Protists are a group of loosely connected, mostly unicellular eukaryotic organisms that are not plants, animals or fungi. There is no single feature such as evolutionary history or morphology common to all these organisms and they are unofficially placed under a separate kingdom called Protista. Therefore, protists are no longer a formal classification, and different members show varying degrees of homology with species belonging to all five eukaryotic kingdoms. However, it is still used as a term of convenience to describe eukaryotic microscopic organisms.
Initially this group included bacteria and fungi, but now it is comprised exclusively of organisms having a fully defined nucleus, with complex cellular structure but lacking tissue-level organization.
History of Classification
Protists include a remarkable number and variety of living organisms that far outnumber bacteria and viruses in their species diversity. It is estimated that there are nearly three times as many undiscovered protists as there are ones that have been described. Their functional diversity and the cosmopolitan nature of the niches they inhabit make them crucial for conservation and the maintenance of biodiversity.
Protists were first classified as a group of organisms by Ernst Haeckel in the 1860s, using the term derived from the Greek word protistos meaning ‘the very first’. It was initially used to indicate that these organisms were probably primitive forms of plants and animals. This term appeared in the backdrop 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 evolutionary lineages and the relationships between different groups of organisms. This has further contributed to the redistribution of protists among the other five eukaryotic kingdoms. Some scientists however classify them based on their ultrastructure and biochemistry. Classification of protists continues to be an area of active research, even as new tools are emerging for the study of phylogenetics.
Characteristics of Protists
Protists show a wide variety of feeding habits, reproductive cycles and modes of locomotion. Some are autotrophs, using pigments to harness solar energy and convert it into simple carbohydrates. Some others are heterotrophs like amoeba 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 nearly every ecological niche – from hot springs to arctic ice caps, from swimming pools to the intestines of mosquitoes. Some are even present in deep ocean geothermal vents.
Most protists have a mitochondria and a well defined cellular structure. However, the cells are rarely organized into higher structures. Even macroscopic species like giant kelp, that can be tens of meters in length, are formed by large clonal aggregations of complex cells that are fully functional and completely independent. There is no specialization in the cells, nor the formation of tissues or organs. This contributes to the phenomenon of polymorphy, where a protist can appear as an independent cell at some point in its life cycle and as a clonal aggregate at others. This is one of the major reasons why giant kelps are not considered plants.
Reproduction is mainly through binary fission or budding, allowing for the continuation of specialized adaptations. However, sexual reproduction when it does occur, can take on varied forms, whether self-fertilized or through cross-fertilization. For example, Plasmodium, the causative agent for malaria, has an asexual as well as a sexual phase in its life cycle. Many protists are also believed to show facultative sexual reproduction.
Types of Protists
Protists can be classified based on their shape, size, the nature and number of nuclear structures, cytoplasmic organelles, presence of endo- or ectoskeletal structures and so on. Given their nature and diversity, however, informal classification is often based on nutrition and motility.
Autotrophic, non-motile protists are often said to resemble plants, though they lack higher levels of tissue organization. It is estimated that members of this group contribute to nearly 40% of the photosynthetic activity on earth. These organisms can be further differentiated based on the pigment used in photosynthesis: chlorophyll (green algae), fucoxanthin (brown algae) and phycoerythrin (red algae) among others. Many of these appear as accessory pigments along with chlorophyll.
Some protists are heterotrophic and motile. They are classified on the basis of their mode of locomotion. Some, like amoeba, use pseudopodia or ‘false feet’ while others use flagella or cilia. These free-living protists are among the major consumers of bacteria and contribute to species diversity in many ecosystems.
Protists that decompose organic material were thought to resemble fungi.
Examples of Protists
Protists are diverse and exist in a variety of ecosystems, often forming the base of the energy pyramid – as either 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 brown color. They are found in shallow oceans that contain adequate light and nutrients and are capable of sustaining 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.
Many free-living forms of protists are parasites and can cause disease in animals and crops. Plasmodium causes malaria in humans and simians. Entamoeba histolytica is the cause of amoebic dysentery. Water molds such as Phytophthora infestans (causative organism for the great Irish Potato Famine) and Plasmopara viticola have caused widespread disease in crops.