Which marine organisms primarily rely on photosynthesis?

Phytoplankton are the primary marine organisms that rely on photosynthesis for their energy needs. These microscopic, plant-like organisms form the base of the marine food web and are capable of converting sunlight into energy through the process of photosynthesis. By utilizing chlorophyll and other pigments, they capture light energy which fuels their growth and reproduction. This not only sustains their populations but also plays a crucial role in producing oxygen and sequestering carbon dioxide in oceanic ecosystems.

The other options consist of organisms that do not perform photosynthesis. Fish, carnivorous zooplankton, and crustaceans are all heterotrophic, meaning they obtain their energy by consuming other organisms rather than producing it themselves. While they may depend on phytoplankton and other photosynthetic organisms for their food indirectly, they do not engage in the photosynthetic process themselves. This distinguishes phytoplankton as the key players in photosynthesis within the marine environment.