Do You Imagine..?

Who are the Players and What do they Do..?

Ammonia (NH3) and ammonia ions (NH4+) are the first nitrogen-based substances to be eliminated in the Traditional Nitrogen Cycle process.  Ammonia ions are positively charged particles that are attracted to other negatively charged particles.  One direct consumer of ammonia are plants.  Plants require nitrogen nutrients for growth and reproduction. They prefer ammonia compounds that are directly absorbed into their tissues.

In the absence or insufficiency of plants to consume available ammonia, the Nitrogen Cycle continues with bacteria in the presence of ammonia and oxygen. These micro organisms break down (or oxidize) ammonia into less toxic products through their metabolic activity.

There are two separate groups of bacteria responsible for breaking down the ammonia into less toxic compounds, first into Nitrites (NO2-) and secondly into Nitrates (NO3-).  These bacteria are referred to as nitrifying bacteria and require a high and stable level of oxygen to function. They are in a group of bacteria called aerobic bacteria, those requiring ample oxygen to live.

Plants can also consume Nitrates as food but only after re-metabolizing the nitrates into nitrogen compounds that they can directly absorb.  In effect, plants must reverse the conversion process that is performed by the bacterial groups with the assistance of photosynthetic energy.  This is an example of the fact plants don't feed on nitrates during dark hours.

The first bacteria group converts ammonia (NH4+) into nitrite (NO2-).  

These bacteria are called “Nitrosomonas”. Nitrite is less toxic than ammonia but more toxic than nitrate. Aquarium nitrite levels must remain near zero.

The second group of bacteria in the nitrogen cycle are called “Nitrobacter”.  These bacteria feed upon or oxidize the nitrites created in the first reaction. The nitrites are, as a result, now converted into nitrates (NO3-).  Nitrates are moderately toxic to aquarium inhabitants and must be minimized. 

The biological filtering activity of the aerobic bacteria ends with the production of nitrates.  This partial nitrogen cycle, now requires water changes to eliminate nitrates and is therefore not complete.

The “Traditional” Nitrogen Cycle ends here, now what..?

In the aerobic denitrification process, the Traditional Nitrogen Cycle, ends with the conversion of ammonia into nitrates.  Nitrates can be eliminated further by plant absorption.  However as mentioned, in the absence of plants, the cycle is now completed by water changes to reduce and maintain lower levels of nitrates. 

The upper limit of "low nitrate levels" is about 40-50 ppm. This level prompts a water change and/or is adequate to for plant nutrition. Sensitive fish and invertebrates often require lower levels of nitrates, below 10 ppm. This is the reason for doing “Traditional” water changes. Their size and frequency is dictated  by the bio-load of the aquarium and the resulting accumulations of nitrates.

Proper water change frequency can be determined by monitoring routine feeding levels and taking water tests to check the nitrate levels.

An Imperfect Solution to Nitrate Removal - Anaerobic Bacteria

There are anaerobic bacteria that can consume nitrates.  These bacterial exist without oxygen.  The unfortunate fact is that they also generate and store ammonia compounds, sabotaging a complete denitrification process.  These bacteria obtain their oxygen, not from the water but from the oxygen molecules in the nitrates, NO2-.

Aquarists using this type of nitrate elimination, firstly must rely on plants to consume the ammonia produced and maintain deep unaerated substrate, void of oxygen.  A problem arises in that plant roots need oxygen from water.  Ammonia gas can accumulate in the lower, uncirculated layers and may potentially poison the aquarium if released.  Planted aquariums are also not always preferred nor are they end goal for many hobbyist’s (fish keepers) and an ammonia release is always a dangerous possibility when using deep, unoxygenated substrates.

The Truly Complete Nitrogen Cycle - Add the Anoxic Bacteria

In general, the denitrifying bacterial groups fit into 3 categories:

• Aerobic - requiring amble oxygen levels
• Anoxic – requiring depleted oxygen levels
• Anaerobic - requiring an absence of oxygen

Without the healthy oxygen levels required by fish in typical aquarium water, aerobic bacteria stop functioning and the Traditional Nitrogen Cycle ceases.

Anoxic bacteria consume all nitrogen compounds including ammonia, nitrites and nitrates.  The only byproduct created is pure, harmless nitrogen gas (N2). This gas is released into the water and evaporates naturally.  The anoxic bacteria fulfill a “Full and Complete” Nitrogen Cycle. This complete nitrogen removal cycle occurs in natural waterbodies such as lakes, ponds and in water treatment facilities. 

Anaerobic bacteria produce their own ammonia that, as previously noted, is toxic to aquarium inhabitants and the aquarium still relies on dense numbers of plants to use the secondary ammonia source.  You don't want any measurable ammonia present in your aquarium water.

Specific (low oxygen) conditions must be met by using anoxic filter arrangements, differing from aerobic filtration methods, to allow anoxic bacteria to grow in an aquarium or fish pond setting. 

A discussion on Anoxic Filtration Theory and the work of Dr. Kevin Novak will follow in Kelberi Aquatics Technical Bulletin #2.