After the previous topic on kalkwasser and pH, I thought it’d be interesting to follow that up with a deeper dive into how corals actually make skeleton. We all know that they take calcium from the water and combine it with carbonate to form skeleton. That is the process we call calcification, and it happens a lot faster when the lights are on.
Usual preface: I’m not a scientist. This is my own hobby-grade interpretation of published research combined with my personal experience, not a review paper. Details vary by species and some of this is still being worked out. But the broad picture is conclusive enough to be meaningful to the curious minds in the hobby.
The basic ingredients
Coral skeleton is mostly aragonite, a form of calcium carbonate. To make it, the coral needs two things from the surrounding water:
- Calcium ions (Ca²⁺)
- Carbonate ions (CO₃²⁻)
The coral does not just pull them out of the water and stick them together. It moves them to a very specific place under its tissue and creates conditions that is characterized by extremely high pH to make the skeleton form quickly.
How calcium gets inside
Calcium first enters the coral by diffusion through the oral cavity, the open mouth of the polyp. From there it moves into the internal fluid (the coelenteron). The final and most energy-demanding step is active transport across the calicoblastic layer, the thin sheet of cells that sits directly against the skeleton. That is the construction site where calcium carbonate is actually laid down.
Calcium drifts in on its own, but the last step across the calicoblastic layer is pumped, and so are the hydrogen ions going the other way. Stripping those hydrogens out raises the pocket to around pH 9.3, which turns bicarbonate into carbonate and makes calcium carbonate settle onto the skeleton. This requires energy.
Studies on Galaxea fascicularis showed that calcium levels drop inside the polyp and coelenteron during the day while they rise under the calicoblastic layer. When the lights go off, the pattern reverses. The coral is actively pumping calcium toward the skeleton while it has energy available from photosynthesis by the algae.
Why light makes a big difference
Calcification is roughly three times faster in the light than in the dark. There are a few reasons for this.
Photosynthesis by the zooxanthellae (the algae living inside the coral) consumes carbon dioxide. When CO2 is removed, the pH in the tissue rises. Higher pH shifts more of the available carbon toward the carbonate form that the coral can use for skeleton.
At the actual site of calcification, the coral pushes the pH even higher. Research showed the fluid under the calicoblastic layer becomes more alkaline than the surrounding water. That higher pH makes it much easier for calcium carbonate to precipitate. In one study the saturation state for aragonite increased almost eight times under these conditions, allowing rapid skeleton formation.
The coral also has more energy during the day. Respiration was measured at twelve times higher in the light than in the dark in Galaxea. ATP levels, the cell’s energy currency, were about 35% higher when the coral was lit. That extra energy powers the pumps that move calcium into the skeleton.
What happens at night
Photosynthesis stops when the lights go out, so energy becomes limited and the pH no longer rises from the removal of CO2. Respiration continues, releasing CO2, which lowers pH.
Without the energy and the pH boost, calcification slows way down. Some of the calcium that was being moved toward the skeleton builds up again in the polyp tissues until the next day. In practical terms, corals do most of their skeletal growth during the light period, consuming the bulk of their calcium and alkalinity while the lights are on.
The role of the algae
The coral supplies the algae with nutrients from its own waste. The algae produce sugars through photosynthesis and pass a large share of them back to the coral. Those sugars fuel the energy-demanding work of calcification. Without the algae, the coral would have far less energy available during the day and would grow much more slowly.
What this means for us
There are two takeaways here: the time of supplementing and night time pH swing.
Calcium levels drop inside the polyp and coelenteron while the lights are on (the calcium is being moved into the skeleton). When the lights go off, the concentration there rises again, mainly by passive diffusion from the surrounding seawater until a new balance is reached.
This suggests the critical time of consumption of calcium and alkalinity is during the light period, peaking just after mid-day. But this is not to say to stop dosing at night. Stability still matters. Corals are still calcifying in the dark, just at a much lower rate, and still infusing calcium passively. Research shows in-the-dark calcification held nearly constant for the first 4 hours, dropped about 25% by 12 hours, and by 24 hours had fallen to roughly 10% of the original rate.
For those that worry about pH swing at night, this suggests that it’s less of a concern than many of us once thought. And in practice, all reef tanks experience pH swings to some degree. That’s just the nature of a closed system. Yes, lower pH shifts more carbon toward bicarbonate and carbonic acid, which makes it harder for calcification. But research shows the coral is already slowing its skeletal growth at night and can partly compensate by raising pH inside its own tissue, the key is that it just costs the coral extra energy to do that. So keeping pH stable does make the job easier for the coral by reducing the work of pumping out hydrogen ions, just not as crucial as we initially thought.
References
Al-Horani, F.A., Al-Moghrabi, S.M. and de Beer, D. (2003). The mechanism of calcification and its relation to photosynthesis and respiration in the scleractinian coral Galaxea fascicularis. Marine Biology 142: 419-426.
Microsensor measurements inside the polyp. Source for the calcium levels reversing between the coelenteron and the calicoblastic layer in light and dark, the raised pH at the site of calcification, the almost eightfold rise in aragonite saturation, respiration running twelve times higher in the light, and the 35% ATP difference.
Al-Horani, F.A., Tambutté, É. and Allemand, D. (2007). Dark calcification and the daily rhythm of calcification in the scleractinian coral Galaxea fascicularis. Coral Reefs 26: 531-538.
A ⁴⁵Ca tracer study following calcification through the day and into darkness. Source for night calcification continuing at about half the daytime rate, for the daily rhythm peaking in mid-afternoon, and for dark calcification being governed by the coral’s stored energy reserves and by the oxygen available in the surrounding water.
Gattuso, J.-P., Allemand, D. and Frankignoulle, M. (1999). Photosynthesis and calcification at cellular, organismal and community levels in coral reefs: a review on interactions and control by carbonate chemistry. American Zoologist 39: 160-183.
A review covering calcification from the cell up to the whole reef. Source for the calcium route: diffusion in through the oral cavity, then active transport across the calicoblastic layer. Also the source for calcification running roughly three times faster in light than in dark.