Photosynthesis occurs in the chloroplasts. These contain pigments such as chlorophyll and beta carotene, it is these pigments that absorb light and are responsible for water decomposition, oxygen production and the production of high energy compounds. During photosynthesis light energy  is converted to chemical energy, which is used to reduce carbon dioxide into carbohydrates.

Factors affecting photosynthesis
There are several factors which affect the rate of photosynthesis of diatoms. These factors include light intensity, and light intensity differences. Light quality is also important, diatoms experience a high rate of photosynthesis when exposed to wavelengths of light between 500 and 550 nm. The temperature of the water is another factor that affects photosynthesis, diatoms are very successful in cold water.