Autotrophic vs. Heterotrophic Bacteria
There's a lot of confusion among aquarists about nitrifying bacteria. This is due in large part to the recent emergence of a wide variety of bacterial products claiming to be nitrifiers or nitrifying aids. The confusion results from the plethora of misinformation presented in advertisements and the aquarist’s general lack of knowledge about bacteria. Most of these products actually contain species of heterotrophic bacteria from the genera Bacillus, Pseudomonas, Escherichia, and others.
True nitrifying bacteria are considered to be those belonging to the family NITROBACTERACEAE. These bacteria are strictly aerobic, gram-negative, chemolithic autotrophs. They require oxygen, utilize mostly inorganic (without carbon) compounds as their energy source, and require carbon dioxide (CO2) for their source of carbon. In the case of the Nitrobacteraceae these energy sources are derived from the chemical conversion of ammonia to nitrite, or, nitrite to nitrate.
Five genera are generally accepted as ammonia-oxidizers and four genera as nitrite-oxidizers. Of these, Nitrosomonas (ammonia-oxidizers) and Nitrobacter (nitrite-oxidizers) are the most important. Species of marine nitrifiers are different from those that prefer fresh water, and yet, are very closely related. Each species has a limited optimum range for survival. They are very slow growing because of the manner in which they must obtain energy. Under optimal growth conditions, they will double in population every 15-24 hours. Heterotrophic bacteria, on the other hand, can reproduce in as little as 15 minutes to 1 hour.
Where does ammonia in the aquarium come from? Some of it is released directly by the fish; by diffusion from the blood across the gill membranes. Excreted urea or uric acid is also converted to ammonia through a process called mineralization. Solid organic, nitrogenous, waste material (a.k.a.- sludge) is also converted to ammonia through mineralization. Sources of this waste material are from fecal material, the decay of plant and animal tissues, and from the decay of excess food. Mineralization is accomplished by any of a number of species of heterotrophic bacteria. Species from the genus Bacillus are the most common.
Ammonia is the primary compound produced by this process. Some species of heterotrophic bacteria can oxidize or reduce nitrogenous compounds directly to nitrites (NO2), nitrate (NO3), or other forms of nitrogen (as NO or N2). In the absence of an organic nitrogen source, many heterotrophs can utilize ammonia instead. This is much more likely to happen in the laboratory, under ideal conditions, than in actual practice. In the aquarium, as in nature, an organic, nitrogen rich, food source is constantly being produced and is readily available for these bacteria to utilize. Heterotrophic bacteria have little or no need to resort to utilizing ammonia as their source of nitrogen.
This ability of heterotrophic bacteria to utilize ammonia has led to the erroneous belief that they are as effective as true nitrifying bacteria in establishing the nitrogen cycle. These bacteria, however, generally cannot utilize nitrites. Experimental data has shown that up to one million times more (103 - 106) of these heterotrophic ‘nitrifers’ are required to perform a comparable level of ammonia conversion that is attained by true autotrophic nitrifiers. When using heterotrophic ‘nitrifiers’, the nitrogen cycle in the aquarium basically follows the same course as when no bacteria are added and the system cycles naturally.
Another negative aspect to heterotrophs is that under certain environmental conditions they can operate in the reverse direction. In other words, they can convert nitrate back to nitrites and ammonia through a process called dissimilation. This is generally an anaerobic process, but, can occur during periods when dissolved oxygen levels are very low (DO £ 2.0 ppm). Dissimilation is a part of the denitrification process. Denitrification is the conversion (reduction) of nitrites and nitrate to gaseous nitrogen (N2, NO, N20).
Heterotrophic bacteria can be either gram-positive (ex: Bacillus) or gram-negative (ex: Pseudomonas). Some are strictly aerobic, but many are facultative anaerobes (they can survive in both the presence or absence of oxygen). Many species tolerate a wide range of environmental conditions: temperature, pH, salinity, etc.
They can also survive adverse conditions by forming spores, increasing the shelf life of the products. Because of this ability, they can be dried, packaged, and sold as viable cultures. Nitrifying bacteria do not form spores and cannot survive drying. This process kills the bacteria leaving no viable bacteria which might inoculate another culture. It has been debated whether or not nitrifying bacteria could survive freeze drying, but, even this is doubtful.
Dry product formulations are by far the most common. These products consist of sludge removing and other nonpathogenic, heterotrophic species of bacteria and their culture media. Formulations may vary to some degree, but, they all perform the same functions. They all contain bacteria species that digest proteins, fats, oils, cellulose, and starch.
These are similar to liquid sludge removers with broader applications. They often contain species of bacteria that function well in fresh, brackish, or salt water. Most liquid type bacterial products available in the pet industry contain little or no true nitrifying bacteria. Instead, they rely on their ability to maintain tolerable levels of ammonia until naturally occurring Nitrosomonas and Nitrobacter bacteria can become established. Both of these liquid and dry products should be considered as biological aids to nitrification.
Some manufacturers recommend the addition of gravel or water from an established aquarium. This is their only source of nitrifying bacteria. An element of risk is involved with this method because the potential exists for introducing disease causing pathogens into the new tank.
Fritz-Zymeâ #7 (freshwater) and #9 (brackish and marine) contain pure cultures of live Nitrosomonas and Nitrobacter bacteria. Fritz-Zymes #7 & #9 give nitrification a tremendous boost by introducing over two million nitrifying bacteria per ounce of product to rapidly accelerate the nitrification process. Ammonia and nitrite levels are quickly and significantly reduced to safe levels. Fish stress induced by high levels of ammonia and nitrites is also reduced and mortalities normally associated with "New Tank Syndrome" are sharply curtailed.Key Points
True nitrifying bacteria are strictly aerobic autotrophs. They can only use nitrogen from inorganic sources such as ammonia and nitrite. Nitrosomonas (ammonia-oxidizers) and Nitrobacter (nitrite-oxidizers) are the most common.
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