Final answer:
Photosynthesis is performed by phytoplankton, algae, and cyanobacteria in the open ocean, which are the primary producers responsible for capturing light energy and synthesizing organic compounds.
Step-by-step explanation:
Photosynthesis is the process by which organisms use light energy to produce their own food. In the open ocean, tiny organisms called phytoplankton are the primary producers that perform photosynthesis, similar to plants on land. These phytoplankton, along with algae and cyanobacteria, capture light energy and synthesize organic compounds through photosynthesis, making them crucial for the survival of many oceanic ecosystems.