Exercise 2 solution (model answer)

Photosynthesis: energy from sunlight; sunlit upper ocean; phytoplankton and seaweeds; supplies the great majority of ocean food.
Chemosynthesis: energy from chemical reactions, for example oxidising hydrogen sulphide or methane; vents and seeps; bacteria and archaea, often living inside tube worms, clams and mussels; locally very productive but small overall.

A vent community lives in the dark because its bacteria get energy from chemicals in the hot fluid rather than from light. It does still need oxygen in many cases, which comes from the ocean's circulation (and ultimately from photosynthesis elsewhere), and the chemicals come from seawater reacting with hot rock. So it is independent of sunlight at the point of production but depends on the ocean's chemistry and circulation.