The question that many beginners in this craft ask ' Natural rivers/lakes do not have a device that constantly inject CO2 for plants to grow, why do we need to do so in our planted tanks?'
The truth is that equilibrium levels of dissolved Carbon dioxide in our planted tanks is quite low (2-3ppm) compared to Carbon dioxide levels found in natural habitats that have abundant aquatic plant growth.
Natural environments that support thriving plant growth typically have elevated levels of carbon dioxide (10 - 40+ ppm) from organic decomposition and underground stores of CO2. In ground water, where off-gassing is constrained, CO2 often builds up to fairly high levels. Virtually all spring water is rich in CO2 for this reason. At spring headwaters, the CO2 level can be around 40-50ppm, tapering off with increased distance and plant growth wanes thereafter. Many of the aquatic plant species in the trade come from such areas.
This is much higher than the 2-3ppm that standard non CO2-injected fish tanks will have (in equilibrium with atmospheric CO2 levels given gas pressure laws).
In nature, the level of CO2 typically builds up throughout the night and is quickly utilized by aquatic plants once there is sunshine. In tiny ponds, CO2 levels and pH fluctuate a huge amount as the sun rises and photosynthesis starts.
Many aquatic plants also grow in fast flowing streams and shallow water where the surface areas are massive relative to the depth and volume of the water and plants can grab CO2 from water that is constantly streaming past.
There are also species of plants from regions where there is less dissolved CO2 in the water. These species will grow well even without injected CO2.
Some plants can also meet their carbon needs in other ways. One way is by splitting it from the carbonates/bicarbonates in the water (Valisneria and Ceratophyllum species are good examples of species that do this). These plants are mostly found in more alkaline waters. However, this process is very energy intensive compared to using dissolved CO2.
Many of the aquarium plants we grow in our tanks are marginals that can meet their CO2 needs via the air for much of the season.
CO2 injection boost growth rates 5-10X times compared to growth in low tech aquariums and helps tremendously in growing carpets & coloured plants. Plants have better form, color and health when grown in an environment where CO2 is optimally provided. Thin stems, stunted leaves, poor coloration, algae on plants are all common signs that CO2 levels are inadequate for optimal plant growth. Growth speed is not the only benefit that CO2 enrichment provides.
40-50% of plant dry mass is made of Carbon
As carbon is the major component of plant tissue, it has the greatest impact by far on growth. Where we commonly think of NPK (Nitrogen, phosphorus, potassium) as important nutrients for plant growth, they make up a small portion of plant mass compared to Carbon. A plant uses 10 times more carbon by mass than all the other " macro nutrients" combined. Think about how much energy folks channel into thinking about fertilization - where actually the much larger impact factor is access to carbon.
In a planted aquarium, injected carbon dioxide (CO2) boosts growth speed as well as quality, and has high impact regardless whether you use low or high lighting. Contrary to popular belief, one does not need to have strong lighting to benefit from CO2 injection.
In nature, most aquatic species are marsh plants that can survive either underwater (submerged growth) or above water (emersed growth). Given a choice, plants generally prefer to be above the water line as this gives them access to gaseous CO2/O2. Because gas diffusion in water is 10000 times slower in water compared to in air, it is much easier for plants to 'breathe' in air. Their CO2/O2 needs are much easier fulfilled when growing emersed. This is exemplified by how Anubias species seeks high ground by growing on rocks.
Water is a challenging environment for plants because gas diffusion in water is much slower than in air. Because of this, many aquatic plants have adaptations that enable better gaseous exchange, such as by changing their growth forms (often dramatically).
For example, submerged leaves are often highly dissected or divided. This has the advantage of creating a very large surface area for absorption and photosynthesis. Submerged forms lack the external protective tissues required by land plants to limit water loss. The epidermal (outermost) layers of aquatic plants usually show very little sign of cuticle formation. Surface cells are able to absorb water, nutrients and dissolved gases directly from the surrounding water while the internal system of tubes (xylem) which normally transports water from the roots to all parts of the plant is often greatly reduced.
Thus, if these submerged forms are removed from the water, many will wilt very quickly. This is because the normal water transport system is poorly developed.
This is clearly shown by observing the same plant, Ludwigia inclinata 'Cuba' in 3 different growth forms. Above Left; emersed form in prolonged dry conditions. Above Middle; emersed form in moist conditions. Above Right; submerged aquatic form.
Given the chance, many of our aquarium plants will grow above the water surface.
If CO2 is so important, do farms inject CO2?
Plant farms that supply aquatic plants for commercial trade will choose to grow aquatic plants in emersed form if possible. This negates the need for CO2 management, and many emersed forms are more hardy (emersed growth have to grow sturdier structures to hold their own weight against gravity, while submerged forms have water to support their weight). Emersed forms are thus easier to grow, and easier to transport. The picture above shows an aquatic plant nursery by Dennerle from Germany.