# Why 30 ppm of CO2?



## cS (Jan 27, 2004)

IUnknown's post got me to thinking: why do we attempt to keep [CO2] at 30 ppm, or any particular concentration for that matter? Do plants require a certain CO2 saturation in order for them to take up? And especially concerning those who own a pH controller where [CO2] is relatively stable, are there any reasons as to why [CO2] shouldn't be kept at 5 ppm, 10 ppm, 20 ppm, etc. as long as it is above 0 ppm and not near toxic levels?

In short...
For those who dose "just in time" (Kropp 2004), nutrients are maintained just at detectable levels and not in excess. Can the same be applied towards CO2? Thomas Barr chose [CO2] = 30 ppm because beyond that he didn't get any response from the plants. I would like to know: why? :mrgreen: Why is 30 ppm better than 20 ppm, 20 ppm better than 10 ppm, and etc.?

I'd appreciate any input. Thanks.


----------



## plantbrain (Jan 23, 2004)

Why?
Because I said so

No, my level is based on 2 different notions.
One is practical application at various lighting and nutrient levels.
The other is based on George Bowes research on light and enrichment with CO2 with aquatic plants specifically.

Beyond 30ppm of CO2, there is no net increase in plant growth at ANY light level accordingly. 600 einsteins/m^2/sec is the max amount of light that most aquatic plants can use and some such as Hydrilla can survive at 12 Ein/M^2/Sec, not much.
Full Tropical sun, is about 2000 Einsteins/m^2/Sec.

So there's 2 very strong arguements for 30ppm. Fish are fine and most of that is an issue of O2, not CO2 poisoning/stress etc at 30ppm amd many have observed this beside myself over many years as well.

I've said for a long time that you do not need so much light. Most plants are low light that we keep.

So you could keep a tank at 1.5w/gal and have less CO2 and not be limited, but at 30ppm, you'll never be limited no matter what the lighting level. Even so, at low light, the results are excellent with high CO2 at 25-30ppm. But at 30ppm, no tank will have CO2 issues so that;s a good level to remember and suggest for *any* tank using CO2 enrichment.

Knowing this allows me to suggest solutions for most anyone no matter what type of set up they have and to solve most al;gal issues folks might have without ever seeing their tank.

But the owner can tailor their routines and tweak them to use less in most cases. They don;'t need to do that, but it can reduce the amount and slow some aspects of growth rates down, I prefer to use light to do that personally and I've discussed why in many other post.

The issue with just enough approaches are centered more around NO3, PO4,Fe/traces perhaps, rather than CO2.

What effect does less CO2 have on a plant? Generally algae and smaller stunted growth at higher light. At less light, the CO2 demand is low enough where it does not become limiting enough to cause plant growth issues in many species.

Non CO2 tanks do work excellent but work for different reasons.
I'd rather keep that seperated from 30ppm discussion vs 15ppm which was the old level suggested by most text and sites prior to myself suggesting 20-30ppm ranges.

30 ppm is just enough for some tanks with high light FYI.

I take the max set of conditions and then sugggest that amount of CO2, NO3 etc and then folks can scale down from there if they want to try the less apporach or they can keep the higher than needed levels and be fine also.

Either method works well, more skill and observation is required for "just enough". But you can approach them in several ways to find the levels that work best for your tank and habits as I've suggested without test kits (except for CO2).

Regards, 
Tom Barr


----------



## cS (Jan 27, 2004)

I understand the background of where all these recommended CO2 ranges come from and its interplay with light. What I would really like to know is the mechanism(s) by which CO2 is being uptaken by the plants' stomata and how that is influenced by the amount of CO2 in the water. But before we get there, let's look at KNO3 first.

When we add dry KNO3, it disassociates into K+ and NO3-. The NO3- floats around the aquarium until the plants grab it. Similarly, in my head, I picture CO2 floating about the tank until the plant's stomata grabs it. Why then is it perfectly healthy to keep residual NO3- (for those who dose just in time) and NOT residual CO2 regardless of light levels?

When we calculate [CO2] from the pH/KH, are we not approximating _free_ CO2, or CO2 that have not been uptaken by plants? From the plant's perspective, why do they respond better when 30 ppm of this stuff is floating around instead of 15 ppm or 5 ppm or 1 ppm? Is it because the more of this stuff is around, the probability of these CO2 molecules coming in close contact with the stomata is increased? Or perhaps it is a matter of concentration gradient across the membranes that dictate CO2 uptake?

---

Sure, experience and research demonstrate that plants do better at higher [CO2] up to a point; but I really want to know HOW and WHY that it is so. "Because I said so" from the plant demigod just isn't gonna cut it. Hehehe. Please don't curse me with algae!

I hope that I have better articulated my confusions and questions. :mrgreen:

P.S. The SpelChek likes George W. Bush. Pretty funny.


----------



## plantbrain (Jan 23, 2004)

Uptake is generallly about Enyzme kinetics.
Rubisco (and all) enzymatic rates have optimal ranges of a substrate such as CO2. Concentration is proportional to the rate of assimilation by Rubsico. 

This has limits, and for CO2 and Rubsico, it's at 30ppm in water for aquatic plants. 

Rubisco "fixes" CO2 and incorporates it in the process of CO2 being made into sugars(reduction).

The more CO2, the more carbon that can be reduced and made into sugar reserves for storage and use by the plant.

Substrate concentration, pH, temperature all influence enzyme reaction rates. Beyond 30ppm, this level evens out and does not increase plant growth.

Regards, 
Tom Barr


----------

