Questions on CO2...

[jgvergo]

I kept planted freshwater aquarium for years and one of the best decisions I made was to add CO2 to the tank. The result was clear...explosive plant growth. CO2 is an essential ingredient for photosynthesis and acts as a fertilizer in freshwater aquariums.

That made me wonder if CO2 would have similar effects in a reef tank. Since zooxanthellae are photosynthetic and have a symbiotic relationship with corals, would adding CO2 improve the health of the zooxanthellae and, by extension, the corals?

The potential downside is that CO2 will lower the PH. However, limiting the addition of CO2 to when the DT lights are on might help reduce PH swings too.

I'm sure I'm not the first to go down this path. What is the consensus on attempting something like this?

 

[Brew12]

I kept planted freshwater aquarium for years and one of the best decisions I made was to add CO2 to the tank. The result was clear...explosive plant growth. CO2 is an essential ingredient for photosynthesis and acts as a fertilizer in freshwater aquariums.

That made me wonder if CO2 would have similar effects in a reef tank. Since zooxanthellae are photosynthetic and have a symbiotic relationship with corals, would adding CO2 improve the health of the zooxanthellae and, by extension, the corals?

The potential downside is that CO2 will lower the PH. However, limiting the addition of CO2 to when the DT lights are on might help reduce PH swings too.

I'm sure I'm not the first to go down this path. What is the consensus on attempting something like this?

Adding CO2 with the lights on would help remove the peak from the pH swing but higher pH's are normally associated with better coral growth. The implication of this is that lower CO2 levels improve coral growth.

 

[jgvergo]

Adding CO2 with the lights on would help remove the peak from the pH swing but higher pH's are normally associated with better coral growth. The implication of this is that lower CO2 levels improve coral growth.

It's clear that CO2 is required for coral growth b/c zooxanthellae are photosynthetic. If we accept that as a fact, I'd like to know a few things:
1) What is the optimal level of CO2
2) How can we measure the level of CO2 (I guess PH is a proxy, but it's certainly not an exact way of measuring it)
3) What is the best CO2 dosing schedule
4) What is the best means to dose CO2 (I would imagine running a CO2 line into a skimmer would really help dissolve it into the water)
5) Is this just a bad idea?

 

[Brew12]

5) Is this just a bad idea?

I'm going with this one.

pH in a reef tank is mainly impacted by 2 factors. Alkalinity and CO2. Most reefers support the concept of alkalinity being best to keep as stable as possible. At this point, CO2 becomes the only variable impacting pH.

I believe I have seen graphs correlating Alk, CO2 and pH but I couldn't find it the last time I looked. It isn't uncommon at all for people to run CO2 scrubbers on their skimmer inlets or run their skimmer air lines outside to get less CO2 rich air. Excess CO2 with low pH is a common problem. I'm not sure I've ever heard of anyone purposely lowering their pH by adding CO2.

 

[Randy Holmes-Farley Verified Community Expert]

I kept planted freshwater aquarium for years and one of the best decisions I made was to add CO2 to the tank. The result was clear...explosive plant growth. CO2 is an essential ingredient for photosynthesis and acts as a fertilizer in freshwater aquariums.

That made me wonder if CO2 would have similar effects in a reef tank. Since zooxanthellae are photosynthetic and have a symbiotic relationship with corals, would adding CO2 improve the health of the zooxanthellae and, by extension, the corals?

The potential downside is that CO2 will lower the PH. However, limiting the addition of CO2 to when the DT lights are on might help reduce PH swings too.

I'm sure I'm not the first to go down this path. What is the consensus on attempting something like this?

It's a complex question that likely has a different answer than a freshwater system. Substantially lowering pH is generally expected to cause a reduction in the growth rate of hard corals, hence much of the concern about elevating atmospheric CO2 levels and coral reef health.

Generally, many many marine organisms can get their carbon from bicarbonate, not just from CO2, which, as you note, is in short supply. But there is a ton of bicarbonate around.

I discuss those issues here:

Photosynthesis and the Reef Aquarium, Part I: Carbon Sources by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2006-10/rhf/index.php

This very recent article summarizes the issues:

Inorganic carbon is scarce for symbionts in scleractinian corals
http://onlinelibrary.wiley.com/doi/10.1002/lno.10550/full

"Corals have pathways to deliver DIC to their symbionts, and though the full details of DIC processing pathways have not been determined they include
the ability to take up CO2 and HCO23 , interconversion ofCO2 and HCO23 using carbonic anhydrases, and transport of DIC through various tissue layers"

"Symbiodinium has a CO2 concentrating mechanism (CCM) for the uptake and concentration of DIC employing carbonic anhydrases and active HCO23 transport"

"Despite these adaptations, there is some disagreement over whether seawater DIC concentrations limit photosynthetic rates in corals."

 

[Brew12]

Not sure if you have read this, but I found it very helpful.
http://www.reefkeeping.com/issues/2004-09/rhf/

 

[Randy Holmes-Farley Verified Community Expert]

It's clear that CO2 is required for coral growth b/c zooxanthellae are photosynthetic. If we accept that as a fact, I'd like to know a few things:
1) What is the optimal level of CO2
2) How can we measure the level of CO2 (I guess PH is a proxy, but it's certainly not an exact way of measuring it)
3) What is the best CO2 dosing schedule
4

IMO, it is almost never useful to dose CO2 (i.e., lower the pH) to a reef tank, and very often it is desirable to remove it and raise the pH.

FWIW, pH is an EXACT way to measure CO2, if you know the carbonate alkalinity. Any two of the three gives the third mathematically.

 

[jgvergo]

IMO, it is almost never useful to dose CO2 (i.e., lower the pH) to a reef tank, and very often it is desirable to remove it and raise the pH.

FWIW, pH is an EXACT way to measure CO2, if you know the carbonate alkalinity. Any two of the three gives the third mathematically.

Thanks Randy. I will cease and desist on this line of thinking