Photosynthesis pt. 2 Calvin Cycle 

          So last time we talked about the 1st step of photosynthesis, the light rxn. Again, this process uses PAR light to make chemical energy in the form of ATP and NADPH. It’s also responsible for splitting water, which generates oxygen. So at the same time this is happening in the cell, these by products are released, free to travel within the stoma of the chloroplast, where the Calvin cycle takes place. ATP and NADPH are required in this process where CO2 goes through several transformations that utilize various enzymes, eventually becoming a much more reduced form of carbon, a small sugar called Glyceraldehyde 3 phosphate, or G3P for short. 2 molecules of G3P are derived from every 1 molecule of CO2.  One will be further converted and swing back to the beginning of this cycle in order to keep it going and the other moves on where it will be further modified.  G3P is used as a building block, a precursor for amino acid, vitamin and fatty acid synthesis, as well as making sugars like glucose that can be stored, or used immediately to drive cellular respiration.

 

          You can see just how important CO2 role is in all of this. Without a consistent source of carbon, plants won’t be able to optimize photosynthesis and the result is often inhibited growth whether or not you can physically tell.  Yes some plants can use Bicarbonate as a carbon source but in most cases only after they expel resources transforming it back to usable CO2. Some plants can import bicarbonate directly, transform it into Co2 via an extracellular enzyme carbonic anhydrase and others may acidify the underside of leaves converting bicarbonate to CO2. Many aquatic plants also try to reach the surface of the water column where they then grow emersed as a way of grabbing as much carbon as they can from the atmosphere. Plant may also absorb CO2 through their roots that is generated from microbes within the substrate, although only a handful process the capacity to obtain all the carbon they need this way.  All of these methods help plants to meet their carbon needs in Co2 scarce environments.  

 

           So where does CO2 come from in a natural environment or in a tank with no added carbon source? CO2 is mixed in at the air water interphase but will only maintain a very low concentrations, generally just a few ppm, depending on several factors, pH and temperature being 2 of them. As previously stated, CO2 is also released from microbes as well as higher organisms such as your fish, but even so, you’re probably not going to see concentrations half of what has been deemed adequate for aggressive photosynthesis. To make matters worse, all compounds including CO2 have to get into plant cells, there is a physical constraint here called a boundary layer that surrounds each leaf. This layer may be ticker in unstirred waters, making it harder for gases and other compounds like co2 to enter the cell.  

 

          Having more CO2 in the water column paired with good water circulation will help get that CO2 into the plant where you want it to be. Without adequate co2 concentrations required for a healthy rate of photosynthesis, plants may begin to photorespire more and more. This is a process where oxygen is pushed into the Calvin Cycle instead of co2, reducing the overall efficiency of photosynthesis and costs your plants energy.  

 

          The biggest, and probably most relevant point I want to make here is this: with more CO2 comes the potential for more assimilation, which means more protein building which in turn means more nitrogen and other nutrients will be removed from the water column to meet growth demands. In other words, high concentrations of CO2 paired with adequate light and good water movement can equal a faster removal of the excess nutrients that cause harm to fish as well as cause algae problems.

 

          If you’re thinking about adding CO2 to your planted tank, here is my advice. While it may seem like a good idea to start slow maybe with the addition of a cheap DIY fermenter system that most start out with and that I’m sure you are all familiar with, it’s not what I recommend for creating a stable long term setup. In my opinion, spending the money on a pressurized system with a solenoid regulator is well worth it. You’re going to be able to control the amount of CO2 much better and most importantly keep things consistent. But what’s the deal with CO2? We’ve all been taught to try and maintain 15 to 30 ppm CO2 in our planted tanks to maximize growth but do all plants really require this? Is adding CO2 100% necessary and should we all be doing it?

 

          In my opinion it’s all about what your goals are for your tank. CO2 will pretty much always benefit your plants in one way or another. Some varieties just don’t need added CO2 to thrive, while others seem to require a decent amount of it. Before you go setting up a tank and investing a ton of money, I think it’s extremely important to look up the plants you want to keep and see what their care level is. This is one of the reasons why I started the Fish and Plant profile series and hope that one day it will be diverse enough to help those in making sound decisions.  

 

          The last thing I want to say is that while CO2 might sound like the holy grail of aquarium additives, it’s only going to truly benefit your system if other required nutrients like K, N and Fe are in abundance as they all play a major role in plant metabolism and energy production. It’s the long term balance that we all want to achieve that going to get our tanks looking great, not just the addition of CO2. This is easier said than done and takes a lot of knowledge and more importantly Practice! So get out there and try something new! Maybe all your tank needs is a little extra carbon.