Coral pH Limits?

[ISpeakForTheSeas]

Hello everyone!
All the threads recently discussing ocean acidification got me wondering: what are the survival/growth limits of corals as related to pH level?

To give some context, from what I've seen, we know that corals can grow at pH levels ranging from ~7.6-8.5, with faster growth typically seen somewhere around 8.2-8.3 and up. As I understand it, that faster growth is at least partially due to lower amounts of hydrogen ions in the water binding with carbonate ions and essentially competing with the corals for skeleton-building resources, but I also understand this is only part of the picture and that this specific phenomenon is largely correlated with higher levels of CO2 (which, while not necessarily directly pH related, is still of interest for my question here, as I'm looking more at generalities relating to corals and pH and less at specifics). Additionally, I understand that some corals (particularly more "boulder"-like corals) are better able to cope with lower pH waters, likely due to their growth forms.

So, what I'm wondering here is, generally speaking (i.e. addressing all/most corals as a group rather than addressing specific coral species, which would likely have considerable variation between them), how low and how high could the pH go before the corals stop growing altogether, and how low and high could it go before the corals start dying from the effects? What are some other pH related phenomena that may impact coral growth/survival (like the high CO2 levels generating higher hydrogen ion concentrations discussed above)? Are there things we could do (in a controlled, aquarium study sort of setting) to work around/overcome these effects and promote coral growth and/or survival beyond their natural limits?

For examples:
-Assuming all other measurable conditions are acceptable, could a coral survive/grow at a pH of 5.8? How about at a pH of 10+? (What are the absolute pH limits for coral growth/survival without intervention?)
-In a high CO2 environment (which would presumably be relatively low pH compared to average ocean/aquarium levels), with the hydrogen ions binding with carbonate ions and reducing resources for coral skeleton growth, could we do something like dose carbonate to continue helping corals grow (or grow faster) despite the limitations caused by the high concentration of hydrogen ions?

I know this is really multiple questions in one, but I'm still getting a handle on the chemistry side of things with marine environments, and I view these things as all being related. So, I figured I would group them together for this thread.

As a note here, I am not looking to discuss politics/policies, energy production, businesses/business practices, climate change, or related topics with this thread (there are plenty of threads about those issues already), I'm literally just looking at the biological and chemical limitations (and potential solutions for those limitations) for corals and their environments as they relate to pH and coral growth/survival.

 

[LegalReefer]

Entirely without any scientific basis or context, but I believe many corals are far hardier than we often want to give them credit for. Different and "unnatural" water qualities and chemical levels can be adapted to given time. The diversity of different water qualities people maintain their aquariums at gives me reason to believe that there are a wide variety of different levels these amazing animals can survive at, though they may not necessarily thrive.

 

[Randy Holmes-Farley]

I do not think we have good info on how high the pH can get because logistically, precipitation of calcium carbonate becomes a problem as pH rises much above 8.5 and makes controlling other factors like alkalinity tricky. I've never seen a single controlled study of that.

 

[ISpeakForTheSeas]

I do not think we have good info on how high the pH can get because logistically, precipitation of calcium carbonate becomes a problem as pH rises much above 8.5 and makes controlling other factors like alkalinity tricky. I've never seen a single controlled study of that.

Any info on how low, by chance?

 

[Reefahholic]

I know Chummingham's Reef was running happily at 8.8 or so. As far as how low….I don’t think many have entered those waters. I’ve personally had pH dip as low as 7.5 and nothing died.

 

[Randy Holmes-Farley]

Any info on how low, by chance?

Lots more info on low since scientists study it to understand future events related to CO2 accumulation.

Calcification (skeletal growth) slows considerably as the pH peak drops lower than about 7.8-7.9.

Below about pH 7.7, coral skeletons can very slowly dissolve.

 

[ISpeakForTheSeas]

Lots more info on low since scientists study it to understand future events related to CO2 accumulation.

Calcification (skeletal growth) slows considerably as the pH peak drops lower than about 7.8-7.9.

Below about pH 7.7, coral skeletons can very slowly dissolve.

To quote the Smithsonian*:
"If there are too many hydrogen ions around and not enough molecules for them to bond with, they can even begin breaking existing calcium carbonate molecules apart—dissolving shells that already exist."
So, the slowed growth and skeleton dissolution that you mention is caused by the hydrogen ions breaking the calcium carbonate bonds of the coral skeletons to be able bond with the carbonate in them, not necessarily due to the pH itself, correct?
* https://ocean.si.edu/ocean-life/invertebrates/ocean-acidification

 

[J1a]

To quote the Smithsonian*:
"If there are too many hydrogen ions around and not enough molecules for them to bond with, they can even begin breaking existing calcium carbonate molecules apart—dissolving shells that already exist."
So, the slowed growth and skeleton dissolution that you mention is caused by the hydrogen ions breaking the calcium carbonate bonds of the coral skeletons to be able bond with the carbonate in them, not necessarily due to the pH itself, correct?
* https://ocean.si.edu/ocean-life/invertebrates/ocean-acidification

PH is a measurement of the concentration of hydrogen ions.

 

[Aspect]

Lots more info on low since scientists study it to understand future events related to CO2 accumulation.

Calcification (skeletal growth) slows considerably as the pH peak drops lower than about 7.8-7.9.

Below about pH 7.7, coral skeletons can very slowly dissolve.

So would soft corals' growth rates be affected less by pH levels?

 

[ISpeakForTheSeas]

PH is a measurement of the concentration of hydrogen ions.

*Facepalm* - the fact that I am not a chemist is very much so on display, haha. Thank you for the information.

That being the case, though, to avoid the skeleton dissolution, could one simply dose carbonate to the tank so that the corals have enough carbonate to bond to their calcium and the hydrogen doesn't immediately bond with them all and begin breaking down the corals?

 

[J1a]

*Facepalm* - the fact that I am not a chemist is very much so on display, haha. Thank you for the information.

That being the case, though, to avoid the skeleton dissolution, could one simply dose carbonate to the tank so that the corals have enough carbonate to bond to their calcium and the hydrogen doesn't immediately bond with them all and begin breaking down the corals?

In a way, yes. This is exactly how a carbonate/bicarbonate buffer works to prevent excessive fluctuations of pH

 

[J1a]

So would soft corals' growth rates be affected less by pH levels?

Soft corals does not build calcium carbonate skeletons. But just like every organisms, there are optimum pH for carrying out whatever biological processes.

 

[Aspect]

Soft corals does not build calcium carbonate skeletons. But just like every organisms, there are optimum pH for carrying out whatever biological processes.

But pH wouldn't really affect soft corals growth rates? After it's in it's optimum range.

 

[J1a]

But pH wouldn't really affect soft corals growth rates? After it's in it's optimum range.

Isuppose if the pH of the water gets too low, soft corals probably need to spend more energy to maintain its internal pH, leaving less energy for growth.

 

[Randy Holmes-Farley]

To quote the Smithsonian*:
"If there are too many hydrogen ions around and not enough molecules for them to bond with, they can even begin breaking existing calcium carbonate molecules apart—dissolving shells that already exist."
So, the slowed growth and skeleton dissolution that you mention is caused by the hydrogen ions breaking the calcium carbonate bonds of the coral skeletons to be able bond with the carbonate in them, not necessarily due to the pH itself, correct?
* https://ocean.si.edu/ocean-life/invertebrates/ocean-acidification

That's a very poor way for them to describe it (IMO) since I do not believe it truly reflects the reality of what H+ does, but pH is just a direct measure of hydrogen ion concentration so there's no difference between saying pH is low and H+ concentration is high.

 

[Randy Holmes-Farley]

Isuppose if the pH of the water gets too low, soft corals probably need to spend more energy to maintain its internal pH, leaving less energy for growth.

I'm sure that it true at some pH (obviously, they will die at pH 1), but I expect the drop off is different (slower) than for calcifying corals.

Just a reminder to folks that some soft corals also make internal calcium carbonate structures (spicules). See figure 2, for example:

Variation in Morphology found within the Soft Corals

Morphological variations within the Soft Corals that provide protection from predation.

www.imagequest3d.com

 

[Randy Holmes-Farley]

*Facepalm* - the fact that I am not a chemist is very much so on display, haha. Thank you for the information.

That being the case, though, to avoid the skeleton dissolution, could one simply dose carbonate to the tank so that the corals have enough carbonate to bond to their calcium and the hydrogen doesn't immediately bond with them all and begin breaking down the corals?

Sort of. One can make the pH at which calcium carbonate stops dissolving lower and lower in pH by raising alkalinity higher and higher.

But a 0.3 pH unit drop in pH requires a doubling of alkalinity to accomplish that task.

That is also not the only reason low pH stresses hard corals.

Aquarium Chemistry: The Chemical and Biochemical Mechanisms of Calcification

All reef aquarists are versed in the fact that such organisms remove calcium and carbonate from the water column in order to provide materials for calcification. Exactly how calcification takes place, however, is rarely considered.

reefs.com

Implications for Reefkeeping: pH
It is well known in the scientific literature, if not in the reefkeeping hobby, that calcification is slowed considerably as the pH is lowered below natural levels. This result is especially concerning and is a hot topic of research because of the decrease in the pH of the oceans as CO2 is added to the atmosphere. The predictions of reduced calcification in coral reefs in the future are substantial. Again, taking the previous sections as a backdrop, we can begin to understand why.

As the pH of the external fluid is lowered, it becomes harder and harder for cells to excrete the excess protons that come about from calcification. That is, they take in bicarbonate, strip off a proton, precipitate the carbonate into their skeleton and then have to do something with that proton. Many of those protons can be used to make CO2 out of bicarbonate, and may thereby boost the rate of photosynthesis.1 Still, not all of the protons may be used this way, and some will be excreted.

In the model of McConneaughey and Whelan, protons are pumped into the calicoblastic epithelium, and then allowed to somehow move down the concentration gradient from the lower pH cell interior (higher proton concentration) to the coelenteron (higher pH, lower proton concentration). If the seawater pH drops, the coelenteron pH will likely also drop since it is exchangeable with the external fluid. As the pH drops in the coelenteron, the efflux of protons from the calicoblastic epithelium cells will be slowed as that process is likely gradient dependent. Finally, as the pH drops inside of the calicoblastic epithelium because protons are not leaving as readily, it becomes harder and harder for the Ca++/2H+ transporter to pump protons out and calcium into the ECF.

In the model of Furla et al where bicarbonate is actively pumped into the ECF, protons must either be actively pumped away, or they must passively diffuse away (the latter probably is not efficient enough to work as the ECF could never then have a pH higher than the coelenteron, something that would mitigate against supersaturation and precipitation of aragonite). In either case, the process may not function as well at lower coelenteron pH as the proton gradient between the ECF and the coelenteron becomes smaller or more likely, bigger in the wrong direction. That is, protons will be less inclined to leave the ECF for the coelenteron. Consequently, the supersaturation of calcium carbonate, which is highly dependent on pH, will decrease, and hence calcification will also decrease.

 

[ISpeakForTheSeas]

Sort of. One can make the pH at which calcium carbonate stops dissolving lower and lower in pH by raising alkalinity higher and higher.

But a 0.3 pH unit drop in pH requires a doubling of alkalinity to accomplish that task.

That is also not the only reason low pH stresses hard corals.

Aquarium Chemistry: The Chemical and Biochemical Mechanisms of Calcification

All reef aquarists are versed in the fact that such organisms remove calcium and carbonate from the water column in order to provide materials for calcification. Exactly how calcification takes place, however, is rarely considered.

reefs.com

Implications for Reefkeeping: pH
It is well known in the scientific literature, if not in the reefkeeping hobby, that calcification is slowed considerably as the pH is lowered below natural levels. This result is especially concerning and is a hot topic of research because of the decrease in the pH of the oceans as CO2 is added to the atmosphere. The predictions of reduced calcification in coral reefs in the future are substantial. Again, taking the previous sections as a backdrop, we can begin to understand why.

As the pH of the external fluid is lowered, it becomes harder and harder for cells to excrete the excess protons that come about from calcification. That is, they take in bicarbonate, strip off a proton, precipitate the carbonate into their skeleton and then have to do something with that proton. Many of those protons can be used to make CO2 out of bicarbonate, and may thereby boost the rate of photosynthesis.1 Still, not all of the protons may be used this way, and some will be excreted.

In the model of McConneaughey and Whelan, protons are pumped into the calicoblastic epithelium, and then allowed to somehow move down the concentration gradient from the lower pH cell interior (higher proton concentration) to the coelenteron (higher pH, lower proton concentration). If the seawater pH drops, the coelenteron pH will likely also drop since it is exchangeable with the external fluid. As the pH drops in the coelenteron, the efflux of protons from the calicoblastic epithelium cells will be slowed as that process is likely gradient dependent. Finally, as the pH drops inside of the calicoblastic epithelium because protons are not leaving as readily, it becomes harder and harder for the Ca++/2H+ transporter to pump protons out and calcium into the ECF.

In the model of Furla et al where bicarbonate is actively pumped into the ECF, protons must either be actively pumped away, or they must passively diffuse away (the latter probably is not efficient enough to work as the ECF could never then have a pH higher than the coelenteron, something that would mitigate against supersaturation and precipitation of aragonite). In either case, the process may not function as well at lower coelenteron pH as the proton gradient between the ECF and the coelenteron becomes smaller or more likely, bigger in the wrong direction. That is, protons will be less inclined to leave the ECF for the coelenteron. Consequently, the supersaturation of calcium carbonate, which is highly dependent on pH, will decrease, and hence calcification will also decrease.

If I understand this correctly, if one were to theoretically add a sufficient amount of carbonate ions to a low pH tank without raising the Alkalinity, the coral skeletons would still dissolve at the lower pH, but - with the added carbonate - the corals would also still at least attempt to grow; they would just be hindered by the increased difficulties associated with the lower pH - such as consistent, slow and steady dissolution of their skeleton and the difficulty of removing the excess protons brought about by the calcification process - yes?

 

[Randy Holmes-Farley]

If I understand this correctly, if one were to theoretically add a sufficient amount of carbonate ions to a low pH tank without raising the Alkalinity, the coral skeletons would still dissolve at the lower pH, but - with the added carbonate - the corals would also still at least attempt to grow; they would just be hindered by the increased difficulties associated with the lower pH - such as consistent, slow and steady dissolution of their skeleton and the difficulty of removing the excess protons brought about by the calcification process - yes?

What you are suggesting is impossible. Carbonate is an alkalinity additive and it also raises pH. you cannot add it and have alk and pH unchanged.

That poor Smithsonian statement is leading you down the wrong path. I don’t think it is worthwhile to try to follow it into an understanding of corals and pH.

 

[ISpeakForTheSeas]

What you are suggesting is impossible. Carbonate is an alkalinity additive and it also raises pH. you cannot add it and have alk and pH unchanged.

That poor Smithsonian statement is leading you down the wrong path. I don’t think it is worthwhile to try to follow it into an understanding of corals and pH.

Good to know. Forgive me for asking so many likely ignorant questions here (most chemistry info I find in my personal research is very technical jargon heavy and - as I don’t have a chemistry background - rather difficult to understand as a result), but does raising Alkalinity always raise pH? I ask because, to this point, I’ve just heard that pH is essentially the ”important” number while Alkalinity is basically just important because it determines how capable something is of resisting pH change (specifically pH lowering). I know that’s a gross-oversimplification of things, but that’s basically how I’ve seen it explained.

Also, to be able to understand the actual relationship between pH and Alkalinity better, would you happen to either be willing to explain it simply here or to point me in the direction of a source or two that might explain things simply (preferably in layman’s terms or nearly so) but in some detail?