Unicellular organisms are living entities that consist of a single cell. They can perform all necessary life functions, such as growth, reproduction, and metabolism, within that one cell. Examples of unicellular organisms include bacteria, archaea, and some types of algae and protozoa. These organisms are often microscopic and can be found in various environments, from soil to water. Despite their simplicity, unicellular organisms play crucial roles in ecosystems. They contribute to nutrient cycling, serve as food for larger organisms, and can even be involved in processes like fermentation. Some unicellular organisms, like certain bacteria, can also be beneficial to humans, aiding in digestion or producing antibiotics.