Hi tel,
Ummm...why do they let you out in public?
No, just kidding mate. Seriously, there are a couple of really profound reasons that we struggle with CO2. Do you remember the scene in The Matrix when Morpheus was taking Neo to see the Oracle? As they got in the lift, Neo became uneasy with the idea of having his life be validated by a fortune teller. He asks "...is she always right?"
Morpheus's responds; "Try not to think of it in terms of right and wrong. She is a guide Neo. She can show you the path..."
In a way, the dropchecker is like an oracle. It's even shaped like a crystal ball. But it's only a guide. It's never right or wrong. The fact of the matter is that it can only tell you what the pH of the water is just outside the opening. Even at that, it can only ever tell you what the pH
was a few hours ago, such is the slow response time.
Now, for someone who has no clue about CO2, having a green dropchecker is invaluable information because it shows that they are
on the path. The dropchecker cannot confirm that this
absolutely is the right level of CO2 for this tank, under these lighting conditions, under these flow conditions and under these nutrient dosing conditions. Here are some of those reasons;
1. CO2 is tightly linked to the photon bombardment reaching the leaf surface. There is actually a quantitative relationship between the number of photon particles striking the leaf versus the number of CO2 molecules required. The reason is that each strike of a photon adds energy to the pigment where the goal is to energize an release an electron which is then used in a chemical reaction. For the chemical reaction to be complete you must have the appropriate number of CO2 molecules to be involved in that chemical reaction, otherwise the reaction is not completed. If I'm baking a cake, I must have a specific ratio of milk, sugar, floure and so on, otherwise the cake will be ruined.
For illustrative purposes lets put some numbers out there. Lets say in a tank, at a certain distance from the surface, on average, 1 trillion photons per second are absorbed by a leaf. For optimal use of the electrons which are released by these 1 trillion impacts, I may need say, 1 billion CO2 molecules to match. That means 1 billion CO2 molecules must pass across the leaf surface and arrive at the reaction centres every second, just to keep pace with the rate of chemical reactions caused by the photon collisions. If I only send 1/2 billion CO2 molecules to the reaction chambers the plant then has to figure out how to make up that extra 1/2 billion per second.
If I can inject enough CO2 into the water column then I can provide enough partial pressure to create enough CO2 density to send billions of CO2 molecules screaming across the surface membrane to those reaction chambers.
2. Here's where we have the next roadblock. CO2 is a gas. It doesn't care about you or your plants. It follows the gas laws. A higher gas pressure in the water than in the atmosphere means that the more we inject, the faster it escapes to the atmosphere. In this sense, the atmosphere acts as a hoover. So even if the dropchecker shows green, it's showing green as the CO2 is on it's way up and out of the tank! A green dropchecker doesn't mean that 30ppm CO2 is just hanging around looking far a plant leaf to satisfy. Those molecules are getting outta there like a rat out of a trap. If a plant leaf happens to be in the way, well, OK fair enough, the molecule might find itself inside the plant. The plant produces an enzyme called Rubisco that specifically attracts CO2 molecules. It is the job of Rubisco to transport the CO2 molecules to the reaction chambers. So guess what? The plant needs to be able to
produce Rubisco. Rubisco is a huge, complicated, slow, lumbering enzyme. It takes a week or two to produce sufficient levels and can only transport maybe three CO2 molecules at a time. The plant has to actually sense the CO2 concentration and then program production of enough Rubisco to match the concentration level of it's environment.
The word Rubisco is an acronym - listen to what this enzyme is actually called:
Rib
ulose-1,5-
bisphosphate
carboxylase
oxygenase.
Rubisco traps either CO2 (or O2) and mixes CO2 with the chemical called Ribulose 1,5 Bisphosphate (RubP) to produce glucose. This is how plants make sugar for growth. The excess sugar is turned into fruit, vegetables and seed. This is THE definitive behaviour of all plants - to produce food from CO2 using this specific reaction. People just dismiss this reaction - probably the most significant reaction on the planet by saying, "Oh my CO2 is good, my dropchecker is green".
OK so what happens if I add more light? More light means more photons per second colliding with the chloroplast and causing more electron movement. Will a green dropchecker still be sufficient? The green only indicates that there was a certain concentration level, but is that level sufficient for the higher level of photon bombardment?
Can you see why flow and distribution are important? I've got to grab the CO2 before it escapes forever and send it careening into the leaf, quickly. If my flow is anaemic an if I cannot deliver sufficient number of CO2 molecules per second, is a green dropchecker still relevant? I'd actually have to increase the injection rate to compensate for the poor delivery. Higher injection rate means higher escape rate but it also means an increase in density for enough time so that Rubisco can capture CO2. This is why you may need more extreme colour in the dropchecker, because the injection rate has to account for greater loss.
If you drive the lighting too high it's very possible that you then need toxic levels of CO2 in the water column just to satisfy the reaction rate. When you lower the light, you lower the photon collision rate, lower the reaction rate, lower the food production rate - and that's why you lower the growth rate. Now, a green dropchecker is OK, because you don't need as much CO2 in the water. The growth demand is lower.
Algae don't care about any of this because they need 1000X less nutrients and CO2 to survive. They "know" that plant needs are higher and that in a highly lit, poor nutrition, poor CO2 environment, the plants will suffer and die. They will have the whole playground to themselves and they'll be able to feed off the remains of the suffering plants like vultures.
It's always assumed by most that a green dropchecker equates to 30ppm CO2, but that's only true in the water column and mostly at the top. Near the surface of the leaf, the CO2 level that the plant has access to is typically 10X lower than what the dropchecker reads. This explains why we get hair algae even with a green dropchecker. CO2 has to be able to penetrate the waxy cuticle on the leaf surface as well as the spongy membranes of the plant. Quite frankly, it's amazing that aquatic plants are able to obtain enough of what they need at all when you consider these obstacles.
A green dropchecker is not a one-size-fits-all indicator. It is another test kit that can can be deceptive if you are not paying attention to the signs around it. Read the plants first. They will always tell you the score. The dropchecker
corroborates your observations. It does not determine them and it must not override them. The rule of thumb is that at lights on the checker should be lime green. If you start to see CO2 related algae then you know that you must either lower the light, or increase flow/improve distribution, or supplement liquid carbon, or increase injection rate, or trim, or, combinations of these.
Cheers,
