Tom and Carissa,

The stuff you two have been saying has got me thinking.

Carissa, when you said


I remembered some things I read a while ago, but I can no longer find to post the link, haha. It referred to how plants actually produce more rubisco in lower CO2 environments in order to become more efficient at growing, and less when CO2 is more abundant. The question that leads me to is this.

Say a non-CO2 tank sits at about 3ppm (equilibrium with air) of CO2 if there is reasonable circulation within the tank providing gas exchange with the air. Now Tom and others have said that plants grow about 10x as fast when you inject CO2 versus not using CO2. The recommended ppm for injected CO2 (at this time) is to keep it around 30ppm. 30ppm is exactly 10x more CO2 versus a non-CO2 tank, so it doesn't seem like that rubisco adjustment is really gaining the plants anything at all.

You would think that if the plants are actually producing more rubisco when CO2 is at lower levels that the plants would have maybe only a 6x increase in growth with a 10x increase in CO2, not be the same. The rubisco should be evening out the CO2 excess or lack and provide a more consistent growth level in any CO2 condition (as long as it is stable.)

That doesn't seem like it is happening though. Right now it just seems like the growth increase is linear, aka the more CO2 you inject, the faster the plants grow, and that's it. 10x CO2 = 10x growth, 5x CO2 = 5x growth, etc. Am I missing something here that is throwing me off?

Have a good one, Jeremy

Jeremy, as I understand the EI fertilizing method, we supply an abundance of everything the plants need, except light. We use the light intensity to set the growth rate. If that isn't too oversimplified, adding more CO2, when there is already "enough" doesn't increase the growth rate at all. Where am I wrong on this?

Ok, so what you are saying is that the changing levels of Rubisco should absorb the change in co2 to some degree. If your logic was correct, you should see plants growing 20x faster at 60ppm of co2. But this doesn't happen.

The reason why is that firstly, plants have a maximum rate of growth that they can achieve, it's not a constant linear increase based on the level of co2 or other fertilizers.

Secondly, plants that are in a non-co2 environment are carbon limited. They can only grow as fast as they can get more carbon. They can only get more carbon as fast as their Rubisco allows, and from what I understand, there is a maximum level of Rubisco that a plant can have at any given time. But plants that are kept in an environment with 30ppm of co2, assuming good circulation, are no longer carbon limited. They are growing as fast as they can under the lighting they are provided (this is what Vaughn alluded to above).

So growth is no longer linked to co2 levels at that point, it's linked to lighting and the maximum rate of growth that particular plant can achieve. This is why growth rates are not strictly linked to co2 levels.

If you take a plant from a high co2 environment and put it into a low co2 environment, you will see a period of time in which the plant's growth is actually at a complete standstill. This is the time that it takes the plant to build up enough Rubisco to enable it to retrieve the carbon from the water again. Some plants will start cannibalizing leaves or pieces of leaves, sacrificing entire leaves for the carbon it needs to build more Rubisco since it's unable to get carbon from the water. Fast growing plants seem to be more affected by this at least in my experience. If you want to try it, get some hygrophila polysperma and keep it in a co2 tank for a month or two, then cut co2 suddenly. I've done it twice. The third time I decided to go non-co2, I just took it all out and threw it in the garbage. It was less work than cleaning it out leaf by leaf over the next month or two.

Excellent question, by the way.

Vaughn and Carissa,

Thank you for your replies. Let me tweak my question a little bit and approach it from another angle, because in my mind I feel like we are all on different pages a little bit and that might be confusing stuff some.

It is my understanding that CO2 is just another nutrient like Nitrogen or Potassium. The only reason that we don't just dose it into the tank with a teaspoon like everything else is because it happens to be in the gas phase under normal circumstances, that's all. So CO2 obeys all the same principals of nutrient uptake within the plant that the other nutrients do. That would mean that CO2 would have the same point of "luxury uptake" at which plant growth is no longer increased by adding more CO2.

In my mind, I am thinking that the best growth from a plant would occur if every single factor to plant growth is available to the plant in "Luxury Uptake" amounts (so obtaining those nutrients as as easy and efficient for the plant as possible), and then the light (overall growth) level is set right at the point where light levels are right at the edge of the "luxury uptake" zone for light as well. Any light more than that doesn't do anything for the plant, and any less light or less of any one or multiple other nutrients would decrease the efficiency, hinder the uptake of other nutrients, etc. and restrict growth to one degree or another. That's what I got from the link that Tom posted a while ago and the things he has said on this thread (and in others) in response to it.

The Relative Nutrient Requirements of Plants

Now my question related to rubisco is only related to levels of CO2 that are less than "Luxury Uptake" and that's it. I am also asking this question, because since CO2 is just a nutrient, I would think we should be able to apply whatever we learn from this to all the other nutrients as well, because it should all work the same way. Levels less than "Luxury Uptake" seem to be what gets everyone in trouble when it comes to growing plants, and changing levels above the "Luxury Uptake" zone doesn't change anything related to plant growth, just like Vaughn stated. So I think we are all in total agreement so far.

This is what I am thinking, I hope it makes sense. For this example, assume there is plenty of other nutrients and light at all times for all situations so that they never become a factor to hinder ideal growth. Say a plant grows ideally with 30ppm of CO2. That plant has plenty of CO2 in the surrounding water, so say it only needs to produce 30 rubisco enzymes per plant in order to suck in all the CO2 it could ever want.

Now from an odds perspective, that first plant has a 1:1 odd of finding a CO2 molecule for each if its' rubisco enzymes. That is perfect and very efficient, so maximum growth is attained. The plant takes in 30 whole CO2 molecules every time its' 30 rubisco enzymes "reach out to grab CO2". That efficiency is rewarded in the form of easy luxurious growth for the plant.

Now the same plant with the same number of rubisco enzymes (30) reaches out to grab CO2 molecules in an environment that only has 3ppm of CO2. That means that those 30 enzymes are now only able to find 3 CO2 molecules. As a result of only finding 10% of what it truly wants the plants growth has to slow down 10x in response, like it does in a non-CO2 tank. Now its' odds of finding CO2 are only 1:10. That is a direct linear relationship between growth and CO2 levels (10% of ideal CO2 = 10% of ideal growth level) when CO2 levels are less then what is desired for "Luxury Uptake", and that seems to be what we see in practice.

Now if a plant actually does increase its' number of rubisco enzymes in lower CO2 water the situation should have changed from linear, so we shouldn't be seeing that when levels of CO2 are less than "Luxury Uptake". The plant would now have a better chance of taking full advantage of the little CO2 it actually could find. Say it increased rubisco within the plant from 30 enzymes to 300 in a 3ppm environment. Now the water is still at 3ppm of CO2, so when those 300 enzymes "go looking for" CO2 their odds of finding CO2 are still only 1:10. The difference is, now a 1:10 (or 10%) success rate actually nets the plant 30 CO2 molecules instead of 3 since there are so many more rubisco enzymes doing the searching then there were before. That's the same total CO2 molecules the plant was getting with the 30 enzymes and the 30ppm water!

Now, the only thing that I would think would prevent the plant with the higher number of rubisco enzymes from being able to now still grow at the same rate as it was when the CO2 was plentiful is that it will take some plant energy/resources to create and maintain all the extra rubisco enzymes it now needs to collect the same amount of CO2, so there will be some loss of efficiency and energy for the plant.

If this is all happening, you would think that a plant growing in 3ppm CO2 water would only be growing maybe 1/2 or 1/3 the speed of the plant in the 30ppm water, not exactly 10x less to correspond directly with there being 10x less CO2.

A plant isn't going to go through all the effort of creating more rubisco enzymes if nothing is really gained from all that effort, so what is missing from this picture/my understanding?

Have a good one, Jeremy

A couple of observations....

1. I read recently somewhere on this site that a plant's growth can increase from 6 - 10x after adding co2. I don't think it's established that it will always equal exactly 10x the growth.

2. Tom's recent post on real time co2 measurements in different areas in the aquarium seems to show that even though a drop checker may say 30ppm, the values at the actual plants can vary widely. So although it may appear that a particular tank is going from 3ppm to 30ppm, in reality at the plant it may only go from 3ppm to 10ppm.

3. If you take a plant from a 30ppm co2 environment and place it in a 3ppm co2 environment, the growth doesn't slow down 10x. It actually stops completely until the plant builds up enough Rubisco to start growing again. This shows that growth in and of itself is just one thing a plant does when other conditions are met. The plant first needs enough carbon to simply maintain itself, then any extra can be used to grow.

4. It would seem that there is a maximum limit on Rubisco that a plant can actually be maintaining. This is what I attribute to the slower growth in a non-co2 tank. For instance, in your example above, I don't think a plant could ever get to 300 Rubisco enzymes. If it could, like you said, there would be little difference in growth between a non-co2 and co2 environment. If we change that example to go from 30 in a high co2 environment to, say, 100; and then we assume that 1/3 of that 100 is being used to maintain all the additional Rubisco, it seems fairly evident that a plant in low co2 would indeed have much slower visible growth. That's the whole idea of being carbon limited, the plant cannot get enough carbon to grow at maximum speed.

Carissa,

Thank you very much, I think I felt a switch click in my brain after reading your response. I think I get it now. Your fourth point is what sealed it for me.


That makes perfect sense, because if the plant could create the 300 rubisco enzymes then it wouldn't actually be limited at all in its' growth by the CO2 levels. That means that the plant would have to fall behind in its' rubisco production and collection of CO2 at lower CO2 concentrations in comparison to higher levels.

I have been reading that thread on "real time CO2 measurements" in the tank as well, and it is very interesting stuff. I had been trying to apply that to this discussion somehow, but it just seemed to put too many variables into everything. Maybe that's actually the point, I am making an oversimplification by latching onto the 10x growth with CO2 vs no-CO2 comments.

Interesting stuff, this aquatic biology/botany. I love it.

Have a good one, Jeremy