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Re: CO2 in air


Air contains approximately 79% N2, 21% O2, 0.9% Ar and 0.035% CO2. Carbon
dioxide dissolves as readily in water as in air which means that at
Standard Temperature and Pressure, CO2 saturated water contains about 0.5
ppm CO2. Unfortunately, CO2 diffuses about 100,000 times slower in water
than in air AND the Prandtl boundary or 'unstirred layer' around a leaf is
about 10 times thicker in water than in air. What all this means is that
submerged aquatic plants are thought to be carbon limited under most
natural conditions. In the laboratory, submerged aquatic plants reach
maximal Ps rates at about 20 to 30 ppm CO2.
There are three ways to overcome this problem;
	1. add CO2. This is the most prevalent method used by this group.
	2. increase flow rates in your tank. This works because to
increase diffusion rates you either increase the steepness of the gradient
(add CO2) OR decrease the length of the diffusion pathway (increase flow
	3. maintain your tank at a pH of 7.5 to 8.0. Under these
condition almost all the DIC in your tank is in the form of
bicarbonate. Species which are able to use bicarbonate (about half) will
thrive under these conditions. There are, however many species that can't
use bicarbonate... these are generally rosette species that naturally grow
in extremely nutrient poor, soft, acidic water... some of these species
even use their root systems to acquire DIC rather than taking up CO2
through their leaves.