Re: what is the carbon source of the phytoplankton?

Hi Andrea
The carbon Source of Phytoplankton is CO2...
I think you already know that phytoplankton are photosynthetic organisms....
That is they produce their own food by photosynthesis....in other terms
They use CO2 and H2O (water) and convert it to Energy and O2 using the light energy (like any 
other plants present on Land).
All phytoplankton are basically Autotrophic...i.e. they produce their own food....
Very few are heterotrophic (especially dinoflagellates (some) which can use the energy in dissolved 
from in the seawater.


I tried to give the simplest explanation to your question

Cheers
Shashank