Which term refers to one-celled microorganisms that can be either beneficial or harmful?

The correct term that refers to one-celled microorganisms that can be either beneficial or harmful is bacteria. Bacteria are incredibly diverse and exist in various environments, ranging from soil and water to the human body. While many types of bacteria are essential for processes such as digestion and nutrient cycling, others can cause diseases and infections in humans, animals, and plants.

Bacteria play a critical role in the ecosystem, with some types being utilized in food production, medication, and biotechnology. Their ability to adapt and thrive in numerous environments illustrates both their beneficial and harmful potential, depending on the species and conditions they are in.

The other terms listed denote different types of microorganisms: viruses are not classified as cells and primarily exist to infect host cells; fungi, while they can include single-celled organisms like yeast, primarily exist as multicellular structures; and protozoa, though they are single-celled, are mainly known for their role in the food chain and sometimes as parasites rather than for their beneficial aspects. Thus, the broad and dual nature of bacteria makes it the correct answer.