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Ammonia and Nitrites and Nitrates, Oh My!
A Quick and Dirty Overview of the Nitrogen Cycle
BackNitrification and Water Chemistry
Although many parameters affect nitrification in the aquarium, four factors play a greater role than others. These are dissolved oxygen (DO), temperature, pH and alkalinity. These will be explored in some detail and others will be briefly touched on in this section.
It is important to note that there are several species of both Nitrosomonas and Nitrobacter, and some species have multiple strains. Though most of the information below can be generalized for all, individual strains or species may exhibit variation from the parameters outlined in this section.
Dissolved Oxygen - As previously noted, nitrification is the biological oxidation of ammonia to nitrate. In the conversion of ammonia to nitrate, approximately 4 mg of oxygen are consumed. Both Nitrosomonas and Nitrobacter require dissolved oxygen to facilitate this process.
The amount of oxygen used in an aquarium can be stated as chemical oxygen demand (COD) and biological oxygen demand (BOD). COD is the amount of oxygen used when the decomposition of organic materials causes oxidization. BOD is the amount of oxygen required by living organisms to facilitate metabolic processes. Dissolved oxygen (DO) supplies these demands. Nitrification decreases rapidly under 3 ppm of DO and is inhibited below 2 ppm, excess COD or BOD can slow or inhibit nitrification. To minimize this potential, regular vacuuming the gravel to remove excess detritus (and subsequent COD from oxidization and excess BOD from heterotrophic bacteria) is important. Nitrobacter are affected more severely by low DO than Nitrosomonas, so a lack of DO may be seen as a spike in nitrites without a preceding spike in ammonia.
Temperature - Both Nitrosomonas and Nitrobacter reproduce through binary division: one bacterium divides into two individuals. Nitrifying activity remains constant from 72 to 96 degrees, but optimal division and growth is achieved from 78-86 F. Nitrification is reduced by 50% at 64F, by 75% at 50 F. Activity ceases completely at 40 F and nitrifying bacteria die at freezing. On the other end of the spectrum, nitrification ceases at 104 F and nitrifiers die at 120 F. Nitrobacter are less tolerant of colder temperatures than Nitrosomonas. It is convenient that the optimal temperature for division and growth falls into the range of temperatures that most tropical aquarium critters live in.
pH and Alkalinity - These two parameters are so intricately linked, they will be discussed together. Please note, basic/basicity will be used to note measurement of pH to avoid confusion of pH with discussion pertaining to alkalinity, which will be the term used to signify measurement of the buffering capacity of the water to increasing hydrogen ions.
pH is a measure of the acidity or basicity of a liquid, in this case water. The most simplistic definition is that it is a measure of the concentration of dissolved hydrogen ions (H+). Acidic water (water with a pH less than 7) contains a high concentration of hydrogen ions. On the other end of the scale, basic water (water with a pH greater than 7) contains few hydrogen ions and a high concentration of hydroxide (OH-). The higher the concentration of hydrogen ions in the water, the more acidic the water is. A pH of 7 is neutral. The pH scale is logarithmic; a change of one point on the pH scale indicates a change in the hydrogen ion concentration of a magnitude of ten. For example a pH of 5 is ten times more acidic than a ph of 6, and a pH of 4 is ten times more acidic than a pH of 5 and 100 times more acidic than a pH of 6. Conversely, a pH of 9 is ten times more basic than a pH of 8, and a pH of 10 is ten times more basic than a pH of 9 and 100 times more basic than a pH of 8. Hydrogen ions are a natural by-product of the metabolic functions of living organisms, thus over time the pH of a closed aquatic system will decrease as the hydrogen ion concentration increases.
Alkalinity is the measurement of water's ability to counteract this natural increase of hydrogen ions and the amount of alkalinity is often referred to as buffering capacity. Although many substances commonly occurring in water contribute to alkalinity (hydroxide and phosphates, for example), the measurement of most concern to hobbyists is carbonate alkalinity, or carbonate hardness. This is a measure of the concentration of carbonate (CO3 2-) and bicarbonate (HCO3-) ions in the water. These ions react with hydrogen ions (H+), effectively preventing acidification, preventing suppression of nitrification by bacteria, and maintaining ammonia in an aqueous solution accessible to nitrifiers rather than allowing the promotion of ammonia to unavailable ammonium (see next paragraph). Please note, carbon dioxide (CO2) does not contribute to alkalinity and in fact destroys buffering capacity due to a tendency to form carbonic acid in aqueous solution. Calcium carbonate and sodium bicarbonate are two substances often driving carbonate hardness.
At a low pH, hydrogen ions (H+) react with ammonia (NH3) to create the non-toxic ammonium ion (NH4+). At a constant low pH, the ammonium itself does not present a hazard to aquatic life. The threat occurs when the pH is raised, alkalinity is increased and the ammonium rapidly reverts to ammonia. The sudden release of ammonia may overload the nitrification capacity of the established colony of nitrifying bacteria, creating a toxic environment.
The high concentrations of hydrogen ions in acidic water react with aqueous carbonates or bicarbonates, removing the carbon from the pool of carbon available to nitrifying bacteria and reducing the buffering capacity of the water to avoid low pH conditions. Carbonates and bicarbonates are the primary source of carbon for nitrifying bacteria. Carbonate hardness has to be replenished on a regular basis to ensure nitrifying bacteria in an aquarium remain healthy and active. Although these bacteria will utilize carbon dioxide as a carbon source to some extent, the nitrification rate is greatly reduced. The optimal means to replenish carbonate alkalinity is through frequent water changes, as this also removes accumulated hydrogen ions that have not been buffered.
It should be noted that nitrifying bacteria are inhibited at a pH below 5.5 and above a pH of 10.5, regardless of other water parameters or conditions. The exact mechanism of this inhibition has not been determined at either end of the scale. Though the upper end of this range is seldom encountered or encouraged by aquarists, the lower end is often seen when a hobbyist recreates a "black water" environment high in organic material. In such cases frequent water changes and an alternate method of ammonia removal (such as a filter containing zeolite) is necessary. Although the ammonia concentration may be low, the toxicity tends to be acute.
Trace Elements - Iron, magnesium, calcium, molybdenum and copper are all required at trace levels for either growth or activity in Nitrosomonas, Nitrobacter , or both. At high concentrations all of these as well as chromium, nickel and most other metals have a negative impact to either growth or activity in one or both species.
Phosphate - Phosphate is required by both species for nitrification. However, phosphate addition is generally not required as phophate is released as a by-product in fish waste and is often present in water supplies. Excess phosphates will contribute to algae and cyanobacteria growth.
Sodium - Sodium is not required for either species. The effect it has on Nitrosomonas is of note, however. At lower concentrations sodium inhibits growth but enhances nitrifying activity, at higher concentrations it enhances growth but inhibits nitrifying activity. Approximately 1.5 mg/l is the threshold where this change takes place.
Amino Acids - High concentrations of amino acids tend to repress or inhibit nitrification. These may be removed with water changes.
Chlorine/Chloramines - Both will kill nitrifying bacteria at high enough concentrations. Before adding water to an aquarium, use a commercial chlorine/chloramine remover available from a pet store.
Light - Light has a minor inhibitory effect. The effect is negligible within the confines of an aquarium.
Medications - Medications may or may not have an affect on these bacteria. The best practice is to have a quarantine tank to move fish to for treatment to avoid threatening the stability of the nitrifying colony.
Other Factors - There are many other factors not mentioned that may have an affect on nitrification. The ones above are discussed as they are either important to the process or are frequently asked about by new aquarists.
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