Here's why:

* Biofilms are complex communities: Natural biofilms are composed of multiple species of bacteria, fungi, algae, and other microorganisms. These organisms interact with each other and their environment in intricate ways.

* Diversity is key: The diversity of species within a biofilm contributes to its resilience and functionality. Different organisms can provide different benefits, such as nutrient cycling, waste degradation, and resistance to environmental stresses.

* Pure cultures are isolated: A pure culture refers to a population of cells that consists of only one species. This is usually achieved in a laboratory setting by isolating a single organism and allowing it to multiply in a controlled environment.

In contrast to a pure culture, a natural biofilm is a mixed culture, where multiple species coexist and interact. This complexity makes natural biofilms fascinating systems to study and understand.