Over the years, environmental impact studies and scientific research have provided and ever-expanding body of good, solid information regarding the importance of Nitrogen Cycle in nature. As a result, aquarists have become better informed of the crucial maturation process that every new aquarium has to go through: the so-called Nitrite Crisis.
The Nitrogen Cycle embodies many complex chemical and biological reactions and processes that maintain a stable level of nitrogen gas (N2) in the atmosphere at around 78%. Part of the cycle involves the action of certain groups of bacteria which convert nitrogen-containing waste products, such as ammonia and nitrites, to other, safer, substances, while other types of bacteria 're-convert' these compounds into nitrogen which is eventually released back into the atmosphere where it originally came from.
Types of Bacteria
Bacteria can be classified in several ways, e.g. depending on shape. They can also be segregated into two main groups if we use their method of obtaining nourishment as the main criterion. When employing this approach, we end up with two groups:
- Autotrophic Bacteria: Autotrophs are organisms, such as plants containing chlorophyll (the pigment that makes many plant species green), which can synthesise their own food from their basic constituents. This synthesis can be chemically stimulated/mediated and carried out in the absence of light (chemosynthesis), or may necessitate the involvement of sunlight (photosynthesis). Among the autotrophic bacteria, there are both photosynthetic and chemosynthetic types. Autotrophic bacteria do not play any role that we can identify in aquarium keeping.
- Heterotrophic Bacteria: Heterotrophs cannot synthesise their food as autotrophs can. Instead, they need to 'feed' on an already-existing source, breaking it down into simpler components and then re-assembling these into different, more complex, compounds. From the aquarium point of view, four kinds of heterotrophic bacteria are of greater importance than any others:
- Several types of hardy, fast-reproducing heterotrophic bacteria are capable of quickly breaking down complex organic waste produced by fish and other organisms. In their turn, these bacteria produce considerable amounts of toxic ammonia.
- One particular genus of heterotrophic bacteria is well-known to most aquarists, whether freshwater or marine: Nitrosomonas. Bacteria belonging to this genus (which are less hardy than the above mentioned heterotrophs) are capable of metabolising ammonia and converting it into nitrites.
- A second genus, Nitrobacter, is less hardy still. These bacteria convert nitrites (themselves toxic) to less dangerous nitrates (their own waste product). Nitrosomonas and Nitrobacter are referred to as nitrifying bacteria because of their nitrogen-binding capabilities.
- A further group of heterotrophic bacteria have been receiving growing attention in recent years. These are the de-nitrifying types (Pseudomonas sp) which will – in some ways – work in reverse to the nitrifying ones, eventually releasing nitrogen, via intermediate, less complex, compounds such as de-nitrogen gas (N2).
The Nitrite CrisesWhen fish are first introduced into an aquarium, they (inevitably) bring in with them, at least, a small quantity of Nitrosomonas bacteria. Left to their own device, these bacteria will take three, four or more weeks before they can establish themselves in sufficient numbers to maintain the ammonia concentration within safe limits.
During this initial period, the ammonia level can climb as high as 10ppm (or even higher). Such a 'dose' will be sufficient to kill all but the very hardiest fish. Clearly, this period presents a critical threat to the newly set up aquarium.
Nitrosomonas bacteria derive their nourishment, not just from ammonia, but also from carbon dioxide, releasing nitrites in the process, and doubling their population every 24-36 hours. If allowed to proceed without check, the nitrite level in the aquarium will, obviously, rise steeply. This is where the Nitrobacter bacteria come in. Their growth is inhibited by high levels of ammonia, so, as these begin to drop as a result of Nitrosomonas metabolism, the activity of Nitrobacter bacteria begins to pick up. In other words, Nitrobacter bacteria 'must wait' until the Nitrosomonas population is sufficiently high to cause a significant drop in ammonia concentration. During this period, the levels of nitrites may rise to 20ppm or higher. Fortunately, high nitrite levels are not as toxic as ammonia and, as long as a healthy growth of Nitrosomonas can be established quickly, this, in turn, will stimulate Nitrobacter into growth and reproduction.
There are several ways of promoting such a situation, one is via the introduction of few, very hardy species, another consists of the use of commercial preparations designed to encourage the growth of nitrifying bacteria. 'Healthy' biologically active gravel/coral sand from an established aquarium may also prove helpful.
Whatever technique is used, it should result in a sharp drop in nitrite levels, with a corresponding (but gentler) increase in relatively safe nitrates. The point at which the rising curve for nitrates crosses the declining one for nitrites is regarded as the 'official' end of the Nitrite Crisis.