Why does calcium deficiency kill hard corals?

[davidflagg]

For me this has been a hard question to google so I'm bringing it here. I've only ever kept softies, so I'm unfamiliar with all the nuances of maintaining hard corals. In my inexperienced opinion, they seem to be ironically way more sensitive than softies despite having tougher exteriors.

What exactly happens when you stop feeding calcium to hard corals? I know euphyllias will definitely die without constant calcium, I'm wanting to know specifically about encrusting or reef-building corals. I'm sure "they'll die" is the answer, but why? Wouldn't they simply stop adding onto their skeletons when there's no calcium left to build with? Why would this starve them? Most of these animals are still photosynthetic or at least capture their own food. In my head it makes sense that you could manage the growth of a hard coral by controlling how much calcium it gets. It reaches a certain size, stop feeding, it stops growing. What exactly is it about calcium deficiency that kills them completely?

 

[Miami Reef]

Good question. Following.

 

[legionofdoon]

Zombie thread, but I've heard that high all and low nitrate causes burnt tips because the coral lays down calcium carbonate faster than it can produce tissue. Maybe low calcium is the metabolic opposite. They produce too much tissue without a place to put it and it short circuits their processes.

 

[rogueshrimp88]

The same reason adults still need calcium. It's not just used for growth but also to mitigate wear and tear. Without calcium the skeletal structure becomes brittle.

 

[Pridedcloth3]

The same reason adults still need calcium. It's not just used for growth but also to mitigate wear and tear. Without calcium the skeletal structure becomes brittle.

But then you bring in LPS in which some species separate from the calcium skeleton and still survive.

 

[Spare time]

But then you bring in LPS in which some species separate from the calcium skeleton and still survive.

I think thats a high risk scenario where the coral is doing it as a last act of survival. My guess is it probably has low survival rates but better than staying in the spot/skeleton where its dying.

 

[rogueshrimp88]

Calcium is still a necessary part of biological life. Just because a soft coral doesn't have a stony part doesn't mean it's not using it, just not as much as a stony coral is or in the same way. Going back to humans as an analog, it's not just bones we use it for. It used for platelets, muscles, nerve transmission, etc. There's likely a biological process that coral use calcium for other than just skeleton building. Those ones that have detached aren't doing to have a good, it's a last ditch effort to escape and possibly survive somewhere else. Most don't survive the process.
Also I might add, trying to use a deficiency of a necessary component for a living creature to stop it from growing beyond what you deem is an appropriate size is animal abuse. It's like putting a goldfish in a 2 gallon bowl and claiming "they grow to the size of their environment".

 

[Tony Thompson]

I think you need to go back a question before you try to answer you last question.

Is mortality of corals certain if there is a limitation of calcium in the water column, and if so what is the minimum level of calcium that would be fatal., are there other factors that determine what that level of calcium is required including species and rates of photosynthesis.

You dont feed calcium in a reef tank, you simply maintain the level commonly found in natural seawater, the same as a number of other elements. This makes the element available to a number of organisms.

Certain coral species can survive in much lower alkalinity and calcium levels than we generally advise for a reef tank. As you mentioned Euphyllia, I have had them survive for very long periods at depleted levels of Alk and Cal that are advised. What I did notice is that compared to regular alk and calcium maintenance a proliferation of small polyp growth along the skeleton but no division of the main coral polyps as is seen when you drive calcification.

I can not see scenario where they would be no detectable levels of calcium in a reef tank under normal conditions.

 

[Tony Thompson]

Just out of interest I found this particular paper that covers

Effects of calcium-free and low-calcium artificial seawater on polyps of a scleractinian coral Galaxea fascicularis​

 

[Tony Thompson]

For those not willing to read the whole article I will give a brief personal comprehension.

The study looked at the release of mucocytes (mucus production) of the particular coral species Galaxea fascicularis. Similar to the species commonly found in the hobby.

The levels of calcium in the water under test was Calcium 0 to 6 mmol l–1
which I calculated to 0 to 240 ppm Calcium

The number of +’s indicates the degree of the response as judged visually, where five +’s represents the maximum response. NE no visible effect. The increased release of mucocytes over the given time period being a proposed indication in coral stress response.

Ca concentration

0 +++++
1 mmol l–1 (40ppm) +++++
2 mmol l–1 (80ppm) ++++
3 mmol l– 1 (120ppm) +++
4 mmol l– 1 (160ppm) ++
5 mmol l–1 (200ppm) +
6 mmol l– 1 (250ppm) NE

Typical levels of calcium suggested in hobby reef tanks 350 - 450 ppm.

​

 

[Randy Holmes-Farley]

In the extreme case of little or no calcium in the water, coral skeletons will dissolve, which would kill any hard coral.

 

[twentyleagues]

Like Randy and others said in a reef tank there is always going to be calcium in the water, It's a matter of where it comes from. If the available calcium gets too low and the ph and alk follow that trend the calcium (alk and ph too) will come from somewhere like sand, rocks, coral skeletons. Obviously you dont want living corals to be giving up their calcium this will cause them to die. Better to keep levels within the proper range so this does not happen.

 

[Tony Thompson]

In the extreme case of little or no calcium in the water, coral skeletons will dissolve, which would kill any hard coral.

What level of calcium would you regard low enough that would start the process of the coral skeleton dissolving?

To stay on point of the O/P, if we take the average level of calcium in typical artificial seawater mix. Is the constant maintenance of calcium levels by way of addition critical to the survival of the average stony coral species. Especially if other driving forces of calcification and the rate of uptake are not at enhanced levels.

Are there situations where calcium levels would be depleted enough in typical synthetic seawater aquarium without maintenance of levels of calcium that would directly cause coral morality due to dissolving skeleton?

 

[Randy Holmes-Farley]

What level of calcium would you regard low enough that would start the process of the coral skeleton dissolving?

To stay on point of the O/P, if we take the average level of calcium in typical artificial seawater mix. Is the constant maintenance of calcium levels by way of addition critical to the survival of the average stony coral species. Especially if other driving forces of calcification and the rate of uptake are not at enhanced levels.

Are there situations where calcium levels would be depleted enough in typical synthetic seawater aquarium without maintenance of levels of calcium that would directly cause coral morality due to dissolving skeleton?

Depends on alk and pH. At pH 8.2 and alk of 6 dKH, you’d need to be below 200 ppm. But at pH 7.7, it can be happening even at 400 ppm.

 

[Tony Thompson]

Depends on alk and pH. At pH 8.2 and alk of 6 dKH, you’d need to be below 200 ppm. But at pH 7.7, it can be happening even at 400 ppm.

Thanks for getting back to me Randy, appreciate it.

Yes I understand the other parameters and the effects, especially the effects of pH on the corals skeleton.

However as the O/P stated this was specifically to do with Calcium levels and the perception of the O/P that calcium needs to be added to keep the coral alive, with no reference to levels of calcium.

This is why I included the reference to the paper.

The pH in these experiments where maintained within a range of pH8 - pH 8.2, the experiments where specifically designed to observe any short term physiological effects of depleted calcium levels (compared to natural seawater levels) specifically with regards the coral polyps of the particular spp. not the skeleton.

One particular consideration was the noted effects on membrane permeability. It would appear that calcium levels as low as 200ppm irrespective of other parameters , formed an observation of possible detrimental effects to the polyps themselves at cellular level.

I found it an interesting read as I am particularly interested in an holistic approach to aquarium keeping. Thankfully its a short read.