Search results
"Marine protists are a polyphyletic group of organisms playing major roles in the ecology and biogeochemistry of the oceans, including performing much of Earth's photosynthesis and driving the carbon, nitrogen, and silicon cycles. In addition, marine protists occupy key positions in the tree of life, including as the closest relatives of ...
Diversity within marine microbiomes spans the three domains of life: microbial eukaryotes (i.e., protists), bacteria, and archaea. Although protists were the first microbes observed by microscopy, it took the advent of molecular techniques to begin to resolve...
Marine Protists. The ocean is teeming with a fascinating world of organisms - the marine protists. Protists are a weird collection of organisms. Most are unicellular, although there are a few...
Protists have fascinated microbiologists since their discovery nearly 350 years ago. These single-celled, eukaryotic species span an incredible range of sizes, forms, and functions and, despite their generally diminutive size, constitute much of the genetic diversity within the domain Eukarya.
- Overview
- Defining the protists
protist, any member of a group of diverse eukaryotic, predominantly unicellular microscopic organisms. They may share certain morphological and physiological characteristics with animals or plants or both. The term protist typically is used in reference to a eukaryote that is not a true animal, plant, or fungus or in reference to a eukaryote that l...
From the time of Aristotle, near the end of the 4th century bce, until well after the middle of the 20th century, the entire biotic world was generally considered divisible into just two great kingdoms, the plants and the animals. The separation was based on the assumption that plants are pigmented (basically green), nonmotile (most commonly from being rooted in the soil), photosynthetic and therefore capable solely of self-contained (autotrophic) nutrition, and unique in possessing cellulosic walls around their cells. By contrast, animals are without photosynthetic pigments (colourless), actively motile, nutritionally phagotrophic (and therefore required to capture or absorb important nutrients), and without walls around their cells.
When microscopy arose as a science in its own right, botanists and zoologists discovered evidence of the vast diversity of life mostly invisible to the unaided eye. With rare exception, authorities of the time classified such microscopic forms as minute plants (called algae) and minute animals (called “first animals,” or protozoa). Such taxonomic assignments went essentially unchallenged for many years, despite the fact that the great majority of those minute forms of life—not to mention certain macroscopic ones, various parasitic forms, and the entire group known as the fungi—did not possess the cardinal characteristics on which the “plants” and “animals” had been differentiated and thus had to be forced to fit into those kingdom categories.
In 1860, however, British naturalist John Hogg took exception to the imposition of the plant and animal categories on the protists and proposed a fourth kingdom, named Protoctista (the other three kingdoms encompassed the animals, the plants, and the minerals). Six years later German zoologist Ernst Haeckel (having dropped the mineral kingdom) proposed a third kingdom, the Protista, to embrace microorganisms. In the late 1930s American botanist Herbert F. Copeland proposed a separate kingdom for the bacteria (kingdom Monera), based on their unique absence of a clearly defined nucleus. Under Copeland’s arrangement, the kingdom Protista thus consisted of nucleated life that was neither plant nor animal. The following decade he revived the name Protoctista, using it in favour of Protista.
Britannica Quiz
All About Biology Quiz
The next major change in the systematics of lower forms came through an advancement in the concept of the composition of the biotic world. About 1960, resurrecting and embellishing an idea originally conceived two decades earlier by French marine biologist Edouard Chatton but universally overlooked, Roger Yate Stanier, Cornelius B. van Niel, and their colleagues formally proposed the division of all living things into two great groups, the prokaryotes and the eukaryotes. This organization was based on characteristics—such as the presence or absence of a true nucleus, the simplicity or complexity of the DNA (deoxyribonucleic acid) molecules constituting the chromosomes, and the presence or absence of intracellular membranes (and of specialized organelles apart from ribosomes) in the cytoplasm—that revealed a long phylogenetic separation of the two assemblages. The concept of “protists” originally embraced all the microorganisms in the biotic world. The entire assemblage thus included the protists plus the bacteria, the latter considered at that time to be lower protists. The great evolutionary boundary between the prokaryotes and the eukaryotes, however, has meant a major taxonomic boundary restricting the protists to eukaryotic microorganisms (but occasionally including relatively macroscopic organisms) and the bacteria to prokaryotic microorganisms.
- The Editors of Encyclopaedia Britannica
Most protists are microscopic, unicellular organisms that are abundant in soil, freshwater, brackish, and marine environments. They are also common in the digestive tracts of animals and in the vascular tissues of plants. Others invade the cells of other protists, animals, and plants. Not all protists are microscopic.
Apr 6, 2020 · Our genetic manipulation systems for aquatic (largely marine) protists will enable deeper insights into their cell biology, with potentially valuable outcomes for aquatic...
