Role of Magnesium

[Fuzzywuzy]

I understand that Magnesium 'poisons' the surface of Calcium Carbonate crystals, preventing further precipitation.

However, have read also the Mg preferentially attaches to Carbonate ions in the water, therefore reducing it's availability to coral.

Is the latter statement true? If one increases Magnesium in the water column, does it mean that while it reduces precipitation of CaCO3, it also denies coral the CO3?

 

[SPR1968]

I can’t answer the specific question, but I can bump the thread to see if we can get you some help

 

[Fuzzywuzy]

Bump!

I can’t answer the specific question, but I can bump the thread to see if we can get you some help

Thanks! I've read one of @Randy Holmes-Farley article wherein he states that magnesium carbonate isn't supersaturated in a normal reef tank. That I understand but I'm wondering what the impact of raising magnesium is - there must be some form of equilibrium where the stability (i.e. preventing CaCO3 precipitation) gained is outweighed by MgCO3 formation such that the alkalinity boost is no longer accessible to coral.

Unless of course those concentrations are so far out there that it doesn't matter?

 

[Ron Reefman]

I understand that Magnesium 'poisons' the surface of Calcium Carbonate crystals, preventing further precipitation.

However, have read also the Mg preferentially attaches to Carbonate ions in the water, therefore reducing it's availability to coral.

Is the latter statement true? If one increases Magnesium in the water column, does it mean that while it reduces precipitation of CaCO3, it also denies coral the CO3?

I have never read that Mg does either of the things you listed, although they maybe true and I just never heard of it. My understanding is that Mg is useful in, and in fact necessary for, coral to use Ca in building a skeleton.

After doing a simple search, Mg does prevent excessive precipitation of Ca. But it's primary use is skeleton building.

Here is a good article: https://orphek.com/magnesium-mg-and-its-role-in-the-reef-aquarium/

 

[Randy Holmes-Farley Verified Community Expert]

I understand that Magnesium 'poisons' the surface of Calcium Carbonate crystals, preventing further precipitation.

However, have read also the Mg preferentially attaches to Carbonate ions in the water, therefore reducing it's availability to coral.

Is the latter statement true? If one increases Magnesium in the water column, does it mean that while it reduces precipitation of CaCO3, it also denies coral the CO3?

There is a strong tendency for magnesium to make soluble 1:1 complexes with carbonate (MgCO3; 44% of carbonate) and carbonate also makes ion pairs with sodium (NaCO3-;16% ) and calcium (21%). I discuss that in my magnesium article linked below and the scientific paper linked at the bottom gives the hard numbers.

But it is not clear whether corals use bicarbonate or carbonate (or both; it is technically hard to distinguish them in this context) and the binding of magnesium to bicarbonate is much less.

Does elevated magnesium (compared to NSW magnesium levels) tend to reduce the "free" concentration of carbonate? yes, but that effect can be offset by higher than NSW alk levels that boost the free bicarbonate and carbonate, or higher than normal pH that boosts free carbonate and lowers free bicarbonate.

Aquarium Chemistry: Magnesium In Reef Aquaria ? Advanced Aquarist | Aquarist Magazine and Blog

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1974.19.2.0223