Relationship between Light and CO2 levels for Aquatic Plant Growth.

Aquatic Plants use a process known as photosynthesis to produce the carbohydrates they need for life. Photosynthesis requires light for energy and CO2 to drive the chemical reactions. 


The process of photosynthesis requires a specific light energy threshold. In other words, there is a point where light has reached a specific intensity to start photosynthesis in Aquatic Plants. If the light is not bright enough, photosynthesis will not occur. Beyond that threshold and up to some high light level, photosynthesis will run faster and faster. 


According to known practice, when light levels exceed two watts per gallon, supplementary CO2 is required for most aquariums.


The relationship between light and CO2 levels is important. The diagram (Fig1) below explains it conceptually. 

Fig1: Relationship between Light and CO2 levels for Aquatic Plant Growth.

Aquatic Plants have a limited amount of stored energy at their disposal for 
1. CO2 Assimilation.
2. Light Utilization.


At low light and low CO2 there is not much energy to play around with for up or down-regulation of the pools of Chlorophyll or enzymes contained in the plant. 


If we then add a little more CO2 to the system the plant can afford to invest less energy and resources in CO2 uptake and that leaves more energy for optimizing the light utilization - Chlorophyll can be produced without fatal consequences for the energy. Hence, although we have not raised the
light, the plant can now utilize the available light more efficiently. 


Exactly the same explanation can be used to explain why increased light can stimulate growth even at very low CO2 concentrations. With more light available, less investment in the light utilization system is necessary and the free energy can be invested into a more efficient CO2 uptake system so that the CO2, which is present in the water, can be more efficiently extracted.




Courtesy: John LeVasseur