Nitrogen Cycle Control in Marine Fish Spawning Systems
Nitrogen-Cycle Control in Marine Fish Spawning Systems
The mechanisms responsible for nitrification in closed systems are perhaps the most important aspects of aquarium keeping. Biological filtration is the most important means of filtration within any body of water-be it an aquarium or roadside puddle. Naturally occurring bacteria will quickly colonize every available surface area within the aquarium creating a thin bio-film. It has been estimated that up to 30% of the total nitrification occurring in closed systems occurs passively on tank walls, within the substrate, on and within filter tubes, etc. The remaining nitrification must occur in a filtration unit specifically designed for such duties.
Fish regularly secrete ammonia (NH3) and carbon dioxide as part of their metabolic activity. That is, ammonia and carbon dioxide are excreted to the surrounding environment as fish respire and secrete waste in the form of liquid urea and feces. Heterotrophic bacteria present within substrate matter, within filter systems and occurring passively on tank walls also contribute to ammonia loads by breaking down organic debris, detritus and excess food present in the system.
Ammonia is the largest threat to marine fishes and proves the most lethal molecule created through the nitrogen cycle. Ammonia exists as two distinct forms that are at equilibrium in water. The relationship between un-ionized or free ammonia (NH3) and ionized or toxic ammonia (NH4J is a function of pH. At higher pH values, such as those found in marine aquariums, a higher ratio of un-ionized (NH3) ammonia is present. Un-ionized ammonia (NH3) is much more harmful to fishes as the molecule is able to pass through the epithelial membrane of the gills. The ionized form of ammonia (NH4+) represents a larger molecule that is prevented from entering the body by its size proving less harmful to fish.
Ammonia is the first step in the nitrification process and is the agent responsible for triggering the cycle's activity. In a sterile environment such as a newly established marine aquarium, fish literally swim in their own urine and must tolerate peaks of ammonia since there are no bacteria present within the system to remove or convert this substance to less toxic by-products.
The first few weeks of establishing a marine system are vital. Regular advice warns that only a few, hardy fish should be added to the system and fed lightly to avoid excess levels of ammonia. As ammonia levels rise due to fish metabolism and uneaten food matter, Nitrosomonas spp. nitrifying bacteria begin colonization and convert the ammonia to nitrite (NO2), the first key part of aerobic nitrification.
As ammonia is converted to nitrite by bacteria, nitrite rises to peak levels, and stimulates a second group of beneficial nitrifying bacteria to colonize. Nitrobacter spp. is triggered to colonize by the presence of nitrite, but its growth is hindered by excess levels of ammonia. This inhibition of growth is the reason nitrite levels peak only after the ammonia spike has fallen. In essence, a see-saw pattern between ammonia and nitrite occurs. Nitrite is still a highly toxic chemical that must be broken down by the process of nitrification. The chemical signal of nitrite is similar to that of chloride salts, which are actively pumped through the gill membranes of fish. The fish is tricked in a sense to pump in toxic chemicals. Nitrite is rendered less toxic at high specific gravity values. At a specific gravity of 1.026 fish are tolerant of slightly higher levels of nitrite.
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