Ammonia (\(\text{NH}_3\)) is a compound made of nitrogen and hydrogen that, along with its ionized form, ammonium (\(\text{NH}_4^+\)), poses a significant threat to aquatic ecosystems. In water, an equilibrium exists between the toxic un-ionized ammonia (\(\text{NH}_3\)) and the less harmful ionized ammonium (\(\text{NH}_4^+\)). The presence of high levels of \(\text{NH}_3\) is particularly harmful to aquatic life, as it can damage gills and internal tissues, leading to stress, poor growth, and potential death. Natural removal methods primarily rely on biological processes to convert this toxic compound into less harmful substances or to remove it from the water entirely.
Harnessing Beneficial Bacteria
The most established natural method for ammonia removal involves biological filtration, a process carried out by specific types of microorganisms. This conversion is a two-step process known as nitrification, which transforms ammonia into progressively less harmful compounds. The first step involves ammonia-oxidizing bacteria (AOB), such as Nitrosomonas species, which convert ammonia (\(\text{NH}_3\)) and ammonium (\(\text{NH}_4^+\)) into nitrite (\(\text{NO}_2^-\)).
Nitrite is itself toxic to aquatic life, necessitating the second step of the process. Nitrite-oxidizing bacteria (NOB), notably Nitrobacter species, then rapidly convert the nitrite into nitrate (\(\text{NO}_3^-\)). Nitrate is the least toxic form of nitrogen in this cycle and is readily utilized as a nutrient by plants and algae.
These beneficial bacteria, collectively known as nitrifiers, are autotrophic, meaning they derive their carbon source from inorganic compounds like carbon dioxide and bicarbonate. They require surfaces, often provided by specialized bio-media or porous substrate, to colonize and form a stable population. To perform their function, nitrifying bacteria are obligate aerobes, requiring a sufficient level of dissolved oxygen in the water.
Optimal conditions for this biological conversion include a dissolved oxygen concentration of at least \(2.0\) mg/L, with maximum efficiency occurring around \(3.0\) mg/L. The process is also sensitive to pH, as alkalinity is consumed during the conversion of ammonium to nitrite. If the pH drops below \(6.7\), the efficiency of nitrification significantly decreases, which can lead to a dangerous buildup of toxic compounds.
Direct Ammonia Absorption by Plants
While bacteria convert ammonia through a chemical oxidation process, living plants offer a parallel and direct method of removal called phytoextraction. Aquatic plants absorb nitrogen compounds directly from the water to fuel their growth and metabolism. Plants generally prefer to absorb nitrogen in the form of ammonium (\(\text{NH}_4^+\)) over nitrate (\(\text{NO}_3^-\)) because ammonium is already in a usable form and requires less energy to assimilate.
This preference for ammonium means plants actively compete with nitrifying bacteria for the available nitrogen source, removing it before conversion to nitrite and nitrate. Many aquatic plants, particularly floating varieties like Water Lettuce (Pistia stratiotes) and duckweed, excel at this process. These fast-growing species are effective because they absorb nutrients primarily through their leaves and stems directly from the water column, supported by direct access to atmospheric carbon dioxide.
An important distinction is that the nitrogen is not converted but is stored within the plant’s biomass. Therefore, for the nitrogen to be permanently removed from the aquatic system, the plant material must be physically harvested and taken out of the water. This regular pruning of fast-growing plants effectively removes the stored nitrogen, preventing its release back into the water upon the plant’s natural decay.
Managing Ammonia Sources
Controlling the input of nitrogenous waste is an important preventative strategy that complements active removal methods. In most managed aquatic systems, the primary source of ammonia is the excretion of metabolic waste by aquatic inhabitants, along with the decay of uneaten food and other organic matter. Protein metabolism in fish results in the excretion of ammonia primarily across the gill membranes.
A common practice to reduce ammonia production is to adopt responsible feeding protocols, providing only the amount of food that can be consumed within a few minutes. Uneaten food quickly breaks down, contributing organic matter and subsequent ammonia to the water. Maintaining appropriate stocking levels similarly prevents overcrowding, reducing the total biological load and the amount of excreted waste.
Regular maintenance, such as vacuuming the substrate, helps remove accumulated organic debris like fish waste and decaying plant material before decomposition occurs. Decomposition of this matter through ammonification is a constant source of ammonia. Proactively removing these sources reduces the overall nitrogen load, making it easier for natural biological and botanical processes to control ammonia levels.