Fish Tanks and Ponds

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Nitrifying bacteria

Odessa barb, Pethia (Puntius) padamya
Odessa barb, Pethia (Puntius) padamya

The bacteria

There are several species of bacteria thought to be responsible for oxidising ammonia to relatively safe nitrate. But primarily it is thought to be nitrosomonas marina, biospira, nitrospira and nitrobacter which are the dominant species found in aquarium filters, other research may point to other species but I don't want to be to pedantic over a name.
It is the nitrosomonas marina, biospira which oxidise ammonia in to nitrite and the nitrobacter, nitrospira which oxidise nitrite in to the less toxic nitrate. All species derive their energy from this oxidation which means they are classed as obligate chemolithotrophs which simply means they derive their energy from inorganic salts rather than an organic source which they cannot use.
Nitrifying bacteria are gram negative, aerobic, rod shaped bacteria approx 0.4 to 6 microns in length. They cannot form spores and have quite a narrow range of environmental factors at which they will thrive.

True nitrifyers V's heterotrophic bacteria

True nitrifying bacteria belong to the family Nitrobacteraceae. Bacteria from this family cannot form spores, they can't be dried out or frozen and have to be kept alive in an oxygen rich environment. So I hear you ask "how do manufactures manage to sell bottled bacteria for cycling tanks?" The answer is they mislead. There is another group of bacteria called heterotrophic "nitrifyers" which are capable of oxidising ammonia to nitrite. However they prefer to obtain their nitrogen from an organic source.

Heterotrophic bacteria cannot convert nitrite to nitrate at all but a few species can convert ammonia to free nitrogen under anaerobic conditions. Studies have shown that it takes at least 1,000 to 1,000,000 heterotrophic bacteria depending on the environment and species to convert as much ammonia as it takes just one single cell of a Nitrobacteraceae, this is over come by the manufacturers through sheer volume of their products each of which contains billions of bacteria.

Finally heterotrophic bacteria can work in reverse. If the oxygen level falls they can utilise the oxygen in nitrate (NO3) and convert it either to nitrite or even ammonia.

Environment

pH: The optimum pH for nitrifying bacteria is between 7.3 to 8 All species have a wider tolerance outside those levels but will become increasingly inhibited the further outside these levels the pH is.
Somewhere between pH 5 to 5.5 ammonia will not be oxidised to nitrite leading to a potentially lethal build up of ammonia if no action is taken to prevent this. If the pH falls below 6.7 there is a significant reduction in nitrification.
The same thing will happen if the pH rises above 10 but this is less important to aquarists since this rarely occurs in the aquarium.

Free ammonia and ammonium:

Ammonia is far more toxic than ammonium the pH and temperature will determine the percentage of ammonia to ammonium found in the aquatic environment.

At 24C:
pH 7 - 0.5% of total ammonia is NH3
pH 8 - 5% of total ammonia is NH3
pH 8.2 - 7.7% of total ammonia is NH3
pH 8.4 - 11.6% of total ammonia is NH3
pH 8.6 - 17.3% of total ammonia is NH3

Free ammonia (NH3) will inhibit nitrification at the following levels: Nitrosomonas marina/biospira 10 mg/ and for nitrobacter/nitrospira just 0.1 mg/l. This has implications if you are trying to cycle a marine tank or a freshwater tank with a high pH for African cichlids for example and using the standard internet recommendation of 4 mg/l of ammonia.
Because the free ammonia level will be approx 0.4 mg/l and this substantially inhibit the rate at which nitrite is converted to nitrate. Because of this it would be better to add the same quantity of ammonia but in several small doses rather than one large dose.
In a healthy well established aquarium ammonia levels should be undetectable using a standard aquarium test kit at all times.

Oxygen

All nitrifying bacteria need a well oxygenated environment or nitrification will cease. Ideally the oxygen level needs to be above 3 mg/l (most fish need a dissolved oxygen level of 6 or 7 mg/l). If the environment is totally devoid of oxygen the bacteria will eventually die. The time an aquarium filter can be left off varies with temp. At cooler temps the metabolism of the bacteria slows down and so they will last longer. It also depends on the dissolved oxygen level of the water when the filter was turned off and whether the filters taps were closed or left open. Nitrifying bacteria will all die in the absence of dissolved oxygen after 24 hrs but it will take several hours to reach this point (longer at cooler temps) so if a filter has been turned off for a day or so it is worth restarting it and letting the remaining bacteria recover. Even if 90% have died filter bacteria will double their numbers every 24 hrs. Slow by bacteria standards but even with just 5% left the filter would recover in just 6 days.

Temperature:

Temperature plays an important role in the nitrification process. The optimum temp range for bacterial growth is around 30C depending on the scientific source used. Temperatures above and below this will inhibit nitrification to varying degrees.

Other important temperature to note are as follows:
1, 18C growth rate decreases by 50%
2, 9C growth rate decreases by 75%
3, 4C no growth occurs.
4, 0C and 49C nitrifying bacteria die

You will notice that at 18C the metabolism of the bacteria is halved. This is the likely temp of many cold water aquariums may drop to at night if they are not heated in some way. It is one of the main reasons why cold water aquaria should only be stocked at half the recommended level of their tropical counterparts.

Other factors: Nitrate:

It is often stated that high nitrate levels will inhibit nitrification, I have been trying to find some scientific data to verify this but have been unable to. BUT high nitrates will mean lower dissolved oxygen levels because water has a finite capacity to dissolve things and more nitrate will invariably mean less oxygen. High nitrate quite often means the water is more acidic and both things together could in theory inhibit nitrification if the nitrate was high enough.

Light:

Most aquarium lighting will have little or no effect on nitrification, marine tank lighting with a high uv such as metal halides or a uv sterilizer will have an impact on a maturing system and should be kept off for the first few days. UV kills bacteria and although nitrifying bacteria usually grow on a solid surface they can become mobile when colonising*new areas.

Interesting statistic:

For every gram of ammonia oxidised into nitrate 4.8 grams of oxygen is used, 7.14 grams of calcium carbonate is used (now you know why pH crash can occur in tanks with to little buffering capacity).

Cycling an aquarium

To achieve the fastest possible results provide the bacteria with an ideal environment: Temp 30C, pH around 7.8 and well oxygenated water. Use an inoculation culture (some media from a mature filter) and provide a source of ammonia at a low level.
These conditions won't suit your fish but changes can easily be made at the end of the cycle when a large water change is made before adding any fish. A cycle carried out this way should take no more than a few days to complete.

Nitrifying bacteria and medication

Nitrifying bacteria are gram negative rod bacteria as are most pathogenic bacteria. They are easily killed with antibiotics and will be inhibited by other commonly used aquarium medications such as methylene blue, malachite green, formalin, potassium permanganate, acriflavine, copper (above 2mg/l) and chlorine.

Aquarium Bacteria

Invisible to the naked eye yet so vitally important for the well being of your aquarium. What am I talking about? Filter bacteria. Without these bacteria your fish would die within a mater of days through ammonia poisoning. So it is essential that you look after these bacteria properly in order for them to thrive and in turn for your aquarium to thrive. Nitrifying bacteria don't have many needs but they can easily be inhibited from performing at their best simply through keeping them in the wrong environment.

References

Fritz industries
Enva water treatment
Iowa state university
Langston university (aquaculture)
WPC (nitrification and denitrification)