CO2 revelations part 2#

[Rex Grigg]

I have been following this thread with interest.

Here are my thoughts.

Little bitty CO2 bubbles can't/won't last long in the water column. The bubbles we are seeing therefor are probably not CO2.

The bubbles, whatever they are, are a great indicator of water flow. We see bubbles we think are CO2 being blown all around the tank and think it's the CO2. But I feel it's more a function of the increased water movement.

[stcyrwm]

Originally Posted by Rex Grigg

I have been following this thread with interest.

Here are my thoughts.

Little bitty CO2 bubbles can't/won't last long in the water column. The bubbles we are seeing therefor are probably not CO2.

The bubbles, whatever they are, are a great indicator of water flow. We see bubbles we think are CO2 being blown all around the tank and think it's the CO2. But I feel it's more a function of the increased water movement.

A lot of this discussion is out of my league but I can say for sure that CO2 bubbles can last in the water column at least long enough to get blown around the tank. I just set up CO2 and I'm using a diffuser disc so I know my bubbles aren't anything else. I think Tom said he had already done this also to rule out other gases.

[dennis]

Since laws dictate that the micro bubbles are not CO2 after ~3 seconds, logically they are N2 and O2.

SCMurphy's comment about increased flow improving growth, along with the saturated O2 levels and micro bubbles of N2/O2 led me to wonder something.....

Either the improvment in growth is due to increased CO2 levels (assuming that freshwater macrophytes are not photosynthetically saturated at 30mg/l CO2) or it is due to presence of elevated O2/N2 levels near the leaf's surface (Pradtl boundry)

Let's remember this, Tom reported improved growth simple by placing a diffuser in the stream from his filter. He had not increased flow so the answer here is not improved/inceased water flow. The flow remained the same in his original situation. Therefore, the improvment in growth comes from either higher CO2 levels or from something displaced/swapped by the micro bubbles.

How would micro bubbles of O2 or N2 affect the plant's grwoth?
If the growth improvment was due to an increase in dissolved CO2 (must be dissolved as the microbubbles can't be CO2, following laws of physics) wouldn't that show up as a decrease in the pH of the tank? Isn't it physically harder for a plant to optain C from CO2 gas rather than from carbonic acid?

If this is all entirely C related, would not one get the same improved growth from a higher concentration of dissolved CO2 in the water as they get from the micro bubbles? Unless, it is easier for the plant to break down CO2 gas (assuming a CO2 bubble actually gets to the plant)?

[jgc]

I have heard that emergent plants do not have the co2 issue as submerged plants. Even if the micro bubbles have no more c02 concentration than the atmophere - perhaps they help submerged plants to grow more like emergent ones. Personally I would love for someone to put an airpump with a micro bubller in front of the water current and see what happens.

Personally I am using a ladder diffuser and can say with pretty much certainty that larger bubbles of co2 do not diffuse completely in 3 seconds. They continue to shrink all the way up the ladder - a path that takes the better part of a minute. The bubbles are still shrinking the last 1/3 of the ladder - where they have already been in the tank for about 30 seconds. But anyway...

[SCMurphy]

Originally Posted by dennis

Since laws dictate that the micro bubbles are not CO2 after ~3 seconds, logically they are N2 and O2.

SCMurphy's comment about increased flow improving growth, along with the saturated O2 levels and micro bubbles of N2/O2 led me to wonder something.....

Either the improvement in growth is due to increased CO2 levels (assuming that freshwater macrophytes are not photosynthetically saturated at 30mg/l CO2) or it is due to presence of elevated O2/N2 levels near the leaf's surface (Pradtl boundary)

Let's remember this, Tom reported improved growth simple by placing a diffuser in the stream from his filter. He had not increased flow so the answer here is not improved/increased water flow. The flow remained the same in his original situation. Therefore, the improvement in growth comes from either higher CO2 levels or from something displaced/swapped by the micro bubbles.

That's not what I said. I said that:
"the dispersion of dissolved CO2 is better and the across tank gradient of dissolved CO2 in ppm becomes more uniform when the micro bubbles are put into a current."

So what Tom did by putting his diffuser under the outflow from his filter was increase the dispersion of the dissolved CO2 throughout the tank. I didn't forget it, I picked right up on it. I did not suggest he increased flow.

I can't find where I did say that a planted tank benefits from more flow rather than less to help plants overcome the diffusion of CO2 and nutrients. I'm sure I did at some point, someplace, since that's something I agree with, but it wasn't in this thread.

It's not the micro bubbles, they are a visual indicator of the current, nothing more. The bubbles have been dismissed, completely, as having any effect on plant growth.

[unirdna]

BTW, Dennis. First post! Welcome to the forum

[SCMurphy]

Originally Posted by unirdna

BTW, Dennis. First post! Welcome to the forum

Doh! Yeah hi Dennis, welcome, btw, Willy Bemis took a job with Cornell, not Yale. He's running the Shoals Marine Lab in the summers. I'm going to go visit him next summer so I can rediscover field work.

[jgc]

I just can not resist beating a dead horse.

Am I incorrect in thinking that 1) the concentration of c02 in the water exceeds the atmospheric concentration. 2) the concentration of c02 in the micro bubbles can not be less than the concentration of co2 in the water 3) hydroponics growers use enriched atmospheric co2 concentrations increase growth in terrestrial plants (or at least one recreational terrestrial plant), and 4) some aquatic plants find it easier to adsorb atmospheric co2 rather than dissolved co2)

Anyway, please correct me if any of those assumptions are incorrect.

It just seems to me that there has been way too little testing done to totally dismiss any possibility that Tom’s (sorry, I initially named the author incorrectly) initially observations of increased plant growth with a micro bubble diffuser placed in the current as complete hogwash.

---
Here are my bets given my previous assumptions. 1) In a non-enriched tank, micro-bubbles of compressed air will increase growth in some plants (those that normally will grow immersed) I am curious to what extent, if any, it will help normally submerged plants. 2) Plants that benefited in #1 will also benefit when co2 is introduced via micro-bubbles in an otherwise co2 enriched tank.

[unirdna]

1) False, atmosphere contains 300ppm CO2

2) False, concentration of microbubbles can be several thousand ppm CO2 (and still resist dissolving). If equilibrium of atmospheric CO2 (300ppm) is at .5ppm with water, then for a tank containing 30ppm CO2, that same gas equilibrium would be 18000 ppm. However, if the bubble is all CO2, it will dissolve completely, unless other gasses diffuse into the bubble. In order to avoid confusion, you need to fully understand what PPM means. PPM is a concentration level with respect to the whole, not a flat number. According to Henry's law, once the CO2 concentration is lowered to 18,000 ppm, it will no longer readily dissolve into water (that contains 30ppm CO2). If you have trouble wrapping your mind around this concept, consider the following. The air has 300ppm CO2. But can you increase CO2 to 300ppm in water by bubbling air into it? Certainly not. Gas laws are esoteric, but they are quite real.

3) True. But, consider the above post. The more CO2 you have in the air, the higher the equilibrium (ppm) that enriched air has with water. So, the water within the plant tissues will more readily "soak" up gasseous CO2 if the concentration in the air is higher.

4) Many aquatic plants attempt to interface with the atmosphere to gain access to more readily available CO2. Emergent plants have a very real advantage.

[SCMurphy]

Originally Posted by jgc

It just seems to me that there has been way too little testing done to totally dismiss any possibility that Rex’s initially observations of increased plant growth with a micro bubble diffuser placed in the current as complete hogwash.

It wasn't Rex it was Tom. We didn't dismiss the increased plant growth, we did a better job of explaining it.

[jgc]

Thank you for the corrections. I will edit my original post to protect the names of Tom & Rex

My points were basically. 1) If emergent plants have an advantage – in a non-co2 enriched tank micro-bubble of air might simulate emergent growth. 2) Emergent growth in co2 enriched air might be greater than in non-enriched air. 3) Persistent micro-bubbles in an enriched tank (even though they are primarily other gasses) should still have a greater than atmospheric concentration of co2.

Will add that even if we inject micro bubbles of air into an enriched tank, co2 dissolved in the water would try to achieve equilibrium with the air in the bubble and the bubble would end up having a greater than atmospheric co2 concentration.

Oh well, will try to step away from the horse again.

[defdac]

Originally Posted by jgc

Will add that even if we inject micro bubbles of air into an enriched tank, co2 dissolved in the water would try to achieve equilibrium with the air in the bubble and the bubble would end up having a greater than atmospheric co2 concentration.

Exactly my thought also.

So even if the micro bubbles are other gases and they land under leaves, they will "soak up" CO2 from water and remove the prandtl-layer and you have in fact a high CO2-atmosphere without a layer near the stomatas maknig the plants grow faster?

[Opiesilver]

Originally Posted by defdac

Exactly my thought also.

So even if the micro bubbles are other gases and they land under leaves, they will "soak up" CO2 from water and remove the prandtl-layer and you have in fact a high CO2-atmosphere without a layer near the stomatas making the plants grow faster?

Now that I agree with. By putting the micro-bubbler in the water current path you also increase the water contact time with the bubbles which helps increase the CO2 absorption rate. That will help with stabilizing and maintaining higher CO2 levels in the water column.

Yes Dennis, it is easier for plants to absorb carbon from carbonic acid (H2CO3) rather than strait CO2. However in the presence of water, H2O, carbonic acid very quickly breaks down into H2O and CO2 while in a non-equalized state. I read a paper a few years ago, which I think came out of Europe somewhere, that calculated that just two water molecules together can increase the rate of decomposition of carbonic acid by a factor of 50 billion. Without the presence of a catalyst, like H2O, carbonic acid is a pretty stable compound. Now this may be a far stretch but this leads me to believe that true aquatic plants have evolved a way to utilize CO2 gas more efficiently than their emergent cousins that rely on a strategy of breaking the water surface for faster growth. We should be able to see very different underwater growth results between the two classes of plants in this kind of experiment.

[unirdna]

Originally Posted by jgc

Will add that even if we inject micro bubbles of air into an enriched tank, co2 dissolved in the water would try to achieve equilibrium with the air in the bubble and the bubble would end up having a greater than atmospheric co2 concentration.

Agreed. Although I don't know that this would result in increased growth. And when I say "I don't know", I do not mean "I belive it would not" , I mean "I don't know". We need to better understand plant physiology before constructing hypotheses. And remember to study up hard, because as we've seen on this thread already, a little bit of knowledge can be dangerous.

I think we've turned the corner on this discussion.

Next step:

1) Do CO2 "enriched" (1.8% for a tank containing 30ppm CO2 in aqueous solution) microbubbles - resting on the underside of plant leaves - improve growth? How? Why? How much? And I say "resting on plant leaves" because those bubbles of gas containing 18000 ppm CO2 are still at equilibrium with the solution. They will not "carry" that CO2 around the tank, and disperse it slowly. They are "keeping" their CO2 gas. Furthermore, as more 02 diffuses into the bubble the PPM of CO2 will be lowered further, creating a bubble that is "hungry" for CO2. This "enriched" bubble discussion is a moot point unless the bubbles come in direct contact with plant tissue.

2) Does increased circulation improve growth? How? Why? How much circulation? How much plant growth? This one, we've suspected all along to be a major contributer, since diffusion alone can be a bit sluggish. However, we should approach the question with the same scientific vigor as more esoteric questions.

[jgc]

I have one last thought, till I leave this to the experts for a while. How long does it take a c02 bubble or an air bubble to achieve equilbrium given the aggitation a suspended (or captured) bubble recieves. Depending on the time, the captured bublles will probably not be at equalibrium, but perhaps several % away from it. Course depending on how much away, it may have no net effect.

The first big if is do enriched bubbles even help, the second question then comes into being - how enriched are the actual bubbles.