4. What is the ecological niche of ph ytoplankton in marine ecosystems? A. Serving as predators B. Breaking down organic matter C. Producing organic matter

The correct answer is C. Producing organic matter.<br /><br />Phytoplankton are microscopic organisms that live in the water column of oceans, seas, and freshwater bodies. They are primary producers in marine ecosystems, meaning they are able to convert inorganic carbon dioxide into organic compounds through the process of photosynthesis. This process produces oxygen and organic matter, which serves as a food source for other organisms in the ecosystem. Therefore, the ecological niche of phytoplankton in marine ecosystems is primarily producing organic matter.