I among these that prefer a NO3 concentration between 2 - 5 mg/L. One of the reasons is that if I read NO3 - I know that I have a N reserve in my water but I´m also run Phosphates well over 0.04 in these cases, There is reports that indicate that NO3 levels around 0.2 mg/L and PO4 below 0.03 can cause P defiency and bleaching among some stony corals. With PO4 above 0.04 - the amount of NO3 is not as critical.
Another reason is the fact as someone state - the only organism I know that really need NO3 is the denitrification bacteria. I prefer to see my aquarium as an ecosystem there bacteria may be the most important inhabitants. In a mature aquarium - there is a lot of different environments with there own set up of bacteria. In the part there oxygen is available - oxygen is used by heterotrophic bacteria in their cellular respiration. But there is also parts that are more or less depleted of oxygen (by the same bacteria) but there is still a lot of organic matter to "eat" Many heterotrophic can in these cases switch from aerobic respiration into anaerobic respiration in order to use the the organic resources. What anaerobic respiration really means? It means that oxygen is not available as an electron acceptor in the respiration - some other molecule must be used. Its here the NO3 molecule comes in as the savior in distress. Many bacteria strains can shift between oxygen and NO3 in the respiration cycle. The oxygen will give more energy but NO3 is first runner up in energy yield. The waste from aerobic respiration is as most of us know CO2 and the waste from anaerobic respiration with help of NO3 is N2 (there is a name for this - denitrification) Its important to note that there is many aerobic bacteria that can switch from using oxygen to be anaerobic - using NO3 in the respiration. OK - i want an anaerobic bacteria population that use NO3 i the respiration - why.
The answer is hidden in the fact the second runner up in the respiration race is bacteria strains that only use sulphate as electron acceptors in the respiration. Our aquariums is rich in sulphates - my own sulphate contend you can see below
This means that if not NO3 is available - the bacteria strains that use Sulphate as electron acceptor will dominate the anaerobic bacteria population totally. - And whats with that?
The most important "what" is the waste they produce - hydrogen sulphide - the rotten egg smell. Its a very toxic compound and IMO one of most common reasons for sudden fish death in mature system. Yes it can be produced even if NO3 is represented but IMO - measurable concentration of NO3 above 2 mg/L limit the hydrogen sulphide producers biomass because the bacteria that denitrify take up rather much of the available area for bacteria growth.
The bacteria that use sulphate in their respiration seems als to be of certain strains that not can use either O2 or NO3 in their respiration.
There is a third reason too - and its about cyanobacteria. My believe is that sometimes is not the mat building cyanobacteria blooms a result of nitrogen starvation but more a result of available phosphorous sources. Many times it had shown up that very low or total zeroed NO3 concertation result in blooming cyanobacteria mats. I´ll think that there is a link between the anaerobic bacteria population between the mats and the substrate. If no NO3 is present - the hydrogen sulphide producers will dominate. Hydrogen sulphide break the bonds between metals and PO4, hence PO4 will be available for consumption. If I git these mats - I always try to stirr the mats in the evening and add NO3 to the water. I´ll think that stirring the mats is good - even if you not suck the out - the mats is not primary the cyanobacteria - they are - IMO - external resources of mainly sugar produced during the daily photosynthesis
This was a long post and probably not exactly right in a scientifically way for a microbiologist but an attempt to explain why I prefer NO3 concentrations between 2 - 5 mg/L
And a attempt to explain why I say yes to the inital question "Do we need Nitrate in a reef tank?"
Sincerely Lasse
Sincerely Lasse