Re: Exactly how does UVB have a direct effect on photosynthesis in phytoplankto

Photosynthesis, which uses light energy to convert water and CO2 into O2
and glucose, consists of two main pathways; photosystem II and photosystem
I.  In short, photosystem II is responsible for breaking up the H2O
molecules into O2 molecules, and releasing free H+ ions.  These H+ ions are
then used in photosystem I.  Details of these systems are well explained on
this University of Illinois site.

UVB affects one component of photosystem II very close to where water is
split.  Specifically, UVB is absorbed by a single protein named D1, you may find
this on some diagrams of photosystem II.  UVB modifies the protein and stops the
transfer of electrons through photosystem 2, which knocks out photosystem 2 and
so knocks out photosynthesis.

I hope this helps!

References: 
Bouchard, Roy, and Campbell 2006 UVB effects on the photosystem II D1 
protein of phytoplankton and natural phytoplankton communities. 
Photochemistry and Photobiology 82: 936-951.