Lemna and oxygen transport

[gclingen at aol.com]

Hello,

     I am conducting a project in which Lemna 
minor being grown in municipal effluent is 
used as food for Tilapia (Oreochromis 
mossambicus X O. aureus). The original 
objective was to do a direct comparison between 
Azolla sp. and Lemna sp. grown in this high 
nutrient environment as food for the same fish.
Unfortunately, a cyanobacteria (I believe it is 
oscillatoria) formed a dense mat under the 
surface of the Azolla, which promptly died. The 
same cyanobacteria seems only to slow the growth 
of duckweed. My current theory is that the 
cyanobateria mat prevented the Azolla from 
getting oxygen from the water, killing the roots.
Does anyone know if duckweed have lacuna or other 
oxygen transporting mechanisms which would allow 
it to live in this oxygen impoverished environment?
I know that rooted aquatic macrophytes have many 
such mechanisms since the mud in which they grow
is almost always anaerobic. Floating plants like
duckweed and Azolla have roots relatively close 
to the surface, where normal O2 diffusion keeps 
oxygen levels high, so I do not know if they 
normally require leaf to root oxygen transport 
mechanisms. At the same time, I would like to 
see if Azolla have oxygen transporting mechanisms. 
Please send any references you may know of to: 
Glenn Clingenpeel
gclingen at aol.com
         
Thank you.
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