Protists are a diverse collection of organisms. While exceptions exist, they are primarily microscopic and unicellular, or made up of a single cell. The cells of protists are highly organized with a nucleus and specialized cellular machinery called organelles.
At one time, simple organisms such as amoebas and single-celled algae were classified together in a single taxonomic category: the kingdom Protista. However, the emergence of better genetic information has since led to a clearer understanding of evolutionary relationships among different groups of protists, and this classification system was rendered defunct. Understanding protists and their evolutionary history continues to be a matter of scientific discovery and discussion.
All living organisms can be broadly divided into two groups — prokaryotes and eukaryotes — which are distinguished by the relative complexity of their cells. In contrast to prokaryotic cells, eukaryotic cells are highly organized. Bacteria and archaea are prokaryotes, while all other living organisms — protists, plants, animals and fungi — are eukaryotes.
Many diverse organisms including algae, amoebas, ciliates (such as paramecium) fit the general moniker of protist. "The simplest definition is that protists are all the eukaryotic organisms that are not animals, plants or fungi," said Alastair Simpson, a professor in the department of biology at Dalhousie University. The vast majority of protists are unicellular or form colonies consisting of one or a couple of distinct kinds of cells, according to Simpson. He further explained that there are examples of multicellular protists among brown algae and certain red algae.
Like all eukaryotic cells, those of protists have a characteristic central compartment called the nucleus, which houses their genetic material. They also have specialized cellular machinery called organelles that execute defined functions within the cell. Photosynthetic protists such as the various types of algae contain plastids. These organelles serve as the site of photosynthesis (the process of harvesting sunlight to produce nutrients in the form of carbohydrates). The plastids of some protists are similar to those of plants. According to Simpson, others protists have plastids that differ in the color, the repertoire of photosynthetic pigments and even the number of membranes that enclose the organelle, as in the case of diatoms and dinoflagellates, which constitute phytoplankton in the ocean.
Most protists have mitochondria, the organelle which generates energy for cells to use. The exceptions are some protists that live in anoxic conditions, or environments lacking in oxygen, according to an online resource published by University of California, Los Angeles. They use an organelle called the hydrogenosome (which is a greatly modified version of mitochondria) for some of their energy production. For example, the sexually transmitted parasite Trichomonas vaginalis, which infects the human vagina and causes trichomoniasis, contains hydrogenosomes.
Protists gain nutrition in a number of ways. According to Simpson, protists can be photosynthetic or heterotrophs (organisms that seek outside sources of food in the form of organic material). In turn, heterotrophic protists fall into two categories: phagotrophs and osmotrophs. Phagotrophs use their cell body to surround and swallow up food, often other cells, while osmotrophs absorb nutrients from the surrounding environment. "Quite a few of the photosynthetic forms are also phagotrophic," Simpson told Live Science. "This is probably true of most 'algal' dinoflagellates for example. They have their own plastids, but will also happily eat other organisms." Such organisms are called mixotrophs, reflecting the mixed nature of their nutritional habits.
Most protists reproduce primarily through asexual mechanisms according to Simpson. This can include binary fission, where a parent cell splits into two identical cells or multiple fission, where the parent cell gives rise to multiple identical cells. Simpson added that most protists probably also have some kind of sexual cycle, however, this is only well documented in some groups.
Classification: from Protozoa to Protista and beyond
The classification history of protists traces our understanding of these diverse organisms. Often complex, the long history of protist classification introduced two terms, still used today, into the scientific lexicon: protozoa and protists. However, the meaning of these terms has also evolved over time.
The observable living world was once neatly divided between plants and animals. But the discovery of various microscopic organisms (including what we now know as protists and bacteria) brought forth the need to understand what they were, and where they fit taxonomically.
The first instinct of scientists was to relate these organisms to plants and animals by relying on morphological characteristics. The term protozoan (plural: protozoa or protozoans), meaning "early animals," was introduced in 1820 by naturalist Georg A. Goldfuss, according to a 1999 article published in the journal International Microbiology. This term was used to describe a collection of organisms including ciliates and corals. By 1845, Protozoa was established as a phylum or subset of the animal kingdom by German scientist Carl Theodor von Seibold. This phylum included certain ciliates and amoebas, which were described by von Seibold as single-celled animals. In 1860, the concept of protozoans was further refined and they were elevated to the level of a taxonomic kingdom by paleontologist Richard Owen. The members of this Kingdom Protozoa, in Owen's view, had characteristics common to both plants and animals.
Though the scientific rationale behind each of these classifications implied that protozoans were rudimentary versions of plants and animals, there was no scientific evidence of the evolutionary relationships between these organisms (International Microbiology, 1999). According to Simpson, nowadays "protozoa" is a term of convenience used in reference to a subset of protists, and is not a taxonomic group. "In order to be called a protozoan, they [protists] have to be non-photosynthetic and not very fungus-like," Simpson told Live Science.
The term protista, meaning "the first of all or primordial" was introduced in 1866 by German scientist Ernst Haeckel. He suggested Protista as a third taxonomic kingdom, in addition to Plantae and Animalia, consisting of all "primitive forms" of organisms, including bacteria (International Microbiology, 1999).
Since then, the kingdom Protista has been refined and redefined many times. Different organisms moved in and out (notably, bacteria moved into a taxonomic kingdom of their own). American scientist John Corliss proposed one of the modern iterations of Protista in the 1980s. His version included the multicellular red and brown algae, which are considered to be protists even today.
Scientists, often concurrently, have debated kingdom names and which organisms were eligible (for example, versions of yet another kingdom, Protoctista had been proposed over the years). However, it is important to note the lack of correlation between taxonomy and evolutionary relationships in these groupings. According to Simpson, these groupings were not monophyletic, meaning that they did not represent a single, whole branch of the tree of life; that is, an ancestor and all of its descendants.
Today's classification has shifted away from a system built on morphology to one based on genetic similarities and differences. The result is a family tree of sorts, mapping out evolutionary relationships between various organisms. In this system there are three main branches or "domains" of life: Bacteria, Archaea (both prokaryotic) and Eukarya (the eukaryotes).
Within the eukaryotic domain, the protists are no longer a single group. They have been redistributed amongst different branches of the family tree. According to Simpson, we now know most of the evolutionary relationships amongst protists, and these are often counterintuitive. He cited the example of dinoflagellate algae, which are more closely related to the malaria parasite than they are to diatoms (another group of algae) or even to land plants.
Still, there are pressing questions that remain. "We simply don't know what the earliest split was among the lineages that led to living eukaryotes," Simpson told Live Science. This point is called the "root" of the eukaryotic tree of life. Pinpointing the root will cement the understanding of eukaryotic origins and their subsequent evolution. As author Tom Williams said in a 2014 article published in the journal Current Biology, "For the eukaryotic tree, the root position is critical for identifying the genes and traits that may have been present in the ancestral eukaryote, for tracing the evolution of these traits throughout the eukaryotic radiation, and for establishing the deep relationships among the major eukaryotic groups."
Protists are responsible for a variety of human diseases including malaria, sleeping sickness, amoebic dysentery and trichomoniasis. Malaria in humans is a devastating disease. It is caused by five species of the parasite Plasmodium, which are transmitted to humans by female Anopheles mosquitoes, according to the Centers for Disease Control and Prevention (CDC). The species Plasmodium falciparum infects red blood cells, multiplies rapidly and destroys them. Infection can also cause red blood cells to stick to the walls of small blood vessels. This creates a potentially fatal complication called cerebral malaria (according to the CDC). The World Health Organization (WHO) states that Plasmodium falciparum is the most prevalent and lethal to humans. According to their recent malaria fact sheet, in 2015 there were an estimated 438,000 deaths due to malaria in the world, the majority of which (90 percent) occurred in Africa. Certain strides have been made in reducing the rates of incidence (occurrence of new cases) and mortality rates in part by supplying insecticide treated mosquito nets, spraying for mosquitoes and improving diagnostics. Between 2000 and 2015 the rate of incidence fell by 37 percent globally and mortality rates fell by 60 percent globally. The WHO has a goal of eliminating malaria in at least 35 countries by 2030.
Protists also play an important role in the environment. According to a 2009 review article published on the Encyclopedia of Life Sciences (eLS) website, nearly 50 percent of photosynthesis on Earth is carried out by algae. Protists act as decomposers and help in recycling nutrients through ecosystems, according to a 2002 review article published in the journal ACTA Protozoologica. In addition, protists in various aquatic environments, including the open water, waterworks and sewage disposal systems feed upon, and control bacterial populations (ACTA Protozoologica, 2002). "If you took all the protists out of the world, the ecosystem would collapse really quickly," Simpson said.