Protists are a group of loosely interconnected, mostly unicellular eukaryotic organisms that are not plants, animals, or fungi. Not all of these organisms have a single feature, such as evolutionary history or morphology, and are informally placed under a separate state called the Protista. Therefore, protists are no longer a formal classification, and different members show varying degrees of homology with species belonging to all five eukaryotic states. However, it is still used as a term of convenience to describe eukaryotic microbes.
Initially, this group consisted of bacteria and fungi, but now it is specifically included in organisms that have a fully defined nucleus, which has a complex cellular structure but lacks tissue-level organization.
History of Classification
Protists include a remarkable number and variety of living organisms that overcome bacteria and viruses in their species diversity. It has been estimated that around three times there have been described as many unseen protesters. Their functional diversity and the cosmopolitan nature of the marks they create make them important for the conservation and maintenance of biodiversity.
Protist was first classified as a group of organisms by Ernst Haeckel in the 1860s, which uses a word derived from the Greek word protists meaning ‘long ago’. It was initially used to indicate that these organisms were possibly primitive forms of plants and animals. The term appeared in the background of the invention of the microscope and the discovery of a wide variety of microorganisms.
DNA sequencing and molecular genetics have made it easier to establish relationships between evolutionary lineages and different groups of organisms. This has contributed to the redistribution of protesters among the other five eukaryotic states. Some scientists however classify them based on their infrastructure and biochemistry. The classification of protists is an area of active research, even as new tools are emerging for the study of phylogenetics.
Characteristics of Protists
Protists show a variety of feeding habits, reproductive cycles, and modes of movement. There are some autotrophs, who use dyes to harness solar energy and convert it into simple carbohydrates. Some are heterotrophs like other amoebae and consume other organisms for food. However, many of them have been known to switch feeding modes from autotrophic to heterotrophic in response to the environment.
They are also seen in almost every ecological zone – from hot springs to arctic ice caps, from swimming pools to mosquito intestines. Some are also present in deep-sea geothermal vents.
Most protists have mitochondria and a well-defined cellular structure. However, cells are rarely arranged in higher structures. Even macroscopic species such as giant kelp, which can be tens of meters in length, are formed by large clonal aggregation of complex cells that are fully functional and completely independent. There is no specialization in cells, nor the formation of tissues or organs. This contributes to the occurrence of polymorphisms, where a protist may appear at some point in its life cycle as an independent cell and in others as a clonal aggregate. This is a big reason why giant kelp is not considered a plant.
Reproduction occurs primarily through binary fission or budding, allowing for the continuation of particular adaptations. However, sexual reproduction can occur in various forms when it occurs, whether it is self-fertilized or through cross-fertilization. For example, Plasmodium, the causative agent for malaria, is an asexual as well as a sexual phase in its life cycle. Many Protestants are believed to demonstrate sexual reproduction.
Types of Protists
Protists can be classified based on their size, shape, the nature and number of nuclear structures, the presence of cytoplasmic organelles, endo or ectoskeletal structures, and so on. Given their nature and diversity, however, informal classification is often based on nutrition and mobility.
Autotrophic, non-motile protists often resemble plants, although they lack high levels of tissue organization. It is estimated that members of this group contribute to about 40% of photosynthetic activity on Earth. These organisms can be further differentiated based on the pigments used in photosynthesis: chlorophyll (green algae), fucoxanthin (brown algae) and ficroethrin (red algae) for others. Many of these appear as secondary pigments, along with chlorophyll.
Some proteins are heterotrophic and motive. They are classified on the basis of their genre. Like an amoeba, some people use pseudopodia or ‘false legs’, while others use flagella or cilia. These free-living guards are among the major consumers of bacteria and contribute to species diversity in many ecosystems.
Protists that decompose organic matter were thought to be similar to fungi.
Examples of Protists
Protists are diverse and exist in a wide variety of ecosystems, often forming the basis of the energy pyramid – either as producers or primary consumers.
Giant kelps are autotrophic algal protists that can form extensive underwater forests. They carry fucoxanthin pigments in addition to chlorophyll, which contributes to their browning. They are found in shallow oceans that have sufficient light and nutrients and are capable of maintaining a complex ecosystem.
Often used as a model organism to study sexual reproduction, these microorganisms are abundant in water bodies – from stagnant ponds to the open ocean. They have abundant cilia and use them in a coordinated manner to move.
When faced with extreme environmental stresses, they sometimes lose their distinctive sandal-like shape.
Many free-living forms of protists are parasitic and can cause disease in animals and crops. Plasmodium causes malaria in humans and simian. Entamoeba histolytica is the cause of amoebic dysentery. Water molds such as Phytophthora infestense (the motive for the great Irish potato famine) and Plasmopara viticola have caused widespread disease in crops.
Protist Definition, Classification, Characteristics, Types and Examples