Nitrate assimilation
Nitrate assimilation is subjected to regulation that may differ depending on the organism. In general, nitrate assimilation is controlled at the transcriptional level by nitrate and nitrite induction and by ammonium repression.
In the pre-genome sequence era, it was known that some, but not all, heterotrophic bacteria were capable of growth on nitrate as a sole nitrogen source. However, examination of currently available prokaryotic genome sequences suggests that assimilatory nitrate reductase (Nas) systems are widespread phylogenetically in bacterial and archaeal heterotrophs.
In cyanobacteria, the transcription factor represses nitrate assimilation genes when ammonium is present whereas it activates transcription of these genes at a high
carbon/nitrogen ratio.
The nitrate-assimilation process begins with the transport of nitrate into the cell. Nitrate is further reduced to nitrite in a two-electron reaction by a cytoplasmic molybdenum containing nitrate reductase followed by a six-electron nitrite reduction to produce ammonia.
The assimilatory reduction of nitrate to ammonium is an energetically expensive process since it requires eight electrons and complex prosthetic groups for the nitrate and nitrite reductase enzymes, in addition to the active nitrate transport. In order to avoid this energetic cost under unnecessary environmental conditions, bacteria have evolved a strict control of the expression of the Nas system. Thus expression of the Nas genes is subjected to dual control based on specific nitrate or nitrite induction, and ammonium repression, by a general nitrogen-regulatory system.
http://www.baharini.eu/baharini/doku...traat_reductie
When adding carbohydrates for the first time not a lot will change to the nitrate level. Ammonia, normally nitrified by autotrophs, is used first . Only when the supply of ammonia is insufficient nitrate will be used. When dosing is continued nitrification will be suppressed due to ammonia availability as the heterotrophs grow a lot faster. Heterotroph ammonium reduction ( assimilation) produces 40 x more bio mass compared to autotroph ammonia reduction. (
http://www.baharini.eu/baharini/doku...onium_reductie ) As a result less nitrate will be produced. The moment the C:N ratio increases due to dosing and increased ammonium take up ( and less or no nitrate production) nitrate may be used for celsynthesis. Nitrate is also taken up for respiration. Nitrate take up for respiration is not suppressed by ammonia. The nitrate level may be depleted fast at high C:N ratio. All this implements a very low nitrification and denitrification capacity.
When biopellets are used in a reactor there is always a high C:N ratio available.
An aquarium system that is kept in balance by carbohydrate dosing has a limited carrying capacity. Such systems are vulnerable for a system crash due to the well known new tank syndrome.
Once started dosing it may be difficult to stop. Dosing should be build off during a period of at least 15 days.
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