Which of the following best describes a primary producer in an ecosystem?

A primary producer in an ecosystem is best described as organisms that synthesize their own food. These organisms, primarily plants and certain types of algae and bacteria, utilize sunlight through the process of photosynthesis to convert carbon dioxide and water into glucose and oxygen. This ability to create their own energy-rich compounds places them at the base of the food chain, making them essential for supporting the entire ecosystem.

Primary producers play a critical role in transferring energy from the sun into the food web, as they are the first trophic level that supports herbivores, which in turn can be prey for carnivores. Their importance also extends beyond energy production, as they help regulate the levels of oxygen and carbon dioxide in the atmosphere, making them vital for maintaining environmental balance.

In contrast, the other descriptions do not capture the essence of primary producers. Those that feed on herbivores are classified as primary consumers, while organisms that consume dead organic matter are known as decomposers or detritivores. The term "compete for resources" pertains to the interactions between organisms rather than their role in energy production. Thus, understanding that primary producers are self-sufficient in food synthesis highlights their fundamental role within ecosystems.