Question

Why does chemosynthesis (occurring deep down in the ocean) use as an energy source instead of sunlight?

Answer 1

because deep down the ocean u have very little or almost no sunlight..

Answer 2

according to the detail you have mentioned, it occur deep down in the sea.
As we know that at bottom layer of sea never get to see sun light they are bounded to accept the process of chemosynthesis to produce energy.