- What are Protists?
- Protists are single-celled eukaryotic microorganisms with a cell body consisting of a well-defined nucleus and other membrane-bound organelles.
- They are neither plants nor animals or fungi.
- The term Protista was first used by Ernst Haeckel in 1866
- These organisms are generally unicellular in nature but can be found as a colony of cells.
- Characteristics of Kingdom Protista
- “Protists are a diverse group of eukaryotic microorganisms
- All single-celled eukaryotes are placed under Protista, but the boundaries of this kingdom are not well-defined. What may be ‘a photosynthetic protistan’ to one biologist may be ‘a plant’ to another.
- The protists' simple organization — either they are unicellular, or they are multicellular (kelp) without specialized tissues. This simple cellular organization distinguishes the protists from other eukaryotes, such as fungi, animals, and plants.
- Just like any other eukaryote, the cells of these species have a nucleus and membrane-bound organelles.
- They may be autotrophic or heterotrophic in nature.
- Protists were traditionally subdivided into several groups based on similarities to the "higher" kingdoms: the one-celled animal-like protozoa, the plant-like protophyta (mostly one-celled algae), and the fungus-like slime molds and water molds.
- Protists live in almost any environment that contains liquid water. Many protists, such as the algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton.
- Other protists, such as the Kinetoplastids and Apicomplexa are responsible for a range of serious human diseases, such as malaria and sleeping sickness.
- Protists exhibit locomotion through cilia and flagella. A few organisms belonging to the kingdom Protista have pseudopodia that help them to move.
- Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation
- Classification of Protista
- They may be divided into-
- The golden algae or Chrysophytes are a large group of algae, found mostly in freshwater, as well as in marine environments.
- They are characterized by a stiff cell wall.
- This group includes diatoms and golden algae (desmids).
- In diatoms, the cell walls form two thin overlapping shells, which fit together as in a soap box. The walls are embedded with silica and thus the walls are indestructible.
- They are microscopic and float passively in water currents (plankton).
- Thus, diatoms have left behind large amounts of cell wall deposits in their habitat; this accumulation over billions of years is referred to as ‘diatomaceous earth’.
- Being gritty this soil is used in the polishing, and filtration of oils and syrups.
- Diatoms are the chief ‘producers’ in the oceans.
- The dinoflagellates are a large group of flagellate protists.
- These organisms are mostly photosynthetic.
- Most are marine plankton, but they are common in fresh water habitats as well.
- They appear yellow, green, brown, blue, or red depending on the main pigments present in their cells.
- The cell wall has stiff cellulose plates on the outer surface.
- Most of them have two flagella; one lies longitudinally and the other transversely in a furrow between the wall plates.
- Their populations are distributed depending on temperature, salinity, or depth. About half of all dinoflagellates are photosynthetic, and these make up the largest group of eukaryotic algae aside from the diatoms.
- Being primary producers makes them an important part of the aquatic food chain. Some species, called zooxanthellae, are endosymbionts of marine animals and protozoa and play an important part in the biology of coral reefs.
- Red Dianoflagellates Example- Gonyaulax undergoes such rapid multiplication that they make the sea appear red tides. Toxins released in such large numbers may even kill other marine animals such as fish.
- Some dinoflagellates are colorless predators of other protozoa, and a few forms are parasitic Example- Oodinium, Pfiesteria
- An algal bloom of dinoflagellates can result in a visible coloration of the water colloquially known as red tide.
- The Euglenoids (or euglenoids) are one of the best-known groups of flagellates, commonly found in freshwater especially when it is rich in organic materials, with a few marine and endosymbiotic members.
- Instead of a cell wall, they have a protein rich layer called pellicle which makes their body flexible.
- They have two flagella, a short and a long one.
- Many Euglenoids have chloroplasts and produce energy through photosynthesis, but others feed by phagocytosis or strictly by diffusion.
- They belong to the phylum Euglenophyta, and their cell structure is typical of that group.
- They are the link between plants and animals. They lack a cell wall but perform photosynthesis in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms.
- Interestingly, the pigments of euglenoids are identical to those present in higher plants. Example: Euglena
- Slime Moulds (Fungi-Like Protists)
- Slime mold or slime mold, a broad term describing fungus-like organisms that use spores to reproduce.
- Their common name refers to part of some of these organism's lifecycles where they can appear gelatinous, hence the name slime.
- Slime molds have been found all over the world and feed on microorganisms that live in any type of dead plant material these organisms are usually found in soil, lawns, and on the forest floor, commonly on deciduous logs.
- In tropical areas, they are also common on inflorescences, fruits, and in aerial situations example- in the canopy of trees.
- In urban areas, they are found on mulch or even in the leaf mold in gutters.
- Many have striking colors such as yellow, brown, and white
- Slime molds are saprophytic protists.
- One of the most commonly encountered slime molds, both in nature in forests in the temperate zones of the earth as well as in classrooms and laboratories is the yellow Physarum polycephalum.
- Most slime molds are smaller than a few centimeters, but some species may reach sizes of up to several square meters and masses of up to 30 grams.
- The body moves along decaying twigs and leaves engulfing organic material.
- Under suitable conditions, they form an aggregation called plasmodium which may grow and spread over several feet.
- During unfavorable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips. The spores possess true walls.
- They are extremely resistant and survive for many years, even under adverse conditions.
- The spores are dispersed by air currents.
- Protozoa or Cornelius protozoans (from Greek proton "first" and zoa "animals"; singular protozoon; (the word "protozoan" is originally an adjective, used as a noun) are microorganisms classified as unicellular eukaryotes.
- All Protozoans are heterotrophs and live as predators or parasites.
- They are believed to be primitive relatives of animals.
- They may be broken down into four primary categories, which are as follows:
- Ciliated Protozoans:
- They are mostly Aquatic animals.
- They are actively moving organisms because of the presence of thousands of cilia.
- They have a cavity (gullet) that opens to the outside of the cell surface.
- The coordinated movement of rows of cilia causes the water laden with food to be steered into the gullet. Example- Paramecium.
- Amoeboid protozoans:
- They may be found in saltwater, as well as freshwater, and wet soil.
- They have pseudopodia (false feet) which help to change their shape and capture and engulf food. E.g. Amoeba.
- Marine forms have silica shells on their surface.
- Some of them such as Entamoeba are parasites.
- Flagellated protozoans:
- As the name suggests, the members of this group have flagella.
- They can be free-living as well as parasitic.
- Trypanosoma species can spread some dangerous diseases such as sleeping sickness.
- This includes diverse organisms that have an infectious spore-like stage in their life cycle.
- The most notorious is Plasmodium (malarial parasite) which causes malaria and has a staggering effect on the human population.