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15 August 2014 Comments::DISQUS_COMMENTS



William A. Tomey, M. Sc. Bid
Consultant N. M. C. P. The Netherlands
Ornamental and Food Fish Breeding,
Shrimp Aquaculture
Exploration of natural aquatic resources
Restoration of aquatic environment

Introduction :

The balance of any aquatic biological system is highly dependent on the decomposition and the utilization of accumulated waste materials. A perfect biological balance and recycling process as in the nature may not be expected, at least not as long as an aquarium is supposed to be a pleasant sight or commercially used. One cannot expect a healthy captive aquatic environment based on a biological sound system and keep this going if we do not control and interfere. This implies that specific measures must be taken for the removal and decomposition of waste materials. Thus we have to remove a considerable part of these waste matters on a regular basis or continuously from the most overpopulated aquarium or raising ponds and at the same time supporting the work of the organisms which decompose these waste substances in the aquatic system. Due to their metabolism all organisms excrete wastes. As a rule vertebrates, thus fish also produce metabolic wastes which are chemically more complex than those of invertebrates. Most marine organisms excrete ammonia (NH3). These waste matters do not originate from animals only, they may also result from dying and decaying algae food substances, etc. Here nitrogenous compounds occur in the form of proteins. Proteins and other complex compounds also accumulate when organisms decay. In natural surroundings the organic substances are decomposed and become inorganic compounds again, which in turn are available for the producers and thus completes the re-cycling process. In this process bacterial action plays the most decisive part, yet all other organisms and their metabolism are involved in it and they thus have an important role in this biological cycle. Nitrogen is one of the elements causing most of the problems and troubles in aquariums.

An aquarium is a small and closed biological system in which the population density of organisms is considerably higher than in natural surroundings. Therefore the degree of pollution in such a system is excessively high. During the process of decomposition of the waste materials a number of intermediate and by-products result, some of which are highly toxic. Accumulation of these matters in the aquatic environment usually result in negative consequences for many of the organisms in the aquarium. This is why it is necessary to clean the water with some kind of filtering device. Which filtering system to choose is of course guided by the number and kinds of organisms that are kept in the aquarium. Some aquatic organisms require much better water quality than others. An aquarium densely populated with fish as is common in commercial installations demands a more effective filtering system than an aquarium with only a few fish and some invertebrates.

Bacteria And Their Environment

Oxygen is indispensable for most living beings, yet it is not available everywhere. A milieu in which oxygen is sufficiently available is called aerobic. If there is no oxygen available it is called anaerobic. In natural surroundings as well as in the aquarium aerobic and anaerobic zones exist. Usually most of the water in the column moving freely about, is nearly saturated with oxygen. In the coral sand of the seabed or especially in live rocks, the conditions are quite different. If a live rock is broken to pieces, one can often see that the stone which is pale outside is black in the middle. With some small differences about the same conditions may exist in bottom soils in natural freshwater recourses. The black colour results from precipitations of sulphur, an unmistakable sign of an anaerobic milieu. The oxygen contents in porous material as live rocks tends to decrease continuously from the outside to its inside. Aerobic species of bacteria need oxygen for their action and to stay alive, while anaerobic species thrive in environments being low in oxygen. These two types of bacteria show a complete different action in the decomposition processes, which will be dealt with later. Moreover, there are also facultative (= optionally) anaerobic bacteria being able to adapt to a changing oxygen levels in their surroundings. A further distinction is made between heterotrophic and autotrophic species of bacteria. Heterotrophic bacteria cover their needs of energy by consuming various organic compounds, while autotrophic organisms either obtain energy from the oxidation of inorganic substances (chemo-autotrophy, e. g. nitrifying bacteria) or make use of light energy via photosynthesis, without setting free excess oxygen (photo-autotrophic bacteria). Usually in aquaria many different species of bacteria are present.

The Decomposition Of Nitrogenous Compounds

The decomposition process of nitrogenous compounds is divided in three main stages :




1) Mineralisation

During this stage of decomposition amino acids (constituents of proteins) are reduced to inorganic compounds. Organic compounds differ from inorganic ones by containing carbon. This carbon derived from the amino acids is consumed by the heterotrophic bacteria, usually of the genus Bacterium, reducing thus organic compounds to inorganic ones. The products derived of this mineralisation process are ammonia and organic acids. These acids have a dangerous influence on the buffer capacity of the aquarium water and may cause the pH moving down in the project of the scale. Ammonia (NH3) is not only introduced into the water by bacteria. It is the most common metabolic waste from all other organisms and depending on the pH, it always stands in a certain relation to the amount of ammonia (NH4). At a pH of 8.3 the relation ammonia:ammonium is about 1:9. The balance can be shown in the following formula :

NH4 + H2O - NH3 + H3O4

Ammonium in its free form is toxic, yet it is an important source of nitrogen for the growth of plant and of course algae. Usually ammonia and ammonium are oxidized to nitrite and nitrate before long.

2) Nitrification

The next stage of decomposition exist in the oxidation of ammonia and ammonium to nitrite (NO2) and nitrate (NO3). These reactions are mainly carried out by two groups of bacteria. Up to this point scientific research suggests that species of the genus Nitrosomonas oxidize ammonia or ammonium to nitrite, while Nitrobacter oxidize nitrite into nitrate (see formula below)

2NH4+ + 2OH- + 3O22NO22 + O2 Nitrobacter

Nitrosomonas 2NO3 -2H+ + 2NO2- + 4H2O

Certainly there are also other organisms and bacteria involved, particularly in sandy substrates, porous materials and live rocks being the habitat of countless species. Heterotrophic bacteria also take part in these processes (Johnson & Sieburth, 1976; Tate, 1977). It has been shown that heterotrophic bacteria are able to convert amino acids directly into nitrite and nitrate, without going indirectly via the ammonia / ammonium pathway (Qastelet al., 1950; Jensen & Gundersen, 1955; Doxtader & Alexander, 1966). In any aquarium always a certain amount of nitrifying bacteria exist, no matter the method of filtration in use. The effectiveness of the action of these bacteria depends on a number of factors :

- the degree of pollution of the water with toxic substances.

- temperature, pH and oxygen content of the water.

- the colonization surface available for the bacteria.

Toxic compounds that pollute the water may be subdivided into two categories :

a) Substances introduced into the aquarium from outside.

b) Substances produced by the metabolism of the organisms in the aquatic environment of the aquarium.

As we have seen the toxic metabolic products, i.e. compounds produced by the Metabolism of the inhabitants of the aquarium. Subsequently include ammonia, ammonium, nitrite and nitrate, in that order. If the concentration of nitrite in the aquarium rises significantly, this may inhibit the nitrification process. More or less toxic substances which have been introduced into the aquarium (e.g. substances that are added to treat fish diseases) may also disturb the nitrification processes considerably. Within circles of fish breeders it is common knowledge that Methylene blue being a component of many fish medicines, kills most of the nitrifying bacteria. This applies also equally to all treatments based on aniline pigments. Yet copper sulphate used in sea water, being known as extremely toxic for invertebrates, does not seem to inhibit nitrification very much.

Temperature, pH and Oxygen Content

The process of effective nitrification depends on the temperature, pH and Oxygen level in the water. Usually its action is more effective the higher the temperature gets. (upto a maximum of 30C). It is similar with pH. In sea water the process of Nitrification works best at pH 9 and in fresh water at pH 7.5. Yet in a marine aquarium only readings between 8.0 and 8.5 are acceptable. With regard to the oxygen concentration, it has to be considered that nitrifying bacteria are mainly aerobic; yet possibly not as strictly as often supposed (Gundersen, 1966).

Bacterial substrate colonization.

Suitable substrates on which bacteria can settle are not of utmost importance for the further development and colonization. In the open body of water there is no way for nitrification. Prior to be effectively decomposed the substances dissolved in water must be absorbed preferably by a large surface covered with detritus. It is thus conditional that a certain amount of detritus is being present in the aquarium and in the filter for effective bacterial decomposition. Bacteria form a thin, slimy film on the substrates they settle. This is where the action of decomposition takes place. Such bacterial coatings are exposed to continual changes and interactions, as it grows permanently, dies and decompose at the same time. Therefore changes in the thickness of the layer occur conform and according to the density of the bacterial population (see. Diagram).

Toxic effects of nitrite on the population.

The Nitrification process produces nitrite as an intermediate and nitrate as the final product. Both compounds are toxic to fish and others live organisms. Nitrite has a toxic effect because it oxidizes haemoglobin into methaemoglobin. Haemoglobin, is the fluid in blood responsible for the intake and transport of oxygen, while methaemoglobin lacks this ability. Nitrite poisoning in fish causes respiratory problems with an unusually high rate of gill movements. Many invertebrates react to nitrite by contracting their bodies heavily; anemones turn their intestinal cavity inside out through their mouth. So far there is no satisfactory explanation why invertebrates react that way. Snails in fresh water close their operculum and remain motionless. Nitrite concentrations exceeding 0.05 mg/l are not acceptable in a marine aquarium. Fresh water fish reacts less sensitive. Some authors writing on aquaristic subjects often regard nitrite as rather harmless probably based on their experience with soft and leather corals, which react indeed much more resistant. This certainly does not hold true for a coral reef aquarium populated with hard stony coral species. High concentrations of nitrite (1000-2000 mg/l) inhibits the development of the tissues of many organisms. Yet delicate corals especially stony corals usually stop thriving already at concentrations which are much lower. Nitrate levels being the follow up nitrite that exceed 10 to 20 mg/l may lead to problems, especially when it comes to the control of filamentous algae. In fresh water fish the presence of such nitrate levels will impede a normal growth considerably. In a marine aquarium representing a well established healthy environment the risk of accumulation of dangerous nitrate levels is negligible. The concentrations of nitrite and nitrate can be tested with electronic devices, or specific test kits available from several producers. These kits are easy to use and they are accurate enough for aquaristic purposes.

3. Dissimilation

In the last stage of decomposition nitrate is reduced. The action of many different types of bacteria is involved in the process of dissimilation from which the following end products may result :

- Dinitrogenoxide (N2O) [a.k.a. nitrous oxide or laughing gas)

- Nitrogen (N2)

- Ammonia/Ammonium (NH3/NH4+)

- Nitrite (NO2)

Dinitrogenoxide and nitrogen are gases. If the decomposition process results in these substances, it is called denitrification. During this process nitrogen escapes from the aquarium into the air. If ammonia / ammonium are the end products of the process, nitrate reduction takes place. In this case the ammonia/ammonium levels may increase so enormously that it can have harmful effects. Fortunately such nitrate reduction only rarely occurs in an aquarium. Here the environmental conditions are of vital importance for dissimilation as well. Temperature and pH influence dissimilation in the same way as they influence nitrification. This is completely different, however, with oxygen. The bacteria involved in dissimilation are anaerobic, or optionally anaerobic. This is why the presence of oxygen inhibits dissimilation. Subsequently, this reaction occurs in an aquarium only in places where oxygen deficiency prevails e.g. inside porous materials as live rocks, in parts of the aquarium bottom substrate or in special denitrification filters in which oxygen cannot penetrate. Some experts hold the opinion, however, that dissimilation can also take place under aerobic conditions. Bacteria living in an anaerobic milieu produce enzymes which reduce nitrate ions, making use of the oxygen from the nitrate respiration. Enzyme production, however, stops as soon as aerobic conditions are established. As a consequence the nitrate concentration in the aquarium rises. Most of the bacteria involved in dissimilation are heterotrophic. So they need organic nutrients which must be available in dissolved carbon compounds. As the different kinds of bacteria take in different nutrients the bacteria populations vary according to the food substances available. In a denitrification filter lactose is often dosed as a food supplement to enhance and stimulate the action of the bacteria. The denitrification process in the aquarium has so far not been the subject of intense research. We obviously still have a lot to learn about this subject. From experience we know that in aquaria decorated with live rocks in combination with protein skimming and sufficient water motion, nitrate accumulation usually does not occur. In aquaria without skimming, nitrate, however, accumulates easily to unacceptable levels. (Arkinson et al (1995)). In the process of decomposition a number of by-products arise. Mineralisation may produce colour substances as derivates of Phenol or Indole, which may give the water a yellow hue. This tinge can only be removed effectively by filtering the water through activated carbon or by using ozone.