species of microbes known as nitrifying bacteria and archaea. These microbes produce enzymes that catalyze the necessary chemical reactions. In the first step, Nitrosomonas or other species of ammonia-oxidizing microbes convert ammonia (NH3) to nitrite (NO2-). Next, Nitrobacter or other species of nitrite-oxidizing microbes convert nitrite (NO2-) to nitrate (NO3-). The end product of nitrification—nitrate—is easily absorbed by most plants and algae, and is therefore an important source of nitrogen to fuel plant growth.
Anammox
Nitrification can only occur when free oxygen is available to oxidize ammonia and nitrite. In low-oxygen environments, like water-logged soils and deep-sea zones, a process known as anaerobic ammonium oxidation (or “anammox”) takes place instead. Like nitrification, anammox reduces ammonia and nitrite, however, the reaction and the products are different. In anammox, certain type of bacteria called planctomycetota produce an enzyme that catalyzes a reaction between nitrite (NO2-) and ammonium (NH4+), producing free nitrogen gas (N2) and water (H2O).
Denitrification
Essentially the inverse of nitrogen fixation, denitrification is the process of reducing nitrate to form free nitrogen gas. This process is driven by denitrifying bacteria and includes a series of intermediary reactions that converts nitrite (NO2-) to nitric oxide (NO), then nitrous oxide (N2O), and finally, dinitrogen gas (N2). Denitrification primarily occurs under anaerobic conditions, although aerobic denitrification is also possible in environments with unstable oxygen levels. Following denitrification, the newly-formed nitrogen gas returns to the atmosphere, and the cycle begins again.
The nitrogen cycle in an aquarium
In large ecosystems, the nitrogen cycle is a self-sustaining, circular process that is responsible both for supplying food to all the organisms living within it, and for cleaning up their wastes. But when it comes to aquariums, we usually just focus on the latter, viewing the nitrogen cycle as more of a linear process for managing wastes. So why the difference?
Many of the steps in the nitrogen cycle happen in an aquarium just as they do in natural ecosystems. But because of the limited scale and bio
diversity of an aquarium, there are a few processes that don’t proceed as efficiently as they do in nature. As a result, aquarists leverage the nitrogen cycle where they can, mostly relying on it to break down harmful nitrogenous wastes and convert them into materials that fish can tolerate. Steps in the nitrogen cycle that can’t be replicated easily in the aquarium environment are substituted with tank maintenance activities, like adding food flakes or pellets, doing water changes, and skimming out debris. As a result, the aquarium nitrogen cycle is usually represented as only a portion of the larger nitrogen cycle, specifically focusing on how toxins like ammonia and nitrite are converted to more innocuous substances, like nitrate and nitrogen gas. In aquariums, we call this biological filtration, or simply biofiltration.
Why are ammonia & nitrite so bad for fish?
Ammonia (NH3) and nitrite (NO2-) are simple nitrogenous compounds that can cause a lot of problems for aquarium inhabitants. Even at very low concentrations, both ammonia and nitrite are extremely toxic to aquatic animals. Fish produce ammonia as a metabolic waste product, which is normally eliminated from their bodies through both urination and respiration. When the concentration of ammonia in the tank is too high, ammonia can’t diffuse through fish’s gills, and it begins to build up in their bodies. Excess ammonia leads to symptoms such as stress, eye or gill inflammation, reduced appetite, lethargy, breathing problems, and even death. Nitrite, on the other hand, poses a danger because it binds to the blood protein hemoglobin and interferes with its ability to carry oxygen. For this reason, fish in high-nitrite environments can’t adequately circulate oxygen through their bodies, leading to symptoms such as stress, irritability, a paler or darker color, frequent surfacing, gasping, greater susceptibility to disease, and death.
Nitrogen cycle steps in an aquarium
Even the most diligent cleaning regimen won’t remove harmful chemicals like ammonia and nitrite from an aquarium. Instead, aquarium owners rely on the nitrogen cycle to manage wastes and maintain good water quality. Here’s how the nitrogen cycle deals with wastes and promotes a stable and healthy environment for your fish:
- Wastes enter the tank. Solid wastes are made up of complex organic compounds, such as digestive wastes from aquatic animals, as well as excess food, dead fish or plants, and shed plant tissues. Other nitrogenous wastes include fish urine and respiratory products, which contain significant amounts of ammonia.
- Wastes break down and release ammonia. During the ammonification process, microbes present in the tank feed on proteins in organic wastes, converting them into ammonia.
- Ammonia is converted to nitrite. In this first step in the nitrification process, Nitrosomonas or other species of beneficial bacteria oxidize ammonia to form nitrite.
- Nitrite is converted to nitrate. In this second and final step of the nitrification process, Nitrobacter or other species of beneficial bacteria convert ammonia to nitrate. If a tank has anoxic zones, it may also support anammox, where microbes convert ammonia and nitrite to nitrogen gas.
- Nitrate leaves the water. Once nitrate is formed, it can leave the tank in any of three ways: through water changes, uptake by plants, and denitrification in anoxic zones. During a water change, a portion of tank water is removed and replaced with fresh water, thereby eliminating nitrate and other wastes from the tank. Live plants and algae can also reduce nitrate levels by absorbing nitrate from the water. Finally, denitrifying bacteria may also reduce nitrate levels by converting it to nitrogen gas, but only if anoxic zones exist in the tank.
The aquarium nitrogen cycle converts toxic ammonia and nitrite to safer compounds, like nitrate and nitrogen gas.
Tips for optimizing the aquarium nitrogen cycle
Now that you know the basic process of the nitrogen cycle, you can apply some of the concepts toward creating a healthier, more self- sufficient tank. Here are some simple suggestions for supporting the nitrogen cycle in your own aquarium:
- Don’t rush a new tank. The nitrogen cycle is driven by billions of invisible microbes, including numerous species of beneficial bacteria and archaea. These microbes are naturally present in water, soil, and on surfaces, and will grow in your tank so long as the right nutrients are present. The catch is that it can take anywhere from one to six weeks for beneficial microbes to grow their numbers enough to consistently convert ammonia to nitrate. This process—known as cycling a tank—can be sped up a bit by using a beneficial bacteria inoculant product, or by adding a piece of biofilter media from an established tank, but it’s still important to wait until the tank is fully cycled before adding fish. Having some patience at the start will help you to avoid problems like new tank syndrome, and ensure a healthy environment for your fish.
- Remove visible debris and wastes. As soon as wastes are deposited in the tank, they will begin to decay and release ammonia. Removing solid wastes from the water helps to head this off at the pass, ensuring that beneficial bacteria aren’t overwhelmed by sudden spikes in ammonia. Manually skimming out solid wastes, employing mechanical filtration, or adding a protein skimmer, are a few ways to prevent excess ammonia in your tank.
- Provide ample usable surface area. Beneficial bacteria will colonize any available surface in the tank, forming a layer known as biofilm. While adding components like rocks, plants, gravel, and decorations can modestly increase surface area, biofilter media packs much more surface area into a smaller space.
- The best biofilter medias don’t just offer lots of surface area, either. They provide a structure that resists clogging even when the biofilm matures into a thicker layer, meaning that more of the surface area is usable. Providing more usable surface area allows a tank to house more beneficial microbes, which in turn increases the tank’s overall capacity for reducing ammonia and nitrite. MarinePure® biofilter media is a man-made ceramic that offers vast surface area and open porosity with Thin Bio-Film Technology™ that supports development of a healthy biofilm.
- Add aquatic plants. Aquatic plants and algae can help to keep nitrate levels under control because they absorb it from the water and use it as a nutrient to fuel their growth. Hobbyists can take advantage of this by incorporating live plants, adding an algae refugium, or even just leaving a little algae behind when cleaning the tank.
- Create anoxic zones. Creating anoxic zones in an aquarium can be achieved by using a biomedia with intricate pore structures, like MarinePure. These low-oxygen areas are valuable because two key processes in the nitrogen cycle only occur in environments with little to no oxygen—anammox and denitrification. Both processes facilitate a more complete, circular nitrogen cycle in the aquarium by helping to convert ammonia, nitrite, and nitrate all the way to nitrogen gas. Since nitrogen gas is able to leave the tank through the water’s surface, the tank may require less hands-on maintenance, like fewer water changes, and less issues with algae overgrowth due to excess nitrate.
Getting familiar with the nitrogen cycle is a great way to understand the rationale behind aquarium care and maintenance. Equipped with the with this knowledge, it’s easier to troubleshoot issues and create a more balanced ecosystem in your tank.