What term describes microscopic, one-celled animals often found in decayed materials and contaminated water?

The term that describes microscopic, one-celled animals often found in decayed materials and contaminated water is protozoa. These organisms belong to the kingdom Protista and are known for their varied shapes and modes of movement, such as using cilia, flagella, or pseudopodia. Protozoa are typically heterotrophic, meaning they obtain their nutrients by consuming organic matter, including bacteria and decaying materials, which allows them to thrive in environments rich in decomposing organic material. Their presence in contaminated water can be significant, as some protozoa can also be pathogens, causing diseases in humans and animals.

In contrast, bacteria, while also microscopic and found in similar environments, are prokaryotic microorganisms, which means they lack a nucleus and are fundamentally different from protozoa. Algae are primarily photosynthetic organisms and can be unicellular or multicellular, but they are not classified as animals and don't fit the description provided. Viruses are much smaller than both bacteria and protozoa and are acellular, meaning they require a host cell to replicate, which differentiates them from the one-celled nature of protozoa.