Bubbles and diffusion

> From: krombhol at felix_teclink.net (Paul Krombholz)
> Date: Sun, 23 Feb 1997 17:21:19 -0500
> Subject: Re: CO2 and O2 diffusion
> >But increasing the flow rate will also carry away oxygen more readily, so
> >it isn't a good test to compare bubble production in stagnant and
> >moving water conditions. (Craig)
> Let's say you had a situation where the plant was not CO2 saturated under
> stagnent conditions,and was producing bubbles, and you then circulated the
> water.  The decreased boundary layer would allow greater CO2 uptake in this
> case.  If the rate of CO2 uptake by the plant increased as much as the rate
> of oxygen loss by diffusion, then you should still get bubbles. That was my
> thinking, but now I see a flaw in that argument.  Since I get no bubbles
> with water movement and a very good supply of CO2, I should not expect to
> see any bubbles with water movement and a lower supply of CO2 and a lower
> rate of photosynthesis.  The conclusion is that bubble production should
> cease at all CO2 levels when the water is circulated.

Plants in stagnant conditions probably don't get enough CO2 to produce
oxygen bubbles unless they take it in through their roots. Environments
where there is a good CO2 source also make a good O2 sink since the
plants need to transmit oxygen into the root system to protect against
the strong reducing environment down there. The oxygen should definitely
diffuse faster in the gaseous environment of the aerenchyma tissue since
it has a smaller molecular size. The aerenchyma tissue forms the air
transport system in aquatic plants. Phloem and xylem form the liquid
vascular system.

In the aquarium where there is an artificial abundance of CO2, and we
observe O2 bubbles, from Paul's arguments we have to conclude that the
plant photosynthesis is not being rate limited by CO2. While CO2 is
increasingly available due to a thinner boundary layer, oxygen production
is not increased linearly hence the aquarium is rate limited by some
other nutrient, light or physical process such as nutrient transpiration

High CO2 water concentrations in natural environments don't occur.
Plants often live in moving water which helps improve the CO2
exchange rates. Some plants like Cabomba, Myriophylum and the
Lace plant have narrow leaf structures as another method of decreasing
the boundary layer size. Plants which live in strongly reducing
muck environments and conduct oxygen into their roots must also be
able to get CO2 as well since the aerenchyma must extend right into
the roots and back to the chloroplast sites.

 Steve Pushak - spush at hcsd_hac.com - Vancouver, BC, Canada

There are no signposts in the sky to show a man has passed that way
before. There are no channels marked. The flier breaks each second
into new uncharted seas. - Anne Morrow Lindbergh