The Nitrogen Cycle

In it's simplest definition, the Nitrogen Cycle occurs as follow:

1. Fish excrete ammonia from their gills and kidneys. Ammonia also is formed from decaying (leaves, uneaten food, etc.)
2. This ammonia is converted to Nitrite by Nitrosomonas type bacteria
3. Nitrite is converted to Nitrate by Nitrospira type bacteria
4. Nitrates, in most cases, are harmless unless at high levels and are consumed by algae, plants or through regular water changes.

Nitrosomonas type bacteria convert ammonia (NH3) to nitrite (NO2)
Nitrospira bacteria convert nitrite (NO2) to nitrate (NO3)

One of the most important aspects of successful koi keeping or any fish keeping for that matter, is biological filtration and its function in the nitrogen cycle. I read recently, that the number one reason novice fish keepers become disillusioned with the hobby is the frequency in which they experience high death rates of their aquatic pets after setting up a new system. Statistically, as much as 75% of the fish sold to hobbyists will die within the first 30 days and 2 out of every 3 new hobbyist abandon the hobby within the first year. This data applies to all types of fish but nonetheless, they're pretty staggering statistics.

One of the most common reason for these kill rates is known as 'new tank syndrome' or, the 'nitrogen cycle.' The fish are simply poisoned by high levels of ammonia (NH3) that is produced by the bacterial mineralization of fish wastes, excess food, the decomposition of animal and plant tissues and let's not forget, the additional ammonia that is excreted directly into the water by the fish themselves. The effects of ammonia poisoning in fish include: extensive damage to tissues, especially the gills and kidney; physiological imbalances; impaired growth; decreased resistance to disease, and; death. Nitrite poisoning inhibits the uptake of oxygen by red blood cells. Known as brown blood disease, the hemoglobin in red blood cells is converted to methemoglobin. This problem is much more severe in fresh water fish than in other marine organisms and can easily cause death.

Nitrifying bacteria are classified as obligate chemolithotrophs. This simply means that they must use inorganic salts as an energy source and generally cannot utilize organic materials. They must oxidize ammonia and nitrites for their energy needs and fix inorganic carbon dioxide (CO2) to fulfill their carbon requirements. They are largely non-motile (can't move around easily) and must colonize a surface (gravel, sand, synthetic biomedia, and the 1001 other filter materials out there) for optimum growth. They secrete a sticky slime matrix which they use to attach themselves. Species of Nitrosomonas and Nitrobacter are gram negative, mostly rod-shaped, microbes ranging between 0.6-4.0 microns in length. They have evolved to become extremely efficient at converting ammonia and nitrite. One disadvantage is, they have a very slow reproductive rate. Nitrifying bacteria reproduce by binary division. Under optimal conditions, Nitrosomonas may double every 7 hours and Nitrobacter every 13 hours. More realistically, they will double every 15-20 hours.

Nitrospira and Nitrosomonas bacteria have limited tolerance ranges and are individually sensitive to pH, dissolved oxygen levels, salt, temperature, and most chemicals. They cannot survive any drying process without killing the organism. In water, they can survive short periods of adverse conditions by utilizing stored materials within the cell. When these materials are depleted, the bacteria die.

There are several species of Nitrosomonas and Nitrospira type bacteria’s and many strains among those species. Most of the following information can be applied to species of Nitrosomonas and Nitrospira in general., however, each strain may have specific tolerances to environmental factors and nutriment preferences not shared by other, very closely related, strains. This is why Genesyz (Lymnozyme) coexists with Nitrosomonas and Nitrobacter bacteria, they don't compete for the same food source.

Temperature

The temperature for optimum growth of nitrifying bacteria is between 77-86° F (25-30° C).
Growth rate is decreased by 50% at 64° F (18° C).
Growth rate is decreased by 75% at 46-50° F.
No activity will occur at 39° F (4° C)
Nitrifying bacteria will die at 32° F (0° C).
Nitrifying bacteria will die at 120° F (49° C)
Nitrospira is less tolerant of low temperatures than Nitrosomonas. In cold water systems, care must be taken to monitor the accumulation of nitrites.

pH

The optimum pH range for Nitrosomonas is between 7.2-8.0.
The optimum pH range for Nitrospira is between 7.4-8.2
Nitrosomonas growth is inhibited at a pH of 6.5. All nitrification is inhibited if the pH drops to 6.0 or less. Care must be taken to monitor ammonia if the pH begins to drop close to 6.5. At this pH almost all of the ammonia present in the water will be in the mildly toxic, ionized NH3+ state.

Dissolved Oxygen

Maximum nitrification rates will exist if dissolved oxygen (DO) levels exceed 80% saturation. Nitrification will not occur if DO concentrations drop to 2.0 mg/l (ppm) or less. Nitrospira is more strongly affected by low DO than NITROSOMONAS.