I don't want to be right ; I want the information to be right.
The study you cited which has as it's goals examination of phosphate uptake not nitrate uptake by bacteria or corals actually says:
. On the contrary, it is well-known, for
example, that nitrate uptake is generally inhibited in the presence of
ammonium (Badgley et al., 2006; Domotor and D’Elia, 1984; Grover
et al., 2003), either through a repression of the nitrate reductase (Berges
and Harrison, 1995; Guerrero et al., 1981; Syrett, 1981; Vergara et
al., 1998) or a competition of ammonium and nitrate uptake
mechanisms for energy resources, leading to the preferential uptake
of ammonium (Terry, 1982a; Terry, 1982b).
. Obviously, if there is ammonium the zooxanthelae and bacteria will use it preferentially as agreed earlier : not just zooxanthellae. Either bactreri8a or zooxanthelae will use all of the dissolved N whether it comes from ammonium or nitrate unitl the law of "the law of the minimum" applies in regard to other nutrients. As long as the ammonium is there it will be used preferentially limiting the need for nitrate by either zooxantheal or bacteria. However, by using the ammonium, nitrate creation via ammonia oxidation is reduced since there is less ammonia to transform to nitrate.
I disagree with the extropolation from the study to a point where sodium nitrate is noted as a preferred source to aspartate or other ammino acids. I don't think anything in the study or discussion supports that notion.
We could go back and forth with studies all day and confuse the issue if you like.
For example, here are two ,found in a few minutes, which present a different view on nitrate uptake by zooxanthellae than your view:
http://www.aoml.noaa.gov/general/lib/CREWS/Cleo/St. Croix/salt_river47.pdf
http://onlinelibrary.wiley.com/doi/10.1002/iroh.19740590102/abstract
Or keep it simple and agree to disagree. I do not agree that there is a reason to think ammonium limits zooxanthellae's uptake of nitrate anymore than it does denitrifying heterotrophic bacteria. Both will use available N to exhaustion unless another limiting nutrient comes into play. Therefore, the notion sodium nitrate is prefered to other sources like aspartate for a source of N assuming only bacteria will use it vs zooxanthellae doesn't work for me.
BTW, this thread concerning organic carbon may be of interest to some reading this thread:
http://www.reefcentral.com/forums/showthread.php?t=2134105&highlight=organic+carbon+dosing
The study cited in this post in the thread gives a good overview of denitrification processes including those performed by heterotrophic bacteria fed with orgnanic carbon:
This study gives an overview ,which was very helpful to me, of 3 major nitrogen pathways including: photoatuotrophic removal via photosynthetic activity: chemoautrotrophic activity by ammonia oxidizing bacteria producing nitrate and the activity of facultative heterotrophic bacteria supported with organic carbon. Though these pathways can occur in combination, the effects of increasing organic carbon content does likely reduce the level of activity by the other two.
The paper asserts the facultative heterotrophs take ammonia directly and may out compete the ammonia oxidizers as the former grow about 5 times as fast. It doesn't deal with amanox bacteria though but those obligate anaerobes are about 14 times slower in growth than the oxidizers ,conditions which make it unlikely ,IMO, that they play any significant role in a typical reef tank.
http://ag.arizona.edu/azaqua/ista/IS...%20Ebeling.pdf
Also ,Wikepedia notes most denitrifying bacteria are facultative, btw.
Nitrate reduction can take months when organic carbon dosing is started per many anecdotal reports . Maybe the bacteria take the N out at the front end of the process using the ammonia which cuts down the nitrate production to a point where they need to use a bit more energy to get at the N in the nitrate and/ or maybe anaerobic activity activity of the bacteria removes it eventually.