My answer will be that the valence of the Carbon atom is (+4) and that of each Oxygen atom is (-2). The CO2 molecule is formed by covalent bonds, in other words: C (+4) + O2 (-4) = 0.
An important part of photosynthesis is light thus the answer would be : <span>Sunlight provide light energy that allow photosynthesis to occur in plankton and aquatic plants.</span>