Help re how submerged aquatic plants uptake CO2

[TwoTankAmin]

I need some help from the scientists in the PC membership.

At the last meeting of my fish club we had a speaker on advanced planted tanks. I kept one of them for about a decade. The speaker stated that the aquatic plants get their carbon in the form of carbonic acid (H₂CO₃). This went against everything I thought I knew on this subject. Here is what I thought was happening in this regard.

CO2 dissolves in water. When it does a small portion turns to carbonic acid and this can act to lower the pH of the water.

Submerged plants can uptake CO2 and some can uptake bicarbonate (HCO⁻₃). Some plants can do either I believe. I have never read of the plants using carbonic acid. To be sure I am being clear here, I am talking about aquatic plants whose leaves never go to the surface where they could absorb atmospheric CO2.

Please educate me on this. Do the submerged plants use what I had though in terms of what forms of carbon they can take in or was the speaker's presentation correct? I tried to find anything via Google Scholar that indicated the plants used carbonic acid. I did learn that "Carbonic acid, which is a weak acid, forms two kinds of salts: the carbonates and the bicarbonates." So it seems to me carbonic plays an indirect role by creating these salts which are usable by bacteria or plants but the carbonic acid itself is not.

Thanks in advance for your answer.

[bekateen]

Hi TTA,

Although I haven't had much education on carbonic acid behavior and plants in aquatic environments, I presume it's similar to what I've learned about carbonic acid in the body/blood. Here's what I've been taught:

The simple part is the chemical reaction:

CO2 + H2O ⇌ H2CO3 ⇌ H⁺ + HCO3⁻ ⇌ 2H⁺ + CO3²⁻

The pKa of carbonic acid is about 6.35, meaning that at a pH of 6.35, about equal amounts of H2CO3 and HCO3⁻ should coexist in solution. But according to animal physiology books, H2CO3 rapidly converts back to CO2 + H2O. So the standing concentration of H2CO3 at any moment is relatively low, and physiologically is considered to be essentially zero.

Respiratory physiologists ignore H2CO3 and essentially treat the CO2 concentration as its proxy. So when many respiratory physiologists try to calculate the H2CO3/HCO3⁻ ratio, instead they just calculate the CO2/HCO3⁻ ratio.

Taking this back to your original questions, I don't know if plants can absorb and utilize H2CO3, but based on animal respiratory science, I'd expect that there is little or no actual H2CO3 in the water environment at any given moment. Thus, even if plants can take up H2CO3, I'd expect it is a small fraction of their total carbon uptake, with the bulk of their carbon uptake being CO2 and HCO3⁻.

I hope that is correct.

Cheers, Eric

[Bas Pels]

As bekaten said, H2CO3 is instable, and falls apart into H2O and CO2. This happens rapidly, and I think the time for a molecule needed from entering a plant to being used by a cell in phyaotsythyesis is olanger thant the H2CO3 survives.

Thus, from a chemist´s opoint of view, regardles if H2CO3 or CO2 is taken in, by the time the carbon is used, it will be CO2

As the other reactions also go very fast, CO2 could enter a plant, be converted into H2CO3 which falls apart into CO2 before it is used.

Therefore, I thik it does not matter, even if a plant could only use CO2 (I don´t know whether this is true or not) it still can take all the carbon compounds in Bekateen named.

[bekateen]

TTA, as follow up to what @Bas Pels and I are saying, see this paper: https://www.ncbi.nlm.nih.gov/pmc/articl ... LXxXZK5U7M

It says that CO2 concentrations in the vicinity of Rubisco (the main enzyme driving photosynthesis) are much lower than the enzyme concentrations. By contrast, HCO3⁻ concentrations are comparable to the Rubisco concentrations, although HCO3⁻ can't be used directly in photosynthesis. But nearby is the enzyme carbonic anhydrase, which converts HCO3 to CO2 + H2O. From the paper, it appears that the plant cells accumulate HCO3⁻ and then use carbonic anhydrase to create a constant supply of CO2 to the Rubisco enzyme for photosynthesis. If my understanding of this paper is correct (I hope it is, because this is a pretty dense article), then there would never be much H2CO3 usage.

Here's another paper (http://www.imedpub.com/articles/carboni ... plants.pdf), which shows the dual role of carbonic anhydrase in converting captured CO2 to HCO3⁻ (so that cell don't lose the carbon) and then use carbonic anhydrase again to reconvert HCO3⁻ to CO2 for carbon fixing by Rubisco.

In fact, since carbonic anhydrase doesn't actually create H2CO3 (this is bypassed in the enzymatic conversion), H2CO3 plays even less of a role than is portrayed by my first post, except that it is the natural intermediary between CO2 and HCO3⁻ during interconversions in the water prior to uptake by the plant cells in the form of CO2. As an aside, the first paper also states that at very low pH, bicarbonate tends to convert to H2CO3 (which is expected by the higher pKa around pH 6.35) and that elevated levels can impair photosynthesis.

Hope this helps.
Eric

[bekateen]

FWIW, a botanist suggested to me that C4 aquatic plants might be more interested in H2CO3 usage than other aquatic plants. As far as wikipedia says, only Egeria densa and Hydrilla verticillata are aquatic C4 plant species. They use PEP carboxylase instead of Rubisco, so maybe for them the enzymatic and substrate-type issues might differ. But otherwise, I don't see justification in what your speaker said, assuming they meant what you interpreted them to have said.

Cheers, Eric

[bekateen]

Here is what I thought was happening in this regard.

CO2 dissolves in water. When it does a small portion turns to carbonic acid and this can act to lower the pH of the water.

Actually, I will correct you on this: All solutes and gases have a saturation point when dissolved in water. Between 20C and 30C, carbon dioxide, saturation occurs between 1.7 and 1.25 g/L of water; bicarbonate ion saturation occurs between 80 and 100 g/L of water (as you know, solute saturation goes up, but gas saturation goes down at higher temperatures). As a matter of fact, typical aquatic habitats are nowhere near saturation with bicarbonate, because the carbon dioxide is saturated at a level insufficient to create super high levels of bicarbonate.

No matter the ranges, the bicarbonate levels represent huge amounts of carbon dioxide. And I say this to correct what you wrote above: when CO2 dissolves in water, "a small portion turns into carbonic acid." In fact, the majority of the carbon dioxide transforms into carbonic acid and then into bicarbonate or maybe even carbonate ion.

The total content of carbon in the water is disproportionately found in the form of bicarbonate or carbonate (mostly bicarb at near neutral pH or slightly acidic pH). For example, in human blood plasma, dissolved bicarbonate accounts for more than 6 times the carbon dioxide than actual dissolved carbon dioxide. It would probably go higher except for the pH buffering systems of the body that regulate pH around 7.4 and prevent dissolved bicarbonate ion concentration from going too high.

And according to a U Hawaii document (http://www.soest.hawaii.edu/oceanograph ... /CO2pH.pdf), 99% of the carbon dioxide in the ocean is in the form of bicarbonate or carbonate ion (but saltwater ecosystems don't really compare to freshwater ecosystems, which don't have the capacity of saltwater systems).

I bet that's more than you wanted to know, but I hope it's helpful.

Cheers, Eric

[dw1305]

Hi all,

.......... but based on animal respiratory science, I'd expect that there is little or no actual H2CO3 in the water environment at any given moment. Thus, even if plants can take up H2CO3, I'd expect it is a small fraction of their total carbon uptake, with the bulk of their carbon uptake being CO2 and HCO3⁻.

I hope that is correct.......

It is. Only about 0.15% of the CO2 goes into solution as H2CO3, and this disassociates into a HCO3- and a proton (H+ ion), acids are "proton donors" and that is why the pH falls.

...........From the paper, it appears that the plant cells accumulate HCO3⁻ and then use carbonic anhydrase to create a constant supply of CO2 to the Rubisco enzyme for photosynthesis.......

Plants take up CO2 gas through their stomata, plants that naturally live in hard water can also use the bicarbonate ion (HCO3-) as their carbon source, via the mechanism in the paper. If any-one is interested in the nuts and bolts of photosynthesis the Calvin-Benson cycle is the bit we are interested in for the vast majority of aquatic plants ("C3" plants).

A factor that is often ignored is that the CO2 from respiration is stored in the plant tissues during the night (and vice versa with the oxygen from photosynthesis) and then this is used for photosynthesis once PAR levels exceed light compensation point.

The amount of CO2 that goes into solution is dependent upon atmospheric CO2 levels, so although CO2 is a soluble gas (think of a carbonated drink), there is only ~400 ppm CO2 in the atmosphere, and ~0.5ppm of CO2 dissolved at 25oC.



The level of DIC/TIC (Dissolved/Total Inorganic Carbon) always remains the same (it is set by the level of atmospheric CO2), it is just the form that changes in a carbonate buffered system. Freshwater isn't always carbonate buffered, but sea water alway is.



When you add CO2 you change the pH value because you have more H+ ions in solution (you can think of it as having a higher level of atmospheric CO2).

cheers Darrel

[TwoTankAmin]

While I realize that H+ is the scientific explanation, for most fish keepers we just see the water as being more acidic, i.e the result of more H+. However, when one is injecting pressurized CO2, one is able to achieve differing levels of dissolved CO2 in a tank.

Because of the difficulties associated with testing for CO2 levels, a way to "measure" this and to control the levels is done in one of two ways. One involves testing for KH and pH and extrapolating from there. There are factors which can effect the result however so in some cases it will be off the mark. Here is a link to chart on Tom Barr's site. The lowest level of CO2 on this chart is .1 ppm and the highest is 600 ppm. This surely indicates a very wide range. https://barrreport.com/threads/co2-ph-kh-table.10717/

What many folks with high tech planted tanks do is to regulate CO2 levels using a pH probe and controller which is used to control the CO2 going into a tank. I specific pH is targeted and the equipment does the work.

To me this indicates two things are happening. First, there is clearly a fairly wide range of dissolved co2 one can have in a tank, including levels that would be toxic to fish. The second is that the plants need more co2 than atmospheric sourced co2 alone can be replenished as fast as the plants use it up. In nature this is not the case since we know such plants are from the wild and that there is no co2 injection happening.

The upshot of this is the change the tho H+ ions present, i.e. something "acid" is changing the pH and that it is directly related to CO2 levels. I assume that the H2CO3, may be short lived as but is unstable is actually there constantly due to the ongoing injection of CO2 into the tank. As fast as it "falls apart" it is replaced by the injection of the pressurized gas.

When I kept my high tech, I did not care about anything besides having healthy plants and not killing my fish. I did a lot of KH/pH testing early on until I determined the bubble rate I needed to be running. For me the CO2 levels were leaner than most, and I let it run 24/7. It seemed to work for me, but then I was not trying to create an Amano type tank. The result was this tank took the most work of all my tanks every week to keep looking good. The dang plants insisted on growing too fast......

I am sure I have this wrong somehow. It's just that as far as i can tell CO2 is not acidic until it dissolves in water.

[Bas Pels]

I read

It's just that as far as i can tell CO2 is not acidic until it dissolves in water.

And I wonder, what is it that makes HCl acidic? It is that it reacts with water, resulting in H+ and Cl-, making the water acidic.

That is, in most common practises, acidic Always means in relation to water.

I know, I´m a chemist myself, there are very good definitions, electron donor or acceptor, for defining acid or base, but in common practises, these do not matter. Anybody interested in methane as an acid? It works, but not in water.

[dw1305]

Hi all,

...........One involves testing for KH and pH and extrapolating from there. There are factors which can effect the result however so in some cases it will be off the mark. Here is a link to chart on Tom Barr's site. The lowest level of CO2 on this chart is .1 ppm and the highest is 600 ppm. This surely indicates a very wide range. https://barrreport.com/threads/co2-ph-kh-table.10717/......

I'd have to say for me that the CO2 will always be off, I've found quite enough ways of accidentally killing my fish without adding another one.

......But, rather than try and estimate the the dKH of the tank water you can use a drop checker with a narrow range pH indicator (bromothymol blue) and a solution of known chemical content (the "4dKH solution"). Only gases can cross the air gap into the drop checker, so it gives you a much more accurate estimate of CO2 levels. The problem is that there is a lag period, so it doesn't estimate CO2 levels in real time, giving you every chance to asphyxiate your fish.

I won't put the full explanation in here, but 30ppm of added CO2 always equates to a drop of ~ one pH unit (have a look at Jose comments in <UKAPS: Question about .....>.

........The upshot of this is the change the tho H+ ions present, i.e. something "acid" is changing the pH and that it is directly related to CO2 levels. I assume that the H2CO3, may be short lived as but is unstable is actually there constantly due to the ongoing injection of CO2 into the tank. As fast as it "falls apart" it is replaced by the injection of the pressurized gas.

That is exactly it. A constant 0.15% of dissolved CO2 becomes H2CO3, more gas means more H2CO3, more H+ ions and a lower pH. Added CO2 is constantly lost through the gas exchange surface, and the larger that surface is, and the greater the concentration gradient between water and air, the quicker this will happen.

To achieve stable CO2 levels the rate of injection and loss must be in balance. I use a bouncy castle analogy, turn the air compressor off and the castle deflates ~ CO2 levels equilibrate with atmospheric levels.

That is, in most common practises, acidic always means in relation to water........ very good definitions, electron donor or acceptor, for defining acid or base...............

I've found it useful as a concept in explaining water chemistry. If you have phosphate buffers you know straight away that K2HPO4 is the base and KH2PO4 the acid.

cheers Darrel

[TwoTankAmin]

But, rather than try and estimate the the dKH of the tank water you can use a drop checker

The problem with this method is it takes about an hour for a change to show up. At least the KH/pH methods is pretty much instant though less accurate. However, it is possible to make a mental adjustment to the result if you know what you are adding to the water which will affect the results.

While Darrel may not want to inject pressurized CO2 into his tanks, there are an awful lot of folks all over the world who do use it. I did for close to 10 years. That tank was one of the most healthy tanks I ever had. I never medicated it for anything It was where my farlowellas spawned.

Using pressurized CO2 in a planted tank is far from being rocket science. Like anything else we do in our tanks, it takes some learning and effort. But so does most of what we have to do in our tanks to keep the residents healthy. The reason I gave up my CO2 added tank was the amount of time I had to give it.

The other part of doing pressurized gas is most who do this also seek to minimize surface agitation in order to prevent the loss of the CO2. I ran my gas directly into the intake of my Eheim canister. The spraybar was eventually re-positioned vertically in the back right corner of the tank to minimize out gassing and to increase circulation in the tank.

One last note about myself. I went through high school without ever taking a chemistry class- biology and physics I did. In college I chose geology to fulfill the two semester required science classes. Most of what I have learned chemistry-wise was the result of keeping fish. (Some of that was on this site and I thank those that contributed whether you were aware of it or not.)

[dw1305]

Hi all,

While Darrel may not want to inject pressurized CO2 into his tanks, there are an awful lot of folks all over the world who do use it. I did for close to 10 years. That tank was one of the most healthy tanks I ever had. I never medicated it for anything It was where my farlowellas spawned.

Using pressurized CO2 in a planted tank is far from being rocket science. Like anything else we do in our tanks, it takes some learning and effort. But so does most of what we have to do in our tanks to keep the residents healthy.....

I think a lot of people will tell you the same. I'll be honest I've got different goals from a lot of "serious" planted tank keepers. I'm not interested in optimal plant growth, aquascaping or aesthetics. I know that having active plant growth is the single most important factor in maintaining water quality and high quality water makes fish keeping a lot easier.

.................One last note about myself. I went through high school without ever taking a chemistry class- biology and physics I did. In college I chose geology to fulfill the two semester required science classes. Most of what I have learned chemistry-wise was the result of keeping fish. (Some of that was on this site and I thank those that contributed whether you were aware of it or not.)

Same for me. Forums like this one have given me much better understanding of what concepts like buffering actually mean in practice. I often knew what the definitions were, but I didn't understand what they actually meant.

cheers Darrel