The Nitrogen Cycle by Travis Heidbrink
The
Nitrogen Cycle
Prepared
by Travis Heidbrink for the Sioux Falls Area Aquarium Society
3 Waste Products Involved in the Nitrogen Cycle:
Ammonia (NH3)
Excreted by all
living organisms in an aquarium.
Highly toxic to
all organisms, but corals utilize it at very low concentrations as a
nitrogen source to feed their zooxanthellae
Because our hobby
level test kits are simply that… hobbyist level, they have
low sensitivity and any presence of ammonia on a hobby level test
kit indicates a toxic level of ammonia in the aquarium.
Ammonia levels in
an aquarium should be maintained at an undetectable level. This
level is typically achieved after an aquarium has been setup and
stable for 2 months.
Nitrite (NO2)
Also toxic to
aquarium organisms, although somewhat less toxic than ammonia.
Not immediately
toxic to organisms at low concentrations, but any presence indicates
a problem in the aquarium or filtration.
Level should
remain undetectable.
Nitrate (NO3)
Not harmful to
organisms in low concentrations, though its presence contributes to
the growth of problem algae.
In the reef
aquarium, the nitrate concentration should remain undetectable, or
be no higher than 2ppm.
In the fish only
saltwater aquarium, the nitrate level should remain below 10ppm.
In the freshwater
fish aquarium the nitrate level should remain below 35ppm.
In the freshwater
planted aquarium the nitrate level should remain below 35ppm if
there are fish in the aquarium and can be allowed to measure
slightly higher if only plants are in the aquarium.
The Nitrogen Cycle Part 1: Nitrification
The process called
“cycling” refers to the establishment of bacterial
colonies in the filter bed that convert ammonia to nitrite and then
nitrite to nitrate. After an aquarium has completed its cycle, the
bacteria will have colonized the filters and substrate (surfaces such
as rock, sand and even glass) in the aquarium, and animal life can
then be supported.
First,
ammonia is introduced into the aquarium. There are many ways this
can happen:
Fresh
uncured live rock/live sand will have die off of organic matter
that will produce ammonia.
A
new fish will excrete ammonia through respiration and its own
waste.
In a tank with
no fish or live rock/live sand, a dead shrimp or other piece of raw
seafood can be added, which will break down and produce ammonia to
start the nitrogen cycle.
Various species
of bacteria from the Nitrosomonas, Nitrosospira, Nitrosococcus
and Nitrosolobus genera as well as others will feed off
of the ammonia and produce nitrite as a byproduct. These bacteria
are aerobic, meaning they need oxygen to survive. These bacteria
will grow in areas in the aquarium where there are adequate supplies
of oxygen. This is essentially any place in the aquarium system
that has constant water flow that is not extremely slow.
The next group of
bacteria involved in the nitrification cycle is responsible for
converting the previously discussed nitrite breakdown product into
nitrate. This is accomplished by numerous bacterial species from the
genera Nitrobacter, Nitrosospira, Nitrocystis, Nitrococcus
and others. Again, these are also aerobic bacteria. This waste
product, the end product of the nitrification cycle, may be removed
by such means as water changes, chemical adsorption, protein
skimming, uptake by algae filtration or other organisms that can
utilize nitrate as a dissolved nutrient source.
The Nitrogen Cycle Part 2: Denitrification
This is the process
where bacteria consume the nitrate and convert it to nitrogen gas,
which gets released into the air through water movement and aeration.
Some suitable substrates for these bacteria are in the low oxygen
areas in the center of porous live rock or in the deeper layers of a
sand bed. These areas provide very low flow rates where the water is
first exposed to aerobic bacteria that consume a large portion of the
oxygen. As the water continues to move deeper into the rock or sand,
it becomes low enough in oxygen where it creates areas where
anaerobic bacteria are able to thrive and produce denitrification
Some of the
bacteria responsible for denitrification are from the following
genuses: Micrococcus, Pseudomonas, Denitrobacillas,
Bacillus and others.
So what can we put in our tanks to help them
process these waste products and keep the ammonia, nitrite, and
nitrate levels at safe concentrations for our inhabitants?
Biological Filters
A biological
filter is simply any type of media, substrate or surface where
bacteria can colonize and have ready access to waste products in the
water from the aquarium.
Complete Biological
Filters:
These filters provide what we call “complete filtration”
because they provide aerobic and anaerobic areas where bacteria can
colonize. The water first passes through the aerobic areas where
ammonia is converted to nitrite and nitrite is converted to nitrate.
The water then passes to the anaerobic areas through diffusion where
the nitrate is converted to nitrogen gas and is then released from
the aquarium.
Live Rock
Live rock provides surface area on the outside of it for aerobic
bacteria to colonize as well as surface area in its pores for
anaerobic bacteria to colonize.
Sand
Sand also provides a surface for bacteria to colonize. Sand performs
a filter similar to live rock because it provides both aerobic and
anaerobic zones where water passes and is fully filtered down to
nitrogen gas.
Incomplete Biological Filters:
These filters are what we
call “incomplete filtration” because they only provide
either an aerobic zone or an anaerobic zone. The filters that
provide only an aerobic zone produce nitrate as the end product,
which can then build up to toxic levels in the aquarium. The filters
that provide only an anaerobic zone can only work if there are other
filters in the aquarium that are producing nitrate as an end product.
These filters will then convert the nitrate to nitrogen gas.
Aerobic Filters:
Wet Dry Filters
These filters consist of a media
such as a sponge, bio-balls, bio-bales, or any other form of media
that provides a large surface area for aerobic bacteria to inhabit.
Water is “trickled” over the media allowing it to become
even more saturated with oxygen, which then allows even more bacteria
to colonize the media, which makes it the most efficient. These
filters are hands down the most efficient biological filters on the
market. The only problem is they produce nitrate as a byproduct and
do nothing to reduce its concentration, which can allow it to build
to toxic levels if nothing else is done to control it.
Canister Filters
These filters consist of a
canister that holds any type of media that provides a large surface
area for bacteria to colonize. Water is then forced through the
canister and media with a pump or power head. Once again, these are
only aerobic filters and do nothing to control nitrate.
Anaerobic Filters:
Nitrate Reductors
These filters consist of a
canister and any type of media that will allow a large surface area
for bacteria to colonize. Water is pumped very slowly through the
reactor causing the water inside the reactor to be low in oxygen
content (anaerobic). In this anaerobic environment, bacteria will
grow that will break down nitrate to use the oxygen that is bound
within it. In order to do this, the bacteria also need a carbon
source. This is typically a dissolving food that is added to the
reactor periodically that slowly dissolves to feed the bacteria.
These filters process only nitrate and will do nothing for ammonia or
nitrate.
Coil Denitrators
These filters consist of a
long length of tubing that water is slowly pumped through. At the
beginning of the tubing, aerobic bacteria will grow and consume the
oxygen. Farther down in the tube the water will become anaerobic and
house anaerobic bacteria that will consume the nitrate. Once again,
this filter will only process nitrate.
Protein Skimmers
Protein skimmers are
not biological filters but I felt it was important to include them
here because they have a larger effect on organic waste than any size
biological filter can ever have. Protein skimmers use a process
called foam fractionation. In this process, small air bubbles are
injected into a chamber where they mix with tank water. Proteins in
the water adhere to the bubbles and as the bubbles rise to the
surface they carry the proteins out of the water and into a
collection container. Through this process DOM (dissolved organic
matter) and POM (particulate organic matter) are removed before they
can even start to break down and turn into ammonia. Therefore, a
protein skimmer removes waste before it can even begin to turn into a
product that requires removal through biological filtration.
Note: At this time, protein
skimming is only possible on saltwater aquaria. This is because the
process of foam fractionation requires a certain amount of surface
tension. Saltwater at the concentration that aquaria are kept at
naturally has the correct amount of surface tension in the water for
foam fractionation to occur. Freshwater aquaria do not have this
surface tension.
And to leave you all with some “food for thought” I
would like to end this with a famous quote from one of the most
knowledgeable people in the industry… Eric Borneman. This
quote deals with maturity issues in an aquarium (mainly a reef
aquarium). This is a must read for everyone and will help to allow
people to understand why something may not be right in their tank
even though “my water tests are perfect”…
When
you get a tank, you start with no populations of anything. You get
live rock to form the basis of the biodiversity - and remember that
virtually everything is moderated by bacteria and photosynthesis in
our tanks. So live rock is the substrate for all this stuff, and also
has a lot of life on it. How much depends on a lot of things. Mostly,
marine animals and plants don’t like to be out of water for a
day at a time...much less the many days to sometimes a week that
often happens. So, assuming you are not using existing rock form a
tank, or the well-treated aquacultured stuff, you have live rock that
has either relatively free of anything alive, or you have live rock
with a few stragglers and a whole lot of stuff dying or about to die
because it won’t survive in the tank. From the moment you
start, you are in the negative. Corallines will be dying, sponges,
dead worms and crustaceans and echinoids and bivalves, many of which
are in the rock and you won't ever see. Not to mention the algae,
cyanobacteria, and bacteria...most of which is dead and will
decompose, or which will die and decompose. This is where the
existing bacteria get kick started...
Bacteria grow really
fast, and so they are able to grow to levels that are capable of
uptaking nitrogen within...well, the cycling time of a few weeks to a
month or so. However, if you realize the doubling time of these bugs,
you would know that in a month, you should have a tank packed full of
bacteria and no room for water. That means something is killing or
eating bacteria. Also realize that if you have a tank with constant
decomposition happening at a rate high enough to spike ammonia off
the scale, you have a lot of bacteria food...way more than you will
when things stop dying off and decomposing. So, bacterial growth may
have caught up with the level of nitrogen being produced, but things
are still dying...you just test zero for ammonia cause there are
enough bacteria present to keep up with the nitrogen being released
by the dying stuff.... does not mean things are finished
decomposing.
Now, if things are decomposing, they are
releasing more than ammonia. Guess what dead sponges release? All
their toxic metabolites. Guess what else? All their natural
antibiotic compounds...prevents some microbes from doing very well.
Same with the algae, the inverts the cyano, the dinoflagellates, etc.
So, let's just figure this death and decomposition is gonna take a
while. OK, so now we have a tank packed with some kinds of bacteria,
probably not much of others. Eventually the death stops. Now, what
happens to all that biomass of bacteria without a food source? They
die. Ooops. And, denitrification is a slow process. Guess what
else...bacteria also have antibiotics, toxins, etc. all released when
they die. But, the die-off is slow, relative to the loss of
nutrients, and there is already a huge population...so you never test
ammonia..."The water tests fine"
But, all these
swings are happening...every time, they get less and less, but they
keep happening. Eventually, they slow and stabilize. What's left? A
tank with limited denitrification and a whole lot of other stuff in
the water. Who comes to the rescue and thrives? The next fastest
growing groups...cyano's, single celled algae, protists, ciliates,
etc. Then they do their little cycle
thing. And then the turf algae. Turfs get mowed down by all the
little amphipods that are suddenly springing up cause they have a
food source. Maybe you've bought some snails by now, too. And a fish.
And the fish dies, of course, because it may not have ammonia to
contend with, but is has water filled with things we can't and don't
test for...plus, beginning aquarists usually skimp on lights and
pumps initially, and haven't figured out that alkalinity test, so pH
and O2 are probably swinging wildly at this point.
So, the
algae succession kick in, and eventually you have a good algal
biomass that handles nitrogen, the bacteria have long settled in and
also deal with nutrients, and the aquarium keeper has probably
stopped adding fish for a spell cause they keep dying and they
started to visit boards and read books and get the knack of the tank
a bit. They have probably also added a bunch of fix-it-quick
chemicals that didn’t help any, either. Also, they are probably
scared to add corals that would actually help with the photosynthesis
and nutrient uptake, or they have packed in corals that aren't
tolerant of those conditions.
About a year into it, the sand
bed is productive and has stratified, water quality is stable, and
the aquarist has bought a few more power heads, understand water
quality a bit, corallines and algae, if not corals and other things
are photosynthesizing well, and the tank is "mature."
That's when fish stop dying when you buy them (at least the cyanide
free ones) and corals start to live and grow and I stop getting posts
about "I just bought a coral and its dying and my tank is two
months old" and they start actually answering some questions
here and there.
So, ecologically, this is successional
population dynamics. Its normal, and it happens when there is a
hurricane or a fire, or whatever. In nature though, you have pioneer
species that are eventually replaced by climax communities. We
usually try and stock immediately with climax species. And find it
doesn't always work. Now, the "too mature" system is the
old tank syndrome. Happens in nature, too. That whole forest fire
reinvigorating the system is true. Equally true on coral reefs where
the intermediate disturbance hypothesis is the running thought on why
coral reefs maintain very high diversity...they are stable, but not
too stable, and require storms, but not catastrophic ones....
predation, but not a giant blanket of crown of thorns, mass
bleaching, or loss of key herbivores.
This
paper and any information within are the property of said references,
Travis Heidbrink, and the Sioux Falls Area Aquarium Society. Any
reproduction of this literature is forbidden without the permission
of Travis Heidbrink or a committee member from the Sioux Falls Area
Aquarium Society.