it is the type of condition and not the strain of facultative bacteria(need oxygen for the processes of respiration and cell metabolism) that limit the reduction of nitrates or the fixation of nitrogen.
In an aerobic condition bacteria will use the oxygen first for the process of respiration. This results in the oxidation of ammonia and nitrites producing nitrates as a biproduct of the chemical equation.
In an anerobic condition the faculative bacteria will reduce nitrates to get the oxygen. The results of the chemical equation will produce nitrogen gas as a biproduct so resulting in the fixation of nitrogen and the reduction of nitrates
Scott,
Not to be pedantic but that's not correct in several points and if technical information is presented it should be accurate ; so,to clarify:
Ammonia oxidizers and nitrite oxidizers are aerobic and function on surfaces exposed to a flow of water with plenty of oxygen. They have nothing to do with the organic carbon sourced by the biopellets. They are chemoautophic or chemolithorophic depending on the specific species and don't need the organic carbon from the pellets This process is generally called nitrification and these types of baceteria are generally called nitrifying bacteria. Nitrate is produced as an end product of the nitrification phase of the nitrogen cylce.
The facultatative heterotrophic bacteria perform
denitrification which is not nitrogen fixation but nitrogen reduction. They need the organic carbon, phosphorous, nitrogen and a source of oxygen either free oxygen or the oxygen from nitrate.
Nitrogen fixation is at the other end of the cylce and is defined:
(
Science: biochemistry) The incorporation of atmospheric nitrogen into ammonia by various bacteria, catalysed by nitrogenase.
This is an essential stage in the nitrogen cycle and is the ultimate source of all nitrogen in living organisms. In the sea, the main nitrogen fixers are cyanobacteria.
Source : Biology on line:
http://www.biology-online.org/dictionary/Nitrogen_fixation
Nitrogen reduction by denitrifying bacteria occurs in two ways:
They use some for food and when they create a mulm with low oxygen availability or otherwise find themselves in a hypoxic enivronment they turn to nitrate (NO3) for the O and reduce it to N, some of which bonds to other N atoms forming N2 nitrogen gas which bubbles out of the tank and reenters the athmosphere and is thus reduced ,ie unbound and no loner fixed to anything but itself as N2.
The bond between two N atoms is very strong and N2 which makes up over 70% of the athmosphere can not be used by living things until that bond is broken and N is allowed to bond with hydrogen as it does in the creation of ammonia NH3/4. Strains of cyanobacteria and related diazotrophs are uniquely able to perfom this task as they posses a unique enzyme ,nitrogenase , which breaks the bond allowing nitrogen fixation to occur.
Pracitcal matters:
Increasing the flow to tumble the pellets will encourage more aerobic activity vs anaerobic activity and will likely be less effective at nitrate removal since the nitrate reduction (beyond nitrogen used as bacterial food ) from nitrate reduction to nitrogen gas will be lessened in a higher oxygen environment.
On the other hand excessive clumping may slow the whole process and lead to anoxic conditions( all the oxygen and nitrate based oxygen is used up) and toxic hydrogen sulfide may be produced as sulfate reducing bacteria will take over in the absence of oxygen and nitrate when organic carbon from the pellets is present.
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The point is setting a proper flow rate can be difficult and is largely guesswork and will likely vary from system to system depending on the amount of nutrients in the water and several other variables.
