Nitrates

[bertoni]

Yes, there are hetertrophic bacterial that consume nitrates but we keep forgetting that ammonia is the preferred nitrogen source for many organisms. The organism is not required to reduce nitrate to ammonia before using it. Ammonia is a lower energy source of nitrogen and can even inhibit or slow down nitrate uptake. So, what appears to be denitrification can also be heterotrphic ammonia uptake or both. This shift from removing nitrogen as nitrate to nitrogen as ammonia is facilitated by carbon dosing.

I agree that organisms will take up ammonia directly, particularly algae. I think you mean this organisms are not required to oxidize ammonia to nitrate, not reduce it. They do need to oxidize it to form amino acids, I think, but "convert" will do, since redox reactions aren't my specialty.

The link with carbon limitation is that carbon is used both to build cells and to make energy, i.e., produce carbon dioxide...

A lot of this description is accurate, although some terms might need some help. I agree that some bacteria can convert ammonia to glutamine, for example. Looking around a bit, I found some articles on bacterial uptake of ammonia. Interesting reading! I'm not sure how well those papers apply to our systems, though, since they are studying planktonic bacteria. I'm not sure how this relates back to the original topic, though, since we were discussing why bio-balls and other media seem to increase the nitrate level of some systems, despite no change in the organic carbon added to the system. I'll have to think about that when I have some more time.