What is a primary characteristic of microorganisms?

Microorganisms are primarily characterized by their small size, which generally makes them invisible or only barely visible to the naked eye. Most microorganisms, including bacteria, viruses, fungi, and protozoa, are measured in micrometers or nanometers, far smaller than what the human eye can see without the aid of a microscope. This characteristic is fundamental in many fields, including microbiology, environmental science, and water treatment processes.

While some microorganisms can be harmful to humans or other organisms, many are beneficial and play essential roles in processes like decomposition, nutrient cycling, and fermentation. The ability to thrive in extreme environments is a property of specific types of microorganisms, such as extremophiles, but this does not represent microorganisms as a whole. Similarly, while some microorganisms can be visible, this isn't typical for the majority. This unique size characteristic is critical when considering how to monitor and manage microbial populations in various settings, such as in water treatment, ensuring that operators can effectively control the water quality.