Which of the following best describes the role of protozoa in ecosystems?

Protozoa play a crucial role in ecosystems as decomposers of organic material. They contribute significantly to nutrient cycling by breaking down organic matter, which includes dead plants, animals, and microbial debris. This decomposition process helps to recycle nutrients back into the soil, making them available for other organisms, especially plants.

While some organisms, like plants and phytoplankton, are responsible for producing oxygen, that role is not typically associated with protozoa. Protozoa can also be part of food webs where they serve as prey for various predators, but they are not usually classified as top predators themselves. The description of protozoa as only detrimental agents overlooks their essential roles in nutrient recycling and maintaining ecological balance. Thus, their primary function as decomposers aligns with the selected answer, highlighting their importance in sustaining healthy ecosystems.