# What form of carbon do water plants use?



## Marcel G

Does anyone know for sure what form of carbon do water plants use?
Some sources say that water plants are able to use aqueous form of CO2, but what exactly does this mean? Other sources say that water plants use carbon in the form of H2CO3.

Thanks in advance for your answers!


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## ceg4048

Hi Marcel,
			 It means nothing. Plants can only use CO2. Their entire carbon fixing architecture is built around CO2. That means all the enzymes, proteins and other compounds are designed specifically around the manipulation of CO2 and the hydration (addition of water) of the carbon to produce the products we know as carbohydrates. Since these internal chemicals are designed specifically to manipulate the CO2 molecule, any other form of carbon must first be converted to CO2.

When we add so-called liquid carbon the plants that are able to process it happen to have, by coincidence, a system of chemicals which break the gluteraldehyde molecule, and one of the products of this chemical action is CO2. The Rubisco inside the leaf then captures the CO2 from the gluteraldehyde decomposition and transport it to the same reaction centers that it would do if the the CO2 came from the water column.

Not all plants can process bicarbonate. I've seen a number like 50% of the aquatic plants have this capability. Vallis is one of the more competent bicarbonate users. Even so, bicarbonate cannot be processed by Rubisco and bicarbonate cannot enter the Calvin Cycle to be turned into sugar. The plants which have this talent use an ingenious strategy because they know that bicarbonate is one of the main ingredients in the Carbonic acid equilibrium equation, which goes like this:

When CO2 dissolves in water the dissolution is described as follows (Equation 1).
CO2(g)<-->CO2(aq)

The (g) means gas such as the bubbles from our reactor. The (aq) means aqueous. Aqueous simply means that the gas is dissolved in water. That's all it means. The double headed arrow (<-->) means that the gas ca dissolve into water and also move from the water back into a gas and escape.

A small percentage of the dissolved gas then reacts with the water molecules to enter Equation 2:
CO2(aq) + H2O<--> H2CO3(aq)

This means that water reacts with the dissolved CO2 to form carbonic acid. Again, there is a double headed arrow, which means that Carbonic acid can break down into dissolved CO2 and water, and the dissolved CO2 can combine with the water molecule to form Carbonic acid. So if I add a lot of H2CO3 it forces the equation to the left. If I add more CO2 then the equation is forced to the right. This is entirely logical The more CO2 we inject in to water, the more Carbonic acid we produce. If we were to inject Carbonic acid into the water then more dissolved CO2 would result.

Here is the part that Vallis and it's colleagues figured out many millions of years before we did (Equations 3 and 4):
H2CO3(aq)<--> (H+)(aq) + (HCO3-)(aq)
(HCO3-)(aq)<--> (H+)(aq) + (CO3++)(aq)

What this means is that the free Hydrogen ions in the water break down the Carbonic acid to create bicarbonate (HCO3-) and continued exposure to more Hydrogen ions breaks the bicarbonate down to Carbonate (CO3--).

Again, the two reactions are reversible, so, if we unbalance the equation by adding H+ to the water it forces the equation to the left in both parts=> Carbonate will be forced into bicarbonate and bicarbonate will be forced back into Carbonic acid.

Now, that unbalances Equation 2, forcing it to the left to create CO2.
Herein lies the genius of plants. They produce H+ by a system we call the Proton Pump. Then, they excrete the Hydrogen ions (H+) out into the water column, thereby forcing a very high localized concentration of H+ in the immediate vicinity of their leaves. The Hydrogen pumped into the water unbalances equations 3 & 4 and then unbalances Equation 2 thereby producing CO2. The CO2 is then absorbed. It's a miracle.

Hope this makes sense!

Cheers,


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## Vanish

A great write up, I do enjoy your in depth explanations.


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## Yo-han

So although probably not measurable (or is it?), does this mean that when we inject CO2 the KH rises?


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## GreenNeedle

No or at least not by any huge amount directly.  Someone else will clarify but I would say the CO2 dropping the Ph lower will not mean the water is actually more acidic which 'could' erode any rock or hardscape within the tank more thus raising KH.  Even if not true I would suggest it would be so minimal as not to worry about anyway.

Short answer is CO2 should not affect KH at all.


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## ceg4048

Yo-han said:


> So although probably not measurable (or is it?), does this mean that when we inject CO2 the KH rises?


You can check by measuring the KH at the point in the day when the pH is at it's highest and then again when the pH is at it's lowest.

Do not think about KH as if it were only an arbitrary number. The alkalinity of a sample of water is the description of the number of acid neutralizing particles. Acidity is the concentration of of H+ protons, and alkalinity is the (equivalent) concentration of HCO3-. They attract each other and therefore neutralize each other. The water only has a set number of HCO3- which describes it's initial KH.  So, as you add more and more H+ protons to the water these protons will occupy an HCO3-. Eventually, you can add so many H+ protons to the water that it overwhelms the number of HCO3-.

With weak acids, that don't disassociate very much, not  very many H+ protons are released into the water, so not many HCO3- are removed. Strong acids will release massive amounts of protons and can overwhelm the buffering ability of the water, the KH, in no time flat.

Cheers,


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## Marcel G

Yo-han said:


> So although probably not measurable (or is it?), does this mean that when we inject CO2 the KH rises?



Clive already answered this, but you're right, Yo-han. The change is so small that it is virtually not measurable (even if there is no buffering capacity of water). See CO2 calculator.


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