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Kingdom Protoctista is a biological classification for unicellular microorganisms with eukaryotic ancestry. Referencing eukaryotic ancestry simply means the organism is made up of eukaryotic cells: those cells that feature a nucleus and surrounding membrane. Virtually all living organisms, with the exception of prokaryotes, are made up of eukaryotic cells. The primary distinguishing factor of Kingdom Protoctista organisms is that protoctists are typically unicellular aquatic microorganisms.
Organisms arranged under Kingdom Protoctista included water molds, slime molds, algae, and similar eukaryotic unicellular microorganisms. Records indicate the smallest organism in Kingdom Protoctista are protists, microscopic single cell organisms such as protozoa. Protoctists live exclusively in aquatic conditions, although certain species exclusively choose freshwater, marine, or watery tissue environments. Large numbers of protoctists are parasitical, living in the watery tissues of host animals and plants.
For decades, scientists arranged all living organisms into five primary biological kingdoms, which were then subdivided into descending categories including phylum, class, order, family, and other categories. Primary kingdoms consisted of animal, plant, fungi, prokaryotes, or protoctista. Elementary school children and even high school and college students were, and frequently still are, taught the five kingdom biological classification scheme in science classes around the world.
As part of the five kingdom scheme, Kingdom Protoctista served as a kingdom of exclusion. Any living organism that was not animal, fungi, plant, or prokaryote was arranged under Kingdom Protoctista by default. Animals and plants form from embryos, while fungi form from spores. Prokaryotes have specific structural characteristics at the cellular level, the most notable being the lack of a nuclei. Members of Kingdom Protoctista do not form from embryos or spores and feature eukaryotic cells rather than prokaryote cells, thus warranting a separate kingdom classification.
Discoveries of unicellular microorganisms dating back to the 1960s and 1970s were found to have multicellular derivatives. Numerous such discoveries changed the way some scientists viewed the old five kingdom scheme. Since the 1970s, some biologists, zoologists, and other scientific researchers have argued for a new multi-kingdom classification scheme. In fact, many such researchers have chosen to use various multi-kingdom schemes adopted by certain segments of the scientific community.
One point of argument in favor of revamping historical understandings of Kingdom Protoctista is the discovery of unicellular organisms with multicellular derivatives. Some of these derivative organisms meet criteria for plant phyla. Other organisms discovered show such unique tissue structures compared to other members of Kingdom Protoctista that researchers argue these organisms deserve a separate kingdom classification.