Calcification and aragonite

[Cory]

I found this article. What does it mean exactly? Particularly Ω


Using living corals collected from Okinawan coral reefs, laboratory experiments were performed to investigate the relationship between coral calcification and aragonite saturation state (Ω) of seawater at 25°C. Calcification rate of a massive coral Porites lutea cultured in a beaker showed a linear increase with increasing Ωaragonite values (1.08–7.77) of seawater. The increasing trend of calcification rate (c) for Ω is expressed as an equation, c = aΩ + b (a, b: constants). When Ω was larger than ∼4, the coral samples calcified during nighttime, indicating an evidence of dark calcification. This study strongly suggests that calcification of Porites lutea depends on Ω of ambient seawater. A decrease in saturation state of seawater due to increased pCO2 may decrease reef-building capacity of corals through reducing calcification rate of corals.

 

[Cory]

I found this
Aragonite saturation state
A measure of the thermodynamic tendency for the mineral calcium carbonate to form or to dissolve. The symbol Ω (omega) is often used as shorthand for "calcium carbonate saturation state". By convention, Ω is usually expressed with respect to aragonite, one of the two most abundant forms of calcium carbonate in the ocean. An aragonite saturation state (Ωar) greater than 1.0 indicates supersaturation, while values less than 1.0 indicate undersaturation with instability favoring dissolution. Recent research shows that calcifying organisms require Ωar well in excess of 1.0 to optimally produce shells or skeletons. Levels below 1.0 are considered corrosive, and skeletons and shells may be subject to dissolution.

 

[Randy Holmes-Farley Verified Community Expert]

Omega is the degree of saturation of a compound, such as aragonite. 1 means it is exactly saturated. Above one means there is more in solution that is stable long term (infinite long term). Less than 1 means aragonite can dissolve.

It depends on many factors, such as calcium (double calcium doubles omega), alkalinity (double alk doubles omega), and pH (0.3 pH unit rise about doubles omega).

 

[Larry L]

Omega is the degree of saturation of a compound, such as aragonite. 1 means it is exactly saturated. Above one means there is more in solution that is stable long term (infinite long term). Less than 1 means aragonite can dissolve.

It depends on many factors, such as calcium (double calcium doubles omega), alkalinity (double alk doubles omega), and pH (0.3 pH unit rise about doubles omega).

What range of omega would be typical for a reef tank? Is it possible (or healthy for the corals long-term) to run a reef tank in the range mentioned in the study (1.08–7.77)? I could see where this would be interesting to know for e.g. a frag farmer who might have fastest growth possible as a primary goal.

 

[Randy Holmes-Farley Verified Community Expert]

What range of omega would be typical for a reef tank? Is it possible (or healthy for the corals long-term) to run a reef tank in the range mentioned in the study (1.08–7.77)? I could see where this would be interesting to know for e.g. a frag farmer who might have fastest growth possible as a primary goal.

It varies with pH and alk, from 1 or a bit less on the low pH(7.8) and alk end (6-7 dKH) to a high of more than 10 at a pH of 8.5 and 11 dKH. Normal seawater at pH 8.2 is about 4.

I do not think growth scales with omega necessarily, since that does not address what is limiting growth. For example, raising calcium from 400 to 800 ppm doubles the omega, but is unlikely to spur hard coral growth since calcium at 400 ppm is not limiting calcification.