Photosynthesis is the primary method of energy acquisition for phytoplankton.
What is phytoplankton?
The autotrophic (self-feeding) members of the plankton community and a crucial component of freshwater and ocean ecosystems are phytoplankton.
Photosynthesis is how land plants like trees and phytoplankton get their energy. As a result, phytoplankton lives on the well-lit surface layers (euphotic zone) of seas and lakes because they need sunlight to survive. Phytoplankton are dispersed over a larger surface area than terrestrial plants, are subject to less seasonal change, and have noticeably higher turnover rates than trees (days versus decades). As a result, phytoplankton reacts to changes in the climate quickly on a global scale.
Hence the correct answer is photosynthesis
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