Nutrient Balance

[Servillius]

I believe that part of the issue is that a stable equilibrium (a reef is currently described as in a state of dynamic equilibrium) is not easily recognized by a novice reef aquarist, especially. As a result, there is a tendency to 'err on the side of caution' and aggressively export nutrients often to the detriment of the system as a whole, especially if imports are not sufficient to properly sustain the organisms. IMO, the 'art' in reef keeping is in determining/maintaining an acceptable balance between the two import and export extremes.

For the last 9-1/2 years I've been running a small 12g 'mixed reef' nano tank partially for enjoyment, but also as an experiment to see if a system this small could self-stabilize without the use of chemical or mechanical filtration . Without going into boring details, for the last 7 years NO3 has been under 1 ppm (often less) and PO4 has read '0' (Salifert, Elos). A Triton test determined that both inorganic and organic levels of phosphate are around half of the recommended levels for a reef tank. Yet, the corals are healthy, colorful and growing. I do maintain a good fish stocking (currently six 2" average size Gobies & Blennies), so my assumption here is that ammonium excreted by them, and the phosphate from 2x/day feedings, is quickly utilized by the good number of mature coral/false corals present...which ultimately results in low NO3 and phosphate in the water column.

From what I have observed over the many years, there are multiple ways to run a reef system and that a natural system is inherently capable of reaching a state of equilibrium given time...if external disruptions are not excessive.

Ralph.

Thanks! I definitely agree. That last line in particular summarized things very well. The flag I’m trying to wave here is that we seem to be giving newer reefers advice that conflicts with a lot of our past experience and encourages them to increase those external disruptions before they have control of the system at all.

 

[Randy Holmes-Farley]

Maybe could test with a ph adaptable marine algae.

Could use a lowered ph containers of water to test the uptake rate ammonia vs nitrate.

Use co2 impregnation to lower ph to below where ammonium not ammonia is formed.
.

While I did not see a study that actually claimed the distinction, every study that I could find relating to ammonia uptake by marine algae refers to ammonium uptake rather than ammonia.

One problem with a pH study is it also alters carbon availability (bicarbonate vs carbonate), and has a known effect on algae for that reason:

Photosynthesis and the Reef Aquarium, Part I: Carbon Sources by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2006-10/rhf/index.php

Photosynthesis of Macroalgae as a Function of pH

One of the side effects of the necessity of taking up carbon dioxide to photosynthesize is that pH may affect the rate of photosynthesis, because the amount of carbon dioxide (as CO2 or H2CO3) in the water varies with pH. Assuming constant carbonate alkalinity, the effect is quite strong. A drop of 0.3 pH units implies a doubling of the carbon dioxide concentration. A reef aquarium at pH 8.5, for example, has one fourth the carbon dioxide of a reef aquarium at pH 7.9, assuming the carbonate alkalinity is the same.

Aquarists may rightly wonder whether organisms are able to photosynthesize efficiently as the pH is raised. The answer is mixed. Some can and some cannot. Those organisms that rely solely on carbon dioxide may not. Those that rely on both carbon dioxide and bicarbonate have a better chance of retaining efficiency at higher pH because a much larger amount of bicarbonate is present, and it does not change as rapidly with pH over the range of interest to aquarists.

Table 1 shows the response of a variety of macroalgae in terms of their ability to photosynthesize at pH 8.1 and 8.7. In seawater with constant carbonate alkalinity, there is 20% as much carbon dioxide at pH 8.7 as at pH 8.1, so an organism relying on carbon dioxide alone might experience a large drop in photosynthetic rate over this range. Clearly, the response varies with species. Chaetomorpha aerea, in particular, may be of substantial interest to aquarists. It is not necessarily the exact species that many grow in refugia (which is unidentified as far as I can tell), but this species of Chaetomorpha shows a 25% drop in photosynthesis when exposed to the higher pH. That drop is not as large as some other species, but may still be important, and it is more than many other species of macroalgae.

Of course, the photosynthesis rate does not necessarily translate to growth rates. If other nutrients are limiting growth (nitrogen, phosphorus, iron, etc.), then it may not matter if the rate of photosynthesis is reduced at higher pH. But because these nutrients are often present in surplus in reef aquaria, it may well be that carbon uptake is limiting in some cases, and in those cases aquarists might benefit from ensuring that the pH is not too high.

 

[mcarroll]

While the idea of nitrogen fixing cyano

+1

On top if that...

I finally got around to looking into it (a little bit) and I'm not too sure that any of the types of cyanobacteria we commonly see in our tanks are actually nitrogen fixers.

 

[Servillius]

+1

On top if that...

I finally got around to looking into it (a little bit) and I'm not too sure that any of the types of cyanobacteria we commonly see in our tanks are actually nitrogen fixers.

That’s entirely possible, but I’ll defend my original point. There are processes in our tanks that can buffer against a lack of nitrogen. There are just too many critters that do this for your tank to be incapable of scavenging some atmospheric nitrogen if it needs it. Maybe it can’t find enough in some circumstances, but none at all would be a shock.

 

[Gareth elliott]

That’s entirely possible, but I’ll defend my original point. There are processes in our tanks that can buffer against a lack of nitrogen. There are just too many critters that do this for your tank to be incapable of scavenging some atmospheric nitrogen if it needs it. Maybe it can’t find enough in some circumstances, but none at all would be a shock.

This I think is a given, i think you need simply look at a frozen body of water where atmospheric gas exchange is greatly reduced and the effects on the chemistry on the water.

http://cmore.soest.hawaii.edu/cruises/biolincs/nitrogen.htm

 

[mcarroll]

That’s entirely possible, but I’ll defend my original point. There are processes in our tanks that can buffer against a lack of nitrogen. There are just too many critters that do this for your tank to be incapable of scavenging some atmospheric nitrogen if it needs it. Maybe it can’t find enough in some circumstances, but none at all would be a shock.

You're right, it's definitely NOT none!

Diazatrophs (N-fixers) are within the menagerie of a coral's holobiont.

I will say just off the top of my head that N-starvation seems to have MUCH milder effects on corals in our tanks than P-starvation. It would be interesting to line up 10-20 examples of N-limited (but NOT P-limited) coral tanks and see if there were significant deleterious effects on any of them. I suspect slow growth and color change might be the worst of it. (Bryopsis seem to have similar capabilities, BTW. )

P-starvation seems to encourage bad things to happen in a heartbeat by comparison.

The thing for our discussion is that I wonder what benefit these capabilities could possibly have for the tank unless something eats your N-fixing coral. Probably "some" effect....but noticeable? If so, in what timeframe?

If you can make your way through it, this article gives a pretty good sense of our corals' capabilities under various conditions:
The Assimilation of Diazotroph-Derived Nitrogen by Scleractinian Corals Depends on Their Metabolic Status

 

[Brew12]

I just want to make sure everyone is speaking the same language. When we talk about N or P limitations it is not an implication that there isn't N and P in the tank. It is that the N and P is not available in a form that the impacted organism can utilize it in.

 

[mcarroll]

I just want to make sure everyone is speaking the same language. When we talk about N or P limitations it is not an implication that there isn't N and P in the tank. It is that the N and P is not available in a form that the impacted organism can utilize it in.

Try to argue with a starving organism.

Unavailable to the organism in question = "none" for all intents and purposes.

We don't require there to be zero atoms of a nutrient in the tank before calling it limited.

We create such artificial conditions in some cases with things like carbon dosing and GFO that "none" is not as impossible as we'd think.

They do give some parameters in the article I linked about different concentration levels and coral performance at N-fixing under those ambient conditions, including different phosphate saturation levels....pretty interesting.

With some variability, corals seem to be able to utilize many forms of nutrient and to acquire it even at quite low concentrations. It's the whole holobiont, or composite organism, at work though....not merely the coral animal.

Other organisms in the tank are probably more likely to actually suffer for nutrient depletion before corals will. I'm guessing most of those organisms aren't of visible size either.

2¢

 

[Gareth elliott]

Found this by accident, was actually looking up acropora variabilis lol.
But happened to be related to the current discussion.

https://page-one.live.cf.public.springer.com/pdf/preview/10.1007/BF00431399