CO2 absorption in Aquarium Plants.

Posted by Dustin Wunderlich on

What’s up Gold and Silverfish?

I wanted to drop this nugget on you all about CO2.  I am going to provide the “Keep it simple” Dustin version below but here is a great little read about CO2 absorption with aquatic plants…..

“Dissolved Inorganic Carbon (DIC) in freshwater occurs as four different species in equilibrium with one another. The four species of DIC are; carbon dioxide (CO2), carbonic acid (H2CO3), bicarbonate (HCO3-), and carbonate (CO3=). The total amount of DIC largely determines the buffering capacity of freshwater, and the ratio of these species with one another largely determines the pH.

Carbon dioxide dissolves readily in water. At air equilibrium, the concentration of CO2 in air and water is approximately equal at about 0.5 mg/L. Unfortunately, CO2 diffuses about ten thousand times slower in water than in air. This problem is compounded by the relatively thick unstirred layer (or Prandtl boundary) that surrounds aquatic plant leaves. The unstirred layer in aquatic plants is a layer of still water through which gases and nutrients must diffuse to reach the plant leaf. It is about 0.5 mm thick, which is ten times thicker than in terrestrial plants. The result is that approximately 30 mg/L free CO2 is required to saturate photosynthesis in submerged aquatic plants.

The low diffusivity of CO2 in water, the relatively thick unstirred layer and the high CO2 concentration needed to saturate photosynthesis have prompted one scientist to state, “For freshwater submerged aquatic macrophyte plants, the naturally occurring DIC levels impose a major limitation on photosynthesis … The DIC limitations on aquatic macrophytes and its corollary, the need to conserve carbon, are becoming increasingly apparent as important ecological features of aquatic environments (George Bowes, ‘Inorganic Carbon Uptake by Aquatic Photosynthetic Organisms, 1985).”

Aquatic plants have adapted to CO2 limitation in several ways. They have thin, often dissected leaves. This increases the surface to volume ratio and decreases the thickness of the unstirred layer. They have extensive air channels, called aerenchyma, that allow gases to move freely throughout the plants. This allows respired CO2 to be trapped inside the plant and in some species even allows CO2 from the sediment to diffuse into the leaves. Finally, many species of aquatic plants are able to photosynthesize using bicarbonate as well as CO2. This is important, since at pH values between 6.4 and 10.4 the majority of DIC in freshwater exists in the form of bicarbonate.

(I found the underlined sentence awesome as I know many of you are running dirt in your tanks.  As you know, when your dirted tank bubbles, that is CO2 doing you some good as well


That passage is from “the Krib”  which is a long outdated but yet highly useful resource initially built by Erik Olsen. I have to give special credit to this site as when I was 14 and just getting into tanks, this was one of the few places I remember being about to go to online and learn stuff.


The Bottom Line is that it is certainly much harder for aquatic plants to get CO2 from the water than it is the air.  During our last Goldfish tank talk we discussed this.

Diana Walstead first introduced me to the concept of the “Arial Advantage” – when aquarium plants grow up above the surface.  On last weeks call it was interesting to hear the different plants people had touching the surface.  Parker’s water sprite,  the baby tears I have growing in a small tub in the green house, and Sally, who is running Crypt Usteriana that touch the water’s surface.

For me, the water sprite and the baby tears are a given. I have played with enough water sprite to know it is a fast grower once it hits the surface.  I have seen baby tears growing natively in SC and it comes out of the water in sweet little patches as well.   The fun one (and I am green with envy of her growth)  is Sally’s Crypt Usteriana.    Sally has runners on a plant that I can barely get to show me a new leaf! (But I know your trick now Sally!

I would invite you to experiment with growing some plants out of the water.  The plant will grow faster and absorb the excess nutrients in your tank faster as the plant will be running at a higher octane with CO2 more readily available to it.

Please let me know what you are doing these days with CO2.  Have you tried it? Do you want to?  I’d love to get some feedback on here about how you have fed your plants CO2.

Share this post

← Older Post Newer Post →