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Ammonia and Nitrites and Nitrates, Oh My!
A Quick and Dirty Overview of the Nitrogen Cycle
Nitrogen is an element essential to life. In its simplest form, it occurs as dinitrogen (N2), the major component of air. How it is converted to a chemical form usable to living organisms, the path it travels through biological processes, and how it is eventually returned to dinitrogen is the nitrogen cycle.
Nitrogen begins its journey into the realm of living organisms via a process known as nitrogen fixation. Fixation occurs mainly when microorganisms having the nitrogenase enzyme extract dinitrogen from the air and convert it to ammonia. These microorganisms may be free-living or may be symbiotic with plants. Regardless, the ammonia produced is quickly utilized by plants or other microbes and converted to proteins or other organic nitrogen compounds. This is known as nitrogen uptake. As a side note, high energy natural events, such as lightning, and industrial processes may also fixate nitrogen.
The organic compounds created are metabolized by living organisms to sustain life processes. The compounds travel through the food chain as critters eat plants and critters eat other critters. The end users that benefit from these organic nitrogen compounds are heterotrophic microorganisms that break them down to ammonia (NH3) and ammonium (NH4+), a process known as nitrogen mineralization. Simplified, this is decay.
Inorganic nitrogen may follow one of two paths at this point. It may be used by plants or free-living microorganisms and returned to an organic state through the nitrogen uptake process, or it may enter the nitrification process.
Nitrification is the conversion of ammonia to nitrate through biological oxidation. It is an aerobic process that involves several stages and numerous species of microorganisms. The two genera that contribute to nitrification the most are Nitrosomonas and Nitrobacter. Nitrosomonas sp. convert ammonia (NH3) to nitrite (NO2-) and Nitrobacter sp. convert nitrite (NO2-) to nitrate (NO3). The nitrate is then either used by plants or free-living microorganisms and returned to an organic state through the nitrogen uptake process, or is denitrified.
Denitrification is the process through which anaerobic microorganisms convert nitrate back to nitrite, and then to nitrous oxide, nitric oxide and dinitrogen. Dinitrogen escapes to the atmosphere to rejoin its friends and family, and to await fixation. Some nitric oxide and nitrous oxide may remain, but dinitrogen is the main product of denitrification.
It is the next to last phase of the nitrogen cycle, nitrification, which is of most interest to the aquarist.
In the Aquarium
Where does ammonia in the aquarium come from? Whereas mammals, birds, reptiles and most terrestrial amphibians excrete excess nitrogen in an organic form, teleost fish, aquatic invertebrates and aquatic amphibians are ammonotelic: they excrete excess nitrogen directly as ammonia. The ammonia is passed out through the gills during respiration. This is the first source of ammonia in an aquarium.
The second source is the result of heterotrophic microorganisms causing the decay of additional organic matter such as uneaten food, dead plant material, undigested fecal matter and that guppy under the driftwood that disappeared last week, but was never found.
At this point, the first stage of nitrification begins. Nitrification is the biological oxidation of ammonia to the final form of nitrate. Ammonia is highly soluble and is absorbed into water. Once absorbed, it circulates until coming in contact with Nitrosomonas bacteria. Nitrosomonas are substrate-dwelling, aerobic autotrophs. Autotrophs are microorganisms capable of synthesizing their own organic substances from inorganic compounds. Nitrosomonas are present in soils and water, and colonize any available surface, including plant leaves and stems. In the presence of dissolved oxygen (DO) and carbon, they convert ammonia (NH3) to nitrite (NO2-).
Left unchecked, ammonia is lethal to aquatic life at 1 part per million (ppm) and causes health problems as low as 0.25 ppm. Symptoms of ammonia poisoning include fish "gasping" at the surface listlessly, a milky appearance as the slime coat degenerates, and blood streaks in the fins and on the body in advanced stages. Even if action is taken and the fish survives, a shortened lifespan and permanent damage may occur.
Nitrite (NO2-) produced in the first stage is then converted to nitrate (NO3) by Nitrobacter bacteria. Like Nitrosomonas, Nitrobacter are substrate-dwelling, aerobic autotrophs and require dissolved oxygen and carbon to metabolize nitrites. In marine systems, there is evidence to support that the oxidation of nitrite to nitrate may be dominated by Nitrospira sp. rather than Nitrobacter.
Nitrite is dangerous to aquatic life at concentrations as low as 0.1 ppm, though this is somewhat species dependent and some species show greater tolerance than others. Symptoms include "gasping" at the surface listlessly and a brownish color to the gills. Nitrites react with hemoglobin, preventing oxygen uptake by blood in the gills, thus suffocating the fish.
Nitrate is the end product of nitrification. It is only removed from the system through nitrogen uptake by plants or microorganisms, by water changes or by denitrification. Denitrification is an anaerobic process by which microorganisms convert nitrate back to nitrite, then to nitrous oxide, nitric oxide and dinitrogen. Anaerobic conditions are the result of a lack of dissolved oxygen in the water, and should be avoided in an aquarium due to the potential of a build-up of hydrogen sulfide gas (H2S). If H2S is released from anaerobic areas, it may cause an unpleasant odor from the water and may be dangerous to the health of the tank's critters.
"Anoxic" is another term occasionally encountered by aquarists. Simply put, an anoxic state is an anaerobic condition where H2S producing microgranisms are absent. Anoxic conditions result in denitrification in the same manner as other anaerobic states.
While ammonia and nitrite are very toxic to fish at low concentrations, fish can adapt to an amazingly high concentration of nitrate, dependent on pH, dissolved oxygen, temperature, and age, size and species of fish. Salmonids suffer ill-effects from nitrate at a concentration of 10 ppm. On the other hand, fish living at over 200 ppm of nitrate is not unheard of and actually occurs in some environments in nature. The ill effects of high nitrate concentrations is often exhibited long-term rather than having immediate consequences like ammonia poisoning or nitrite poisoning.
The final form of inorganic nitrogen common in aquariums is the ammonium ion. Ammonium is formed when ammonia picks up an extra hydrogen molecule, converting the NH3 (ammonia) to NH4+ (ammonium ion). Ammonium is non-toxic and is more prevalent at a low pH (this will be discussed in more detail in the Nitrification and Water Chemistry section). Ammonium will often catch another molecule to form an ammonium salt. Many hobbyist grade ammonia test kits do not distinguish between ammonia and ammonium, and may give a false high reading for ammonia. This is why a tank that has been in service for a long period of time without a water change may show an ammonia reading, but the fish are still alive. The water has become acidic due to the lack of buffering capacity and the resulting ammonium is falsely reading as ammonia.
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