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Carbon in the Planted Aquarium
By: Greg Morin, Ph.D., President/CEO, Seachem Laboratories, Inc.
Carbon is the backbone of all life. Every organic molecule of every living organism is
predominantly carbon based. Given this simple fact, it becomes clear why carbon plays a pivotal role in
the planted aquarium. Aquatic plants extract CO 2 (carbon dioxide) from their environment and employ it in
a process called photosynthesis. Photosynthesis combines CO2, water and light energy to produce simple
carbohydrates and oxygen (O2) (see Figure 1). The first and simplest carbohydrate produced from
photosynthesis is 3-phosphoglycerate. It is from this simple molecule that larger and more complex
carbohydrates arise (by way of a variety of enzymatic processes).
Growth rates of aquatic plants are strongly correlated1 with availability of carbon and the plant’s
affinity for carbon uptake. Studies1 have shown that plants with the greatest carbon affinity have the
greatest growth rates, whereas those with lower carbon affinity have correspondingly slower growth rates.
Because carbon availability is normally the limiting factor to growth, addition of CO2 to a planted aquarium
will always result in large increases in growth (assuming other critical elements are not lacking). Without
additional CO2 the growth rate will be dependent on the rate at which atmospheric CO2 equilibrates into
the water. CO2 will dissolve into CO2–free water to a degree that is dependent on the air pressure,
temperature, pH and bicarbonate/carbonate content of the water. The final concentration of CO2 in the
water depends entirely on those factors. Once that concentration is achieved the level of CO2 will not
change unless the plants remove it or one of the other factors is altered. Plants remove CO2 at a rate
much greater than the rate at which it equilibrates into the water. So at the height of CO2 utilization the
plants limit their own growth by using up all available CO2. Because CO2 is an integral component of the
bicarbonate buffer system a drop in CO2 will necessarily result in a rise in pH. As the pH rises the influx of
additional atmospheric CO2 will be diminished by its conversion to bicarbonate. This is offset somewhat
by hard water plants that can utilize bicarbonate directly. However, without routine water changes or
buffer additions (Alkaline Buffer™ or Liquid Alkaline Buffer™) this path will eventually lead to complete
depletion of the KH (carbonate hardness) which will result in dramatic pH swings from day to night (5.7 –
9.6).1
CO 2 injection bypasses this predicament by delivering a constant source of CO2. Because the
introduction of CO 2 will lower pH one has two options: (1) Monitor and calibrate the rate of CO2 addition to
precisely match the usage by the plants or (2) use a pH feedback metering system. (2) is ideal because
as the pH falls below a certain point the CO 2 turns off, thus avoiding catastrophic pH drops.
If one is not quite ready for the initial investment in a CO 2 injection system but would still like to
enjoy some of the benefits of adding additional carbon there is an alternative: Flourish Excel™. Flourish
Excel™ provides a simple organic carbon molecule (similar to what is described above in the
photosynthesis discussion) that plants can use as a building block for more complex carbohydrates.
Because Flourish Excel™ is an organic carbon source it does not impact pH.
The chemical structure of Flourish Excel™ is quite similar to some of the products of
photosynthesis (see Figure 2) such as Ribulose 1,5-bisphosphate and 2’-carboxy-3-keto-D-arabinitol 1,5
bisphosphate. Flourish Excel™ possesses the same basic 5-carbon chain seen in these molecules. The
route through which Flourish Excel™ is used by plants involves two main processes: a) adsorption and b)
transformation. Because the active component of Flourish Excel™ (see Figure 2, polycycloglutaracetal) is
charge neutral and of relatively low molecular weight it is readily adsorbed directly across the cellular
membranes of most plants. Once present within the cell there are two possible modes of action. It may be
biologically converted into CO 2 and then utilized in that fashion. Or, it may be converted into any number
of more complex organic compounds needed for the life processes of the plant (e.g. sugars, starch,
amino acids, etc). These conversions (in either mode of action) are mediated by any of a variety of
enzymes present (oxygenases, carboxylases, phosphorylases, etc). In order to determine the precise
mechanism (i.e. down-conversion to CO 2, or up-conversion to longer chains) further studies involving
radioactive C 14 tracers would be necessary. However, with that said, our studies to date show that
Flourish Excel™ imparts a measurable, quantitative growth benefit to plants. Thus, it is clear that the
plants are utilizing the Flourish Excel™.
Our research has shown that Flourish Excel™ imparts not only a clear qualitative increase in plant health
and vitality but also a clearly measurable increase in growth. Recent studies (see Figure 3) have shown
growth enhancements using Flourish Excel™ that range from 200% - 500% (growth above normal growth
seen without Flourish Excel™). These are only preliminary results of a currently ongoing studied aimed at
determining more precisely the relative growth response to Flourish Excel™ in comparison to a standard
control and a CO 2 based control. The anecdotal evidence to date suggests that CO2 injection will promote
growth enhancements above the growth enhancements seen with Flourish Excel™ alone. However, one
can still obtain a cumulative benefit by using Flourish Excel™ in conjunction with CO 2 as the two work
quite well together.
1. Walstad, Diana, Ecology of the Planted Aquarium, Echinodorus Publishing, 1999, pp. 94-97.
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