THE NITROGEN CYCLE | IN DEPTH
by Leonard Ho
o Nitrogen: The Essential Element
is an element vital to all life processes on Earth.
To appreciate the importance of nitrogen in our biosphere, simply
realize nitrogen comprises 78% of the atmosphere, and is
embedded in every living tissue!
It is a component of amino acids, proteins and nucleic acids.
With the exception of carbon, nitrogen is the most universal element of
life Put simply: Life could not exists without nitrogen. Aside from organic development, nitrogenous compounds are also required by some organisms for metabolic
functions and respiration.
Nitrogen exists in many states. In its most common gaseous state of N2, nitrogen forms very strong covalent bonds that can only be broken when great force or energy is applied (e.g. seismic event or lightening), or by particular bacteria species which use nitrogenous compounds for metabolism. The fact is various bacteria are critical to every step of the nitrogen cycle. It is these bacteria and their specific roles that are of particular interest to reefkeepers.
o The Nitrogen Cycle: Defined
The nitrogen cycle is defined as the pathways for which nitrogen is recycled. As with all elements, nitrogen is constantly combined and uncombined with other elements to form essential and nonessential compounds for life. We term this progression from one compound to the next its pathway. Tracking nitrogen’s pathway is critical to understanding the role nitrogen plays in the chemical functions of an ecosystem. Consequently, we can apply this knowledge to the management of key processes in our captive reef ecosystems to ensure a balanced, healthy environment for our reef’s inhabitants.
To simplify the concept, we will trace the nitrogen cycle from its source of introduction to the eventual recycling of nitrogen within (and without) our reef aquariums.
Simplified Illustration of the Nitrogen Cycle
The introduction of any organism, whether it
be coral or fish, constitutes an introduction of
nitrogen into the aquarium.
Introduction of Nitrogen
Nitrogen is introduced into the aquarium in a variety of ways. Here are the three major contributors of nitrogen:
Nitrogen Released Via Decomposition
As mentioned, living organisms possess a large mass of assimilated nitrogen. This localized organic concentration is immobilized (“locked up”) in the tissue cells until the organism dies, at which time the nitrogen is released back into the environment via aerobic decomposition. This process is known as eutrophication. Note that decomposition requires a lot of oxygen. When large organisms die, decomposition demands large amounts of oxygen that may be very taxing of the system. This is the very reason why ORP (oxygen redox potential) levels drop significantly when decaying matter is present.
Organisms constantly reproduce, grow, and die in any ecosystem; our reef
aquariums are no different. Keep
in mind that fish and corals are not the only organisms that inhabit our
aquariums: phytoplankton, zooplankton, microalgae, macroalgae, worms,
crustaceans, bivalves, and sponges are just a few of the other life
forms found in our aquariums. There exists a continual cycle of life and death.
When these organisms die and decompose, assimilated nitrogen is
released back into your aquarium.
Because food inputs are nothing more then introduced organic masses, they go through a similar process of releasing nitrogenous byproducts into the environment. Foods are either consumed or unconsumed:
§ Foods consumed will pass through the organism. The organism will process the food to assimilate the nutrients it needs, and discard the rest as excrement. Although the organism will incorporate a considerable quantity of nitrogen, their excrements are still rich in nitrogenous compounds, which is quickly liberated into the water through advanced decomposition.
Foods that go unconsumed will eventually die (if not dead
upon introduction) and decay, contributing previously assimilated
nitrogen back into the environment via decomposition.
Decomposers include bacteria and fungi, with bacteria performing the greatest burden for decomposition in reef ecosystems.
Foods are either consumed and processed by
fish and other organisms, or go unconsumed.
Both will invariably add nitrogen to the system.
Fish and corals are not the only organisms
that inhabit our aquariums. Small organisms,
such as these fan worms, contribute
significantly to the nitrogen cycle.
o Ammonification & Nitrification
The decomposition process produces large quantities of ammonia (NH3) through the process of ammonification. Heterotrophic microbes utilize the organic compounds of decomposing matter as their carbon source. Ammonia (NH3) is the byproduct of this consumption. Ammonia, in its neutral state, exists as ammonium (NH4+).
Ammonium has several divergent
pathways from this point forth. Plants
and algae can assimilate ammonia and ammonium directly for the
biosynthesis. The remaining bulk of decomposed byproducts is utilized by
bacteria in a process called nitrification.
Nitrification is the oxidation
(affixation of oxygen) of ammonium by chemolithotrophic bacteria species. During
this process, specific species of nitrifying bacteria strip the ammonium
of its hydrogen molecules as an energy source.
Oxygen molecules are then affixed to the stripped nitrogen,
forming the oxide nitrite (NO2).
Another group of bacteria utilize the enzyme nitrite
oxidase that is then responsible for converting
nitrite into nitrate (NO3).
In order for nitrification to occur, three constants must be present:
The biochemicial processes of nitrification is described below:
In short, the processes described thus far progresses in the following manner:
decomposition à NH3 à NH4+ à NO2 à NO3
o Recycling of Nitrogen
First and foremost, let us dispel
the misinformation that the nitrogen cycle “ends” at some point
along the pathway. The prevailing concept about the nitrogen cycle is that it ends with the
formation of NO3, or with the process of denitrification.
Neither is true.
As the term suggests, the nitrogen
cycle is an unremitting succession of pathways for nitrogen.
It is never "completed." There is no end to the
nitrogen cycle; Matter can neither be created or destroyed, and
nitrogen is no different. Instead, nitrogen
is continuously recycled from one form to another.
So if the nitrogen cycle doesn’t end, what happens to the aforementioned byproducts of nitrification? There are two primary routes for nitrate reduction: Assimilative and Dissimilative pathways.
"There is no end to the nitrogen cycle;
Matter can neither be created or destroyed,
and nitrogen is no different. Instead,
nitrogen is continuously recycled from one
form to another. "
Assimilative reduction is process whereby nitrate is reduced to organic nitrogen for the construction of organic matter. Assimilation may occur in either the presence or absence of oxygen, and only enough nitrate is reduced to fulfill the organism's requirements. Photosynthetic plants, algae, and zooxanthellae (the symbiotic dinoflaggellates found in photosynthetic corals, anemones, and some sponges) assimilate NH3, NO2, and/or NO3 for biosynthesis of proteins, amino acids, and nucleic acids. Of these, nitrate is the most utilized compound because it is very low in toxicity, and is readily acceptable. When utilizing nitrate, the organisms performs the following reduction:
NO3 à NO2 à NH2OH à NH3 à R-NH2 (organic N)
Nitrite is the least utilized compound for biosynthesis since it is the most toxic. In the presence of acid, nitrite forms nitric acid, a known and dangerous mutagen. In advanced organisms such as fish, nitrite will bind with red blood cells and hinder their capability of transporting O2, asphyxiating the organism.
Animals that consume plants and algae subsequently
assimilate nitrogen for biosynthesis of their own tissue (N.B. Plant
matter is the ultimate source of nitrogen for most animals).
When both plants and animals die and decompose, the assimilated
nitrogen is released back into the environment, and the whole cycle
begins over again.
Various organisms, like this macroalgae
(Halimeda sp.), can bioassilimate nutrients
directly from the water.
Dissimilative nitrate reduction is the antithesis of assimilative nitrate reduction. Whereas the assimilative pathway is generally aerobic and only uses enough nitrogenous compounds to meet an organism's requirements (i.e. no excess is produced), the dissimilative pathway is generally anaerobic and produces copious amounts of excess byproducts. Denitrification is the key dissimilative pathway for nitrate reduction.
Denitrification is the terminology
used to describe the conversion of nitrogen oxides (NO2 and
NO3) back into gaseous nitrogen (N2, N2O,
or NO). Denitrification
results in nitrogen being lost from the local environment (e.g. water) to the atmosphere.
This process, as most processes are in the nitrogen cycle, is
accomplished primarily by bacteria species.
However, unlike nitrification, denitrification is an anaerobic
process, meaning it occurs in the absence of oxygen. Denitrifying bacteria metabolize nitrogenous compounds (with
the assistance of the molybdenum-containing enzyme, nitrate reductase) in the
reverse way that nitrifying bacteria does: they turn oxides back into
nitrogen gas or nitrous oxides for energy generation.
These gases then volatize, returning back into the atmosphere.
Because the enzyme nitrate reductase is synthesized only when O2 is repressed, anoxic conditions are obligatory for most denitrifying bacteria. This is why the denitrification process predominantly occurs in deeper substrates and in areas of stagnant flow where oxygen levels are depressed. And this is why deep sand beds are effective as a nitrogen export mechanism. As water slowly diffuses deeper, aerobic organisms strip all available oxygen for respiration. In the deep, oxygen-deprived layers, denitrifying anaerobes are given the opportunity to convert nitrogen compounds into nitrogenous gases.
The net loss of nitrogen to the atmosphere will be regained via introduced foods and water. Some nitrogen will also inevitably be fixated and incorporated back into the aquarium by bacteria and cyanobacteria.
Deep sand beds are suited for the process
of denitrification because the fine substrate
media stagnates water flow, creating an
ideal anaerobic condition for anaerobes.
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o And so ....
.... these processes repeat endlessly in our reef aquariums, unseen yet nevertheless of utmost significance.
The nitrogen cycle is the continuous recycling of nitrogen as an essential building block for life. The understanding of these mechanisms is critical to the application of methodologies that promote a balanced, healthy, captive ecosystem.