Plants, CO2/O2 etc.

Hello Folks...

I've been reading this list for quite awhile, but have never posted; thanks
to everyone for the very sound advice/opinions. Over the last several days
there have been a few posts relating to the production/consumption of CO2
and O2. I am a forest ecologist/tree physiologist, and my research has
focused on various aspects of plant energy relations for years, including
measurement of photosynthesis. The relationship between the processes of
photosynthesis and respiration is near and dear to my heart, and really
must be understood correctly in order to interpret the ecology of any
system (i.e., a planted aquarium).

Stated simply:

 The process of photosynthesis consumes CO2, and produces O2 and
carbohydrate. The ENERGY required to drive this reaction is obtained, of
course, from light. The PROCESS can continue in the absence of light for
some time, at least until the chemically-fixed energy derived from light is

The process of respiration consumes O2 and carbohydrates, and produces CO2.
Respiration is a continual process (albeit at differing rates), occurring
in both light and darkness.

The important point is this:

Since respiration occurs continuously, and photosynthesis occurs
(essentially) only in the presence of light, what we CONSIDER to be
"photosynthesis" (that is, the assimilation of CO2 resulting in plant
growth and development) happens only when the photosynthetic capture of CO2
is proceeding at a rate that EXCEEDS the respiratory production of CO2.
Under this condition, there is a NET consumption of CO2 (and therefore a
net production of O2) but BOTH processes are occurring simultaneously.
Understanding this, one can recognize that there can be conditions where
there is photosynthetic capture of CO2 occurring BUT not at a rate
sufficient to completely offset respiration rates (resulting in a NET
production of CO2). This situation occurs commonly in terrestrial plants,
and, I assume, in aquaria. Under this condition, the plant is "loosing
ground" (consuming more carbon than it is producing), but NOT loosing
ground as fast as it would in the absence of any photosynthetic capture at
all. The relationship between CO2/O2 production and consumption in a
planted aquarium will be a function of the relative balance between these 2
processes at any given time. Therefore, there are almost certainly times,
even in the light, where there is more CO2 being produced by our aquarium
plants than is being consumed, even when the plant is not "rotting". In
fact, I suspect that this gradual "loosing ground", resulting from some
limiting factor (unfavorable light intensity, nutrient deficiency, etc.) is
what causes the gradual failing of plants so commonly reported by beginning
plant enthusiasts.

Sorry for the long-winded post, but the idea that "photosynthesis occurs in
light, and respiration occurs in the dark" is a concept I battle constantly
with students, and, surprisingly, with a number of otherwise well-informed

Well, back to my tank...

(I too should be grading papers....but boy, the tank is more fun...)


    John Shane                                        (802) 656-2907
University of Vermont                       jshane at moose_uvm.edu
 Forestry Department                        FAX --   (802)656-8683