Marine microbiology is the study of microorganisms and non-organismic microbes that exist in saltwater environments including the Open Ocean, coastal waters, and estuaries on marine surfaces and in sediments. Defining marine microbiology means determining what exactly qualifies as a marine microorganism. Microorganism usually means any living entity too small to see with the naked eye and is often used for any single-celled organism. However, some single-celled organisms are visible alone or when gathered in communities and some multicellular animals are microscopic. Marine microbiology deals with all very small life and life-like biological phenomena like non-organismic microbes, bacteria, archaea, protozoans, single-celled algae and very small multicellular plants, fungi, and animals.