Re: How do plants perform Photosynthesis under water?

In a nutshell, plants perform photosynthesis underwater in the same way 
plants perform photosynthesis in the air.  Light can penetrate into water 
some distance and the underwater plants capture energy from this light 
using chlorophylls.  The light energy is eventually used to make ATP or to 
make reducing equivalents (NADPH), with the production of oxygen of course.  
Thus, photosynthesis underwater is just like terrestrial photosynthesis.

I have a feeling that you may really be asking about the dark reactions of 
photosynthesis, the fixation of carbon dioxide to sugars.  Here the 
processes are again the same between plants on land and underwater.  
Reducing power captured during the light reactions is used to reduce carbon 
dioxide (CO2) to sugars (CHOH).  Just as fish can breathe underwater, so 
can plants.  By contact with the atmosphere, water becomes saturated with 
carbon dioxide.  Much of this is in the form of bicarbonate ions.  The 
underwater plants take up the inorganic carbon and use it in the Calvin 
cycle, the dark reactions of photosynthesis.  The process is the same as in 
terrestrial plants.  The only real issue is the uptake of inorganic carbon 
from the water.  But this is taken up by the plant easily.

If you want to learn more about photosynthesis you can visit this website 
and sites linked from it:
http://photoscience.la.asu.edu/photosyn/default.html

Hope this helps.

Cheers,

Dr. Mark.