CO2 at night - Tropical Fish Keeping - Aquarium fish care and resources
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post #1 of 5 Old 10-19-2007, 01:13 AM Thread Starter
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CO2 at night

I have been wondering for some time why information on turning your CO2 off at night is so ubiquitous on the internet. This doesn't make any sense at all. I am a biology major and plants don't use CO2 during the day. It's used at night, during the Calvin cycle.

During the day (when light is present) the light-dependent stage of photosynthesis occurs. This is when the plant takes H2O, sunlight, and the molecules ADP and NADP+ and converts them to potential-energy molecules like ATP and and NADPH.

At night (when light is not present) the light-independent stage of photosynthesis occurs. This is when the plant takes in the CO2 (after opening the stomata) and turns H2O and the ATP and NADPH that it made during the day, into sugars that the plant stores and uses for growth and energy.

So, what we think should really be reversed. Optimal plant growth should be achieved by turning your CO2 ON at NIGHT and OFF during the DAY. Does anyone have any questions?
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post #2 of 5 Old 10-19-2007, 06:49 PM
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With my limited understanding of the CO2 process and photosynthesis light is required to turn the CO2 into carbohydrates. Without light, CO2 will not be turned into carbohydrates and in turn the plants can not use it to perform photosynthesis. Even cactuses that can only collect CO2 at night due to heat and evaporation can only store CO2 as an acid and depend on light to perform photosynthesis and use the CO2 then to form carbohysrates of which they use as food.

I think the main idea behind turning off CO2 at night is through many experiments I have seen that show that CO2 levels can triple or even increase by a factor of 10 at night if CO2 is left on all night. This could lead to a tank full of very happy plants and very unhapy fish.

There are plants that will take in CO2 at night but they store it in different ways and then use it when the sun comes out. The above article explained that one for me as I did not kow this before.

If there is something else out there that says plants use CO2 at night with no light and in an aquatic envoronment I would love to see it. There is always something new to learn in this hobby and it could be a matter of what plants you have in your tanks as to when they use CO2. So far I have been able to find nothing that says CO2 is used as pure CO2 by plants without the presence of light to turn it into carbohydrates which the plants then store and use the carbohyrates at night to grow.
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post #3 of 5 Old 10-19-2007, 11:19 PM Thread Starter
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You are correct that the plant does eventually make carbohydrates, but not out of, strictly speaking, CO2. Like I said, the plant converts ADP and NADP+ into ATP and NADPH +H; during the day with light. The light excites the pigments which in turn sets off a chain of chemical reactions resulting in their formation.

At night, the plant takes those molecules that it just created during the day, and, with the addition of CO2, turns them into carbohydrates.

Photosynthesis does not occur JUST during the day!

There are the two stages to complete photosynthesis.


That is a perfect diagram of the photosynthetic cycle.

There are the two photosystems that take in the light and water and create oxygen and the ATP and NADPH +H that is required for the Calvin cycle. The Calvin cycle does not require light.

The Calvin cycle is a continual process of converting ATP and NADPH +H, in the presence of carbon dioxide and water, into carbohydrates, ADP, and NADP+. This, in turn, sets the light reactions for a whole new day of starting the process over again.

On your second link, is read the following:

"The dark reaction takes place in the stroma within the chloroplast, and converts CO2 to sugar."

The overall equation is:
6CO2 + 6H2O (+ light energy) = C6H12O6 + 6O2

NOTE: OVERALL. It does NOT say when those reactions take place. (Which is also at night.)

Very misleading.

Any more challengers? :D
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post #4 of 5 Old 10-19-2007, 11:28 PM Thread Starter
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I guess to be more clear, I should say that it is not all one cycle that takes place every dawn to dusk. There are two cycles, one during the day, that stops at night, and one at night that stops during the day. It is not just one cycle that occurs in the presence of light.

ATP is Adenosine Triphosphate which is the energy "currency" of all living cells. The Dark Reactions (Calvin cycle) use this energy molecule instead of the energy from light to make the sugar at the end.
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post #5 of 5 Old 10-20-2007, 04:04 AM
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Okay, I agree to a point. The Calvin part of photosynthesis can occur without the presence of light. I think the main question here, back to your original question is why turn of your CO2 at night. The simple reason is that CO2 levels are higher at night and plants use O2 at night.

Turning off CO2 at night prevents toxic levels of CO2 at night so we turn it off. My CO2 levels double and go up as much as 8x higher at night and I have wiped out a tank of fish overnight when I first started using CO2. It caused my pH to drop by 2 full points, from 7.2 to 5.2 and my CO2 levels went from 10ppm to over 180ppm in less than 6 hours. I know people who have used pressurized CO2 where their CO2 levels went from 35ppm to over 300ppm overnight which killed everything living in the tank including the plants. The pH of the tank was 3.5 which just slightly higher thanCarbonic Acid. It also ate airline tubing and ruined their filters and powerheads. Thankfully they were testing the same theory about CO2 and O2 use at night and did not have fish in the tank, only a few snails.

In short, we don't turn it off for the plants, we do it to protect out fish.

Yes you can run CO2 at night safely if it is done right but why take a chance when the effect on plants is minimul. After all, there is always CO2 in the water anyway, about 3ppm if I remember right. My tanks have never tested below this so plants still get CO2 to do the Calvin part of the process anyway.

I am curious though, why do CO2 leves increase at night if plants use so much of it in the Calvin process after the lights go out?

Oh and as far as I have been able to find, the Calvin cycle does not stop in the light. It happens continually. The process happens independently of light, not only in the absence of it, unless you have found something that says otherwise. Even the CAM plants will take on CO2 and perform the Calvin process if conditions allow them to during the day.
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