THE BACTERIA OF THE NITROGEN CYCLE AND THEIR IMPORTANCE IN THE AQUARIUM
The bacteria of the nitrogen cycle are those strains of bacteria which carry out the degradation of the nitrogenous organic substance; transforming the latter into inorganic nitrogen compounds.
This organic matter is attacked and degraded into increasingly simpler compounds by a long series of organisms; among which, in addition to fungi, protozoa, nematodes and other animals, a significant part is represented by various bacterial strains.
These can be divided into:
- Heterotrophic bacteria
- Autotrophic bacteria
Starting from organic nitrogen, their degradation work and the substances resulting from this degradation can be represented as in the image below:
And, more in details:
HETEROTROPHIC BACTERIA
Heterotrophic bacteria constitute one of the first steps in the degradation of the organic nitrogenous substance, transforming it into intermediate organic compounds (such as proteins and amino acids) which are gradually simpler, until arriving at an inorganic nitrogenous compound: ammonium/ammonia *
* It makes sense to talk about ammonium/ammonia and not just one of the two compounds, because they are in mutual equilibrium and do not exist individually in water.
For our purposes, heterotrophic bacteria do not have particular needs, they are ubiquitous (they are found everywhere), they colonize our tanks without any intervention being necessary on our part, they reproduce very quickly (even every 20 minutes!) and feed thanks to the organic substance that is inevitably present in our tanks (sediment, residual food, fish droppings, dead leaves, etc.).
Their initial and necessary establishment and development in the tank therefore does not pose any problems and does not require our specific intervention.
Rather we will generally have to take care of the fact that they do not develop in an uncontrolled manner, carrying out the due and regular cleaning operations, removing the residues and dirt accumulated in the tank (bottom, filter, etc.) and changing the water.
This is essentially to avoid:
- An excessive production of organic nitrogenous by-products (such as amino acids), which can lead to the development of very harmful algae (such as black brush algae of the Audouinella genus)
- The development of bacteria and pathogenic microorganisms (bacteria, but also fungi and protozoa).
- Their excessive consumption of oxygen dissolved in water
- A too high production of Ammonium/Ammonia (a harmful compound in the tank beyond very small concentrations).
AUTOTROPHIC BACTERIA
Autotrophic bacteria, on the other hand, constitute the second phase of the degradation of nitrogenous compounds, operating only on inorganic nitrogenous compounds and differentiating themselves into two macro-groups:
- Ammonia Oxidizing Bacteria (AOB): converting ammonium/ammonia into Nitrite
- Nitrite Oxidizing Bacteria (NOB): converting Nitrite into Nitrate
AOB bacteria carry out the reaction: NH3 + 1.5 O2 => HNO2 + H2O E= -234 KJ/mole (energetic gain)
So, they transform Ammonium/Ammonia into Nitrite, obtaining energy from it
NOB bacteria carry out the reaction: NO2- + 0.5 O2 => NO3- E = - 54 KJ/mole (energetic gain)
So, they transform Nitrite into Nitrate, obtaining energy from it
For simplicity however, from now on, we will refer to both these bacterial families with the term: "nitrifying bacteria".
It should be noted that the degradation/transformation of the aforementioned inorganic nitrogenous substances serves to obtain the energy necessary to fix the Carbon (CO2) and for the synthesis of the other organic compounds necessary for the life and proliferation of bacteria.
The autotrophic bacteria of the nitrogen cycle therefore play a fundamental role in the ecosystem of the aquarium, as:
- They detoxify the toxic compounds Ammonium/Ammonia and Nitrites, transforming them into a much less toxic compound (Nitrate).
- they stabilize the tank from a microbiological point of view, greatly reducing the development of other harmful/pathogenic bacterial strains and the probability of uncontrolled algal bloom
It is therefore extremely important for the correct maintenance of the aquarium's state of health, to try to maintain its population of nitrifying bacteria in the best possible way, knowing their characteristics and vital needs well and thus supplying them with what is necessary.
This is even more necessary in consideration of the fact that nitrosating/nitrifying bacteria are not ubiquitous in the environment, they reproduce much more slowly than heterotrophic bacteria (approximately every 12 hours) and have specific needs, which mean that, unlike the heterotrophic bacteria, their settlement and their proliferation in the tank are constantly at risk.
So, let's see what the characteristics and needs of these bacteria are and what we need to worry about and take care of, on our part, to allow these bacteria to carry out their very important job in our tanks.
Characteristics: 
They are ovoid-shaped bacteria between about 0.6 and about 2-3 microns.
One of their unique characteristics is that they live inside a so-called "Biofilm".
That is, unlike heterotrophic bacteria, they are sessile bacteria, which live inside a mucous matrix (the biofilm), generally between 200 and 500 µm thick.
This biofilm, produced by the bacteria themselves, serves to keep the bacterial colonies immobilized against a surface present in an area where the environmental conditions exist (we will see shortly which ones) for their proliferation.
The biofilm must be thick enough to protect them, but thin enough to allow oxygen, bicarbonates and nutrients to enter and waste substances (mainly H+ ions and nitrate) to escape.
Similarly, the biofilm must reside in an area that is constantly subjected to good water recirculation, which supplies oxygen and nutrients and washes away the waste substances produced.
AOB abd NOB nitrifying bacteria have very specific vital needs, which it will be good to try to satisfy in our tanks. In particular, concerning:
pH
The ideal pH is around 7.6-7.8, and, for decreasing pH, their metabolic activity decreases linearly until it stops below pH around 6.0.
Since in any case, the pH in our tanks must be set according to other considerations and needs, we can say in this regard that we simply have to worry about not lowering the pH excessively and unnecessarily.
Or where there is no absolute need for special needs
Temperature
The ideal temperature is around 35 degrees.
But what has been said for the pH also applies to the temperature; having to fix it according to the animal and plant species bred/cultivated.
Oxygen
AOB and NOB bacteria require very large quantities of dissolved oxygen.
For every gram of ammonia oxidized, 4.6 grams of oxygen is consumed.
Oxygen which must be supplied through an excellent water recirculation on the surface of the bacterial biofilm
Alcalinity
AOB and NOB bacteria produce Nitrous/Nitric acid as the last metabolite, which lowers the pH in the bacterial biofilm.
To avoid this and its consequences (green above, in the paragraph relating to optimal pH), the acid produced must be promptly removed from the bacterial biofilm (by constant water flow) and/or must be compensated by a good quantity of bicarbonates (always provided by a constant flow of water).
Inorganic Carbon - CO2
Like all living beings, nitrosating/nitrifying bacteria also need carbon; but, unlike heterotrophic bacteria, they need inorganic carbon.
These bacteria then obtain the carbon they need to grow and reproduce from the CO2 dissolved in water.
Even about 80% of the energy obtained from nitrosation / nitrification is used to fix the dissolved CO2 and obtain the necessary carbon.
This CO2 must also be supplied by a constant flow of water through the bacterial colonies.
Iron and Trace elements
They are both necessary and to be supplied in small amounts
Phosphorus
It is necessary and to be supplied in small amounts
IDEAL SUBSTRATE (FILTER MATERIAL)
Having said this, and remembering that nitrifying bacteria are sessile bacteria that live inside a biofilm they produce, we must therefore fully understand which is the best environment/substrate suitable for attacking their biofilm and for the growth and maintenance of bacterial colonies .
This must guarantee some fundamental parameters; and precisely:
Total surface
Obviously, it must be as large as possible (larger surface area available = greater possibility of biofilm development
Esposure to water flow
The surface available to the bacteria must also be CONSTANTLY, uniformly and correctly wet, or uniformly subjected to a constant flow of water, capable of supplying oxygen and nutrients and washing away the waste substances generated.
Furthermore, the correct flow of water constantly exfoliates the colony, allowing it to be optimally maintained.
Non-blockability
The surface area available to the bacteria must be such as to allow the surface of the bacterial biofilm, which acts as an exchange surface, to ALWAYS be kept free from dust/debris, etc. Thus avoiding the development of heterotrophic bacteria which would damage the work and life itself of the autotrophic bacteria contained in the biofilm.
This is done both by directly consuming the dissolved Oxygen necessary for the autotrophic bacteria, and by obstructing the surface of the biofilm and preventing the exchanges of incoming oxygen, nourishment and bicarbonates, as well as outgoing waste substances.
Filtering materials with areas in which the flow stagnates, with the consequent possible accumulation of surface particles/debris, must therefore be avoided.
These considerations relating to the accommodation surface have meant that, for many years now, a whole series of artificial substrates have been developed to allow the massive and optimal settlement of colonies of nitrifying bacteria.
These artificial substrates are also well known in the aquarium hobby and even more so in aquaculture and wastewater treatment plants, which need high bacterial filtration efficiency
The substrate/filter material must consist of through holes, in which the water can flow freely over the surface.
There must therefore be no blind cavities, which make it difficult for the flow/recirculation of water inside.
Furthermore, the holes must be large enough to allow the growth of the biofilm on all its internal walls up to its optimal thickness (200 – 500 micron => 0.2 – 0.5 mm) without this damaging or impeding the flow of water through the filter material.
On the basis of these considerations it is clear that the best substrate (filtering material) for the aerobic biological filtration of water is obviously made up of a material with controlled and constant porosity, with passing pores with a diameter of a few millimeters.
Inevitably this will have to be a specially designed and produced synthetic material, since porous natural materials (such as pumice) are not very suitable for purely aerobic filtration, due to their conformation (often non-passing pores, too small and inhomogeneous diameter, etc.) .
Therefore, in alxyon, in addition to pumice-based materials (such as Pro-Filter BR1), which can be used for mixed aerobic-anaerobic filtration, we have designed and created Pro-Filter BF1: a product specifically optimized for purely aerobic water filtration.
Pro-Filter BF1 consists of a foam of a non-toxic plastic material that is perfectly water resistant, with totally open pores and loops with a controlled and uniform diameter of about 2 mm.
Pro-Filter BF1 is cut into 15 mm cubes to guarantee the best occupation of the filter volume in normal domestic aquaria and therefore the highest available surface area for bacterial colonies.
It is also opaque black, so as not to transmit light, which is harmful to nitrifying bacterial colonies.
This allows Pro-Filter BF1 to provide a very large surface area (over 1400 m2/m3) that can be fully exploited by nitrifying bacterial colonies as it is totally exposed to the flow of water, thus offering the ideal environment for the engraftment of nitrifying bacterial colonies mainly aerobic.
The aforementioned structural characteristics guarantee total exposure of the surface of BF1 to the flow of water and its homogeneous distribution within the structure of BF1; managing to guarantee the optimal supply of oxygen, nutrients and bicarbonates and the removal of the acid and waste substances produced by the bacteria.
It therefore does not get clogged (if normally/correctly maintained) with the growth of bacterial colonies and allows the optimal flow for the correct maintenance and regeneration of the biofilm, which is constantly exfoliated on the surface and, therefore, correctly reciprocated/regenerated.
All this makes alxyon Pro-Filter BF1 the most efficient aerobic filter media currently available.