:  The Nitrogen Cycle
The Nitrogen Cycle: Nitrogen Gas (N2(g)), Ammonia (NH3), Nitrite (NO2(aq)), Nitrate (NO3(aq), Protein
The Nitrogen Cycle describes the movement of nitrogen, a nonmetal element, through an ecosystem. Aquariums have their own unique issues with respect to this cycle. Nitrogen gas, ammonia, nitrites, nitrates, and proteins are all part of the story. Ultimately, all organisms must get nitrogen from some source to build their proteins, ann activity central to life on earth. Bacteria and blue-green bacteria play key roles at critical points in the cycle.

Nitrogen gas (N2): Earth's nitrogen reservoir is the nitrogen gas in air, at almost 80% by volume. The only marine organisms that can use nitrogen gas in a chemical reaction are specific types of bacteria. These species of bacteria can convert nitrogen gas to ammonia, a process called nitrogen fixation. Two bacteria that have been studied are the planktonic Azotobacter and the benthic Clostridium. Thus, the aqueous nitrogen gas of seawater is inert but becomes active when it is converted to ammonia (highly poisonous) by bacteria.

Ammonia (NH3): Ammonia produced by nitrogen fixation in an aquarium is usually of negligible quantity compared to that produced by the metabolism of protein by animals and bacteria. Aqueous ammonia gas (NH3 (aq)) is the most toxic and dangerous of all the nitrogen cycle compounds. Most of the ammonia reacts with water to form ammonium ions (NH4 (aq)), rendering it less toxic. Some species of aerobic bacteria, Nitrosomonas, and Nitrosococcus (purple sulfur bacteria) found in marine sediment, are chemoautotrophic and consume ammonia (NH3/NH4+). Their oxidation of ammonia produces nitrite, NO2-. This process is the first step of nitrification, called nitrosification. Ammonia and nitrite levels should be or nearly be 0 in the seawater of an aquarium.

Nitrite (NO2-): Nitrite is metabolized by other nitrifying genera of bacteria: Nitrobacter (alpha proteobacteria), Nitrospina (delta proteobacteria), and Nitrococcus (gamma proteobacteria/purple sulfur bacteria). These aerobic bacteria oxidize the nitrite to nitrate. Nitrates are far less toxic than ammonia or nitrites. The conversion of nitrite (NO2-) to nitrate (NO3-) is the 2nd step of nitrification. While nitrite levels should be 0, nitrate levels in aquarium seawater can safely range from 0 to 10 mg/L for corals, up to 50 mg/L for fish.

Nitrates: (NO3-): The removal of nitrates from sea water is accomplished via uptake by algae or by anaerobic bacterial reduction of the nitrate to nitrogen gas. Under anaerobic conditions, bacteria such as Pseudomonas, Micrococcus, and Paracoccus convert nitrates to nitrogen gas, in a process called denitrification. These bacteria are facultative anaerobes, and can grow in the sand, where water flow is minimal and oxygen levels are low. They participate in the removal of nitrogen from the aquarium system, because the gases they produce from nitrate reduction escape to the atmosphere. Note: one of the products of denitrification is ammonia, which is then consumed by nitrifying bacteria. Also, denitrification can only occur under anaerobic conditions. Nitrates are also used by marine algae, both microscopic and macroscopic, to produce protein. Phytoplankton, coralline algae, Caulerpa, Ulva are only a few of the organisms that rely on nitrates for their source of nitrogen. The amount of nitrate available to these organisms directly determines their growth potential. Algae can be used to biologically control the nitrate levels in a marine aquarium.

Protein: Proteins make up the structural material that binds bodies together. Enzymes, the catalysts of chemical reactions, are also proteins. Excess protein produced by animals is metabolized to carbohydrate and ammonia is released as a waste product.

Inputs to the Aquarium Nitrogen Cycle: Animals digest food protein to make their own protein. They metabolize excess protein to produce other biochemicals, and release the waste product ammonia to the water around them. Foods high in protein, such as frozen Mysis or brine shrimp can add aquarium bioload. Decomposition of excess food by such bacteria as Bacillus adds ammonia to aquarium water. High levels of nitrosifying bacteria are needed to process the excess ammonia.

New Aquariums: If key species of the nitrogen cycle are absent from an aquarium, a chemical bottleneck will occur. Their component builds up in the water, possibly to toxic proportions. For example, if Nitrosomonas and Nitrosococcus are absent, ammonia will build up to toxic levels, but there would be little or no nitrate or nitrite in the aquarium system. Normally, a period of several weeks is required after initial setup before animals are introduced. This allows an aquarium system to grow the bacteria/algae necessary to complete the entire nitrogen cycle. Live rock and sand greatly accelerates the acclimation process. Once all bacterial "players" in the nitrogen cycle are present, nitrogen should cycle through the aquarium system and end up in the algae. Harvesting algae from the aquarium and consumption of algae by fish moves the nitrogen through the food chain.

Protein Skimmers: Protein skimmers benefit an aquarium by removing dissolved organic substances from water, including proteins. This prevents the proteins from being metabolized to carbohydrate and ammonia by bacteria, which could toxify an aquarium.

Final Notes:
  1. All new marine aquarium systems must be allowed to "cycle" for at least 2 weeks and possibly "seeded" with live rock and live sand to accelerate the growth of nitrogen cycling bacteria. During this time there should be no animals in the aquarium. After two weeks, only a few animals at a time should be added with days between additions. This will allow the aquarium populations of nitrogen cycling species to grow to the necessary levels required to keep nitrogen compounds at minimal levels.
  2. Refugium biological filtration systems with live rock and gravel beds increase the populations of nitrogen cycling bacteria and will likely increase the carrying capacity of any marine system. The more rock and gravel surface present, the greater the habitat for nitrogen cycling bacteria to grow. Denitrification will require anaerobic conditions, which a deep gravel bed in a refugium provides.
  3. Quality test kits should be used weekly to determine the status of a marine system nitrogen cycle, including ammonia, nitrite, and nitrate.
  4. One of the simplest means of remediating unacceptable ammonia, nitrite, or nitrate levels is regular water changes with purified (not tap) water.
  5. In summary:
    • Aerobic processes add nitrates to the system (nitrification).
    • Anaerobic processes (denitrification) and aerobic algae (nitrate uptake) remove nitrates from the system.
    Filters and media must balance aerobic and anaerobic+uptake processes to properly process nitrogen in a marine system. There must be aerobic and anaerobic regions in an aquarium for these processes to occur. Live rock and sand is currently believed to be the best system for both of these to occur.