Kingdom Protoctista is a classification for single-celled microorganisms with eukaryotic cells, typically unicellular aquatic microorganisms. It includes water molds, slime molds, algae, and protists. Protoctists live exclusively in aquatic conditions and many are parasites. The five-kingdom classification scheme excluded organisms that were not animal, fungus, plant, or prokaryote. The discovery of unicellular organisms with multicellular derivatives has led to a discussion of renewing the Protoctist Kingdom.
Kingdom Protoctista is a biological classification for single-celled microorganisms with eukaryotic ancestors. Referring to eukaryotic ancestry simply means that the organism is made up of eukaryotic cells: those cells that have a nucleus and a surrounding membrane. Virtually all living organisms, with the exception of prokaryotes, are made up of eukaryotic cells. The main distinguishing factor of Kingdom Protoctista organisms is that protoctistas are typically unicellular aquatic microorganisms.
Organisms organized under the Protoctist Kingdom included water molds, slime molds, algae, and similar eukaryotic unicellular microorganisms. Records indicate that the smallest organism in the Protoctist Kingdom are protists, microscopic single-celled organisms like protozoa. Protoctists live exclusively in aquatic conditions, although some species exclusively choose freshwater, marine or aqueous environments. A large number of protoctists are parasites, living in the aqueous tissues of host animals and plants.
For decades, scientists have organized all living organisms into five primary biological kingdoms, which have then been divided into descendant categories including phylum, class, order, family, and other categories. The primary kingdoms consisted of animals, plants, fungi, prokaryotes, or protoctists. Elementary school children and even high school and college students were taught, and often still is, the five realms biological classification scheme in science classrooms around the world.
As part of the five kingdoms scheme, Kingdom Protoctista served as an exclusionary kingdom. Any living organism that was not animal, fungus, plant, or prokaryote was placed under Kingdom Protoctista by default. Animals and plants are formed from embryos, while fungi are formed from spores. Prokaryotes have specific structural features at the cellular level, most notably the lack of a nucleus. Members of the Protoctist Kingdom do not form from embryos or spores and feature eukaryotic cells rather than prokaryotic cells, thus warranting a separate classification of the kingdom.
Discoveries of unicellular microorganisms dating back to the 1960s and 1970s have been found to have multicellular derivatives. Numerous such discoveries changed the way some scientists viewed the old five kingdoms scheme. Since the 1970s, some biologists, zoologists and other scientific researchers have advocated a new multi-kingdom classification scheme. In fact, many of these researchers have chosen to use various multi-realm schemes adopted by some segments of the scientific community.
A point of discussion in favor of renewing the historical understanding of the Protoctist Kingdom is the discovery of unicellular organisms with multicellular derivatives. Some of these derived organisms meet the criteria for plant phyla. Other discovered organisms show tissue structures so unique compared to other members of the Protoctist Kingdom that researchers argue that these organisms deserve a separate classification of the kingdom.
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