[Dan_P]

Originally Posted by Belgian Anthias

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.

Many systems are controlled at this level. No arguement here

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.

Without a reference to the study, I will take your word for it, though this seems reasonable. Complementary chromatic adaptation has been described for cyanobacteria and there seems to be a link between color of the organism and nitrogen source. No surprise then about the level of control but the notion that a high C to N ratio is a trigger is interesting. Again, no reference, so, I will take your word for now.

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

Link does not work but this is well known.

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 )

This is published in the aquaculture literature

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.

The potential risks of robbing Peter to pay Paul does not seem to be addressed in the aquarium carbon dosing discussions. Maybe most aquarists get away with a reduced denitrication capacity. In low nutrient systems like reef aquaria, maybe the risk is not so high.

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.

The carbon dosing system vulnerability conjecture needs support. It is reasonable and something that I have wondered about but could be absolutely wrong. Yes, switching back and forth, between non-dosing to dosing or dosing to non-dosing, with minimal accumulation of ammonia or nitrate would take time for the population of organisms to readjust. But to say that one regimen has a higher “carrying capacity” needs support. It might be amenable to an stoichiometric calculation.