Which term refers to any living entity that forms part of an aquatic ecosystem?

The term that refers to any living entity that forms part of an aquatic ecosystem is "aquatic organism." This encompasses a wide variety of life forms that inhabit water bodies, including fish, amphibians, aquatic plants, algae, and microorganisms. Aquatic ecosystems can be freshwater or saltwater, and thus, the term is inclusive of all organisms that thrive in these environments.

In contrast, the term "marine entity" specifically denotes organisms found in saltwater environments, such as oceans and seas, and does not cover freshwater organisms. "Terrestrial organism" describes land-based life forms, like plants and animals that live on land, and is not relevant to aquatic ecosystems. Lastly, "freshwater life" refers only to organisms found in freshwater systems, which excludes those that live in saltwater environments. Therefore, the most comprehensive and inclusive term is "aquatic organism," making it the correct choice for describing any living entity in an aquatic ecosystem.