the Future and notes on CO2 mist methods

Tom Barr

Staff member
Jan 23, 2005
I think both dissolved and gas phases are present in every case using the CO mist method.

It's difficult to isolate one without dissolving some gas into solution.
So setting up a control for gas phase only is tough.

But exposure to air etc over brief time frames can give insight into gas phases and dissolved CO2 without any mist can give insight into that methods. For both cases, in isolation for each prospective phase.

Also as far as whether the method "works":
What can be done is compare the O2 levels between the dissolved CO2 and then maintain that same dissolved CO2 level, and add mist.

If there is an increase in O2 levels, then we can say the method itself works.
I've found a 20-40% increase in O2, thus that is easily transferred to 20-40% increase in plant growth as O2 production and plant growth are tied together closely in aquatic submersed systems and is a standard unit of measure for growth. These rates of O2 production can be correlated with dry weights.

The theory part I'd first started with was that gas can transfer across a boundary layer much faster than than a liquid, and that is true in theory. I'd also suggested that the CO2 mist had a higher concentration than the water as a whole, this too is likely.

One thing that may be occurring is that the mist bubbles break up the boundary layer surrounding the leaves and helps to exchange the dissolve CO2.

This may be addressed by using air or an inert gas such as Helium(air has some CO2) which is relatively simple and cheap to add and non toxic. That would break up the boundary layer, but would not increase growth due to added CO2.

If there is no net gain in O2 production, then we can likely rule out the boundary layer issue as a reason. Physical chemistry kinetics clearly shows a much fast transfer rate, about 10,000X faster for a gas phase versus a liquid phase in diffusion.

The same is true as to why the atmosphere does not provide plenty of CO2 for submersed plants, but seldom is limiting for terrestrial plants. The rate of exchange, or flux, is too slow to keep up with demand.

While I can isolate the boundary layer issue, and show that there is some effect in increased plant growth, and I can show that is will produce localized effects in a larger tank, I cannot show what fraction of gas vs the dissolved form the plant gets.

I am unaware of any method that may answer that question.
Isotopic labeling will not do it. What labels are there would show that the CO2 was gas or liquid when the plant assimilates it?

Nothing I know of.
We can collect the mist itself at various times and flow rates and measure it's content.

That+ ruling out the boundary layer effect+the O2 readings between treatments, + localized effects can lend more support to the theory.

Another approach to see about the gas phase that isolates just that, although only for a little fraction of time, is exposing the plants to air when doing a water change and misting them with water to keep them from drying out.

Wait, then refill the tank and measure the O2 levels/pearling at midday.
You can also do this same water change method(typically 80-90% to exposure the plants well) right before the lights come on. If you see no bubbles forming after 10-60 minutes, then you know it's not degassed tap water, the increase in pearling you see in the middle day is due to the exposure to air and the CO2 uptake.

You should also see a gradient of pearling as well, you can do a 50% water change and exposure some plants, and not others and see the exposed plant pearl a lot more than those that were not.

Some suggest that the since the plants are closer to the light, they will pearl more than those lower, a simple solution is to use something that pearls a great deal but lacks roots like Riccia, and take it out of the tank for a period of time and keep it moist, then return to the tank vs the same Riccia that was not taken out and is at the same depth/light intensity.

Tom Barr