Question

what is protozoa and the major groups of protozoa? What is their biological importance?

Answer 1

The group of organisms known as 'protozoa' are defined by a few of their shared characteristics. Protozoa are non-phototrophic, unicellular, eukaryotic microorganisms with no cell walls. This diverse group of over 65,000 species generally share these basic attributes. Looking deeper, this group can be extremely complex and variable. In fact, the protozoa are often described as the pinnacle of unicellular complexity.

Unlike the relatively simple bacteria, protozoa can have many different intracellular organelles performing specific tasks. Some species of protozoa have structures that are analogous to mouths, GI tracts, and anuses. This probably goes against everything you've been taught about microbes being simply bags of proteins and enzymes.

Many protozoa cause diseases in animals and humans. Some, like Plasmodium, which causes malaria, can be devastating to people worldwide. Others, like Trichomonas, cause sexually transmitted diseases that are relatively benign and 100% curable. The vast majority of the species, though, are completely harmless. But, as is usually the case in microbiology, it's the dangerous ones that get the most attention. The protozoa can have very diverse lifecycles with multiple morphological stages, depending on species. Most protozoa have a cyststage, which is dormant and highly resistant to environmental stress. In the disease-causing species, these cysts are often the mode of infection, frequently acquired by fecal-oral contamination. The trophozoite stage is the active, reproductive, and feeding stage. The image below shows a purple trophozoite emerging from an oblong brownish-red cyst. The trophozoite is the stage that typically causes disease by pathogenic protozoa. This trophozoite can be very specific, infecting only one species, like humans. Or, it can be more general, infecting whole groups, like any mammal, for example. Since the protozoa are so diverse and only distantly related, they have been separated into four major groups based on motility and the structures used to generate movement. We're going to look at all four groups individually, highlighting the key points and providing examples of important species within the group.

Groups of protozoa

The protozoa group comprises more than 65,000 species. All the protozoan species belong to the kingdom Protista. Many kinds of protozoa are symbionts. Some of the protozoan species are parasites and some are predators of bacteria and algae. Some examples of protozoans are dinoflagellates, amoebas, paramecia, and plasmodium.

Based on the mode of locomotion, protozoa have been divided into four types-

Amoeboids

Amoeba

An amoeboid (ameba or amoeba) is a type of cell or organism that is capable of changing its shape, mainly by extending and retracting pseudopods. They are normally found in the soil and in aquatic habitats. They move by using pseudopods. They typically ingest their food by phagocytosis. They extend their pseudopods to engulf a prey. They do not possess a mouth or cytostome.

There is no specific place on the cell where phagocytosis takes place. The food sources of amoebae differ. Some of them feed on bacteria and other protists. Some others feed on dead organic material. Some also feed by absorbing dissolved nutrients through vesicles. The examples of amoeboids are Amoeba proteus, Chaos carolinense (the giant amoeba), Naegleria fowleri (the brain-eating amoeba), Entamoeba histolytica (the intestinal parasite of commensals and humans), and Dictyostelium discoideum (the multicellular social amoeba).

Flagellates

Euglena

Flagellates are organisms which have one or more whip-like organelles called flagella. They may be solitary, colonial, free-living or parasitic. Parasitic forms live in the intestine or bloodstream of the host. An example of a parasitic flagellate is Trypanosoma, which has an interesting life cycle as it uses two hosts; humans and tsetse fly. Many other flagellates like dinoflagellates live as plankton in the oceans and freshwater. Some flagellates are autotrophic while others are heterotrophs.

Flagellates are divided into two classes:

Phytomastigophorea: The Phytomastigophorea includes protozoans that contain chlorophyll. They can produce their food photosynthetically, like plants. Examples include Euglena and Dinoflagellates. Euglena is regarded as both an alga and a protozoan.

Zoomastigophorea: It is the phylum commonly called zooflagellates. Zooflagellates include protozoans which are colorless. They ingest organic substances by osmotrophy (uptake of dissolved organic compounds through plasma membrane) or phagotrophy (engulfing prey in food vacuoles). They may be free-living, symbiotic, commensal, or parasitic. Examples include hypermastigids, holomastigotoides, and trichomonads.

Ciliates

Paramecium

The ciliates are a group of protozoans which possesses hair-like organelles called cilia. Cilia are used in swimming, crawling, attachment, feeding, and sensation. Most ciliates are heterotrophs. They eat organisms such as bacteria and algae. They sweep the food by their modified oral cilia into their oral groove (mouth). The food is moved with the help of cilia through the mouth pore into the gullet, which forms food vacuoles.

Some ciliates do not have a mouth and they feed by absorption (osmotrophy), and some others are predatory and feed on other protozoa, especially ciliates. Some ciliates also parasitize animals. Examples of ciliates include free-living forms like Paramecium caudatum, Stentor polymorpha, Vorticella campanula, and parasitic forms like Balantidium coli.

There are three types of ciliated protozoa. They are free-swimming ciliates, crawling ciliates, and stalked ciliates. All of them use cilia for locomotion and capturing food. Examples of free-swimming ciliates include Litonotus and Paramecium. Examples of crawling ciliates are Aspidisca and Euplotes.

Sporozoans

Plasmodium

Sporozoans are non-motile, unicellular protists, usually parasites. These protozoans are also called intracellular parasites. An example is Plasmodium vivax, that causes malaria in humans. The earlier stage sporozoan forms show some movement. They do not possess locomotor organelles in their later stage.

Four main groups of sporozoa (based on spore structure) include:

Apicomplexa: The Apicomplexa, also called Apicomplexia, are a large phylum of parasitic protists. They are spore-forming unicellular parasites. Most of them have a unique organelle that is made up of a type of plastid called an apicoplast, and an apical complex structure. The organelle is used by the organism for penetrating into a host cell. Flagella or pseudopods are found only in certain gamete stages. This group includes organisms like coccidia, gregarines, piroplasms, haemogregarines, and plasmodium. All organisms of this phylum have an infectious stage, the sporozoite. All the species of this group, except Nephromyces, a symbiont in marine animals, are endoparasites of animals.

Microsporidia: The microsporidia constitute a group of spore-forming unicellular parasites. They were at a time known to be protists, but are now known to be fungi. They have a polar tube or polar filament in the spore with which they infiltrate host cells. Microsporidia do not have mitochondria, and instead possess mitosomes. They also do not have flagella. Most organisms in this group infect animals and insects and a few infect humans. Microsporidia can also infect hosts which are themselves parasites.

Ascetosporea: They are a group of protists that are parasites of animals, especially marine invertebrates. Two groups which come under this are the haplosporids and paramyxids. Haplosporid spores have a single nucleus and an opening at one end, covered with an internal diaphragm. After emerging, it develops within the cells of its host, usually a marine invertebrate. However, some infect other groups or freshwater species. Paramyxids grow within the digestive system of marine invertebrates, and produce multicellular spores.

Myxosporidia: The Myxosporea are a class of microscopic parasites, belonging to the Myxozoa (group of parasitic animals of aquatic environment). They have a life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate, usually an annelid, and an ectothermic vertebrate, usually a fish.

Based on the mode of nutrition, protozoa are divided into the following two types-

Free-living protozoa

Euglena

The free-living protozoa are those which do not infect or live on hosts for their survival. They may produce their food photosynthetically, or eat bacteria, yeast and algae. Example: Euglena

Parasitic protozoa

They depend on their hosts for survival. They take in fluids from the body of their hosts. Example: Plasmodium

Based on the mode of respiration, protozoa are classified into two groups-

Aerobic Protozoa

Amoeba Proteus

Most species of free-living protozoa are aerobic. They cannot live without oxygen. Aerobic protozoa are tiny and so are capable of getting oxygen from the liquid medium by diffusion. Example: Amoeba proteus

Anaerobic Protozoa

Giardia

They can survive in the absence of oxygen and are not commonly found amidst eukaryotic organisms. Normally, anaerobic eukaryotes are either parasites or symbionts of multicellular organisms that have originated from aerobic ancestors. Examples: Giardia and Trichomonads.

Ecological and industrial importance of protozoans-

Protozoans play important roles in the fertility of soils. By grazing on soilbacteria, they regulate bacterial populations and maintain them in a state of physiological youth—i.e., in the active growing phase. This enhances the rates at which bacteria decompose dead organic matter. Protozoans also excrete nitrogen and phosphorus, in the form of ammonium and orthophosphate, as products of their metabolism, and studies have shown that the presence of protozoans in soils enhances plant growth.

Protozoans play important roles in wastewater treatment processes, in both activated sludge and slow percolating filter plants. In both processes, after solid wastes are removed from the sewage, the remaining liquid is mixed with the final sludge product, aerated, and oxidized by aerobic microorganisms to consume the organic wastes suspended in the fluid. In the activated sludge process, aerobic ciliates consume aerobic bacteria, which have flocculated (formed loose aggregates, making them easily separated from liquid). In the percolating filter process, substrates are steeped in microorganisms, such as fungi, algae, and bacteria, which provide food for oxidizing protozoans. In the final stages of both processes, solids settle out of the cleaned effluent in the settlement tank. Treatment plants with no ciliates and only small numbers of amoebae and flagellates produce turbid effluents containing high levels of bacteria and suspended solids. Good-quality, clean effluents are produced in the presence of large ciliated protozoan communities because they graze voraciously on dispersed bacteria and because they have the ability to flocculate suspended particulate matter and bacteria.

Protozoans probably play a similar role in polluted natural ecosystems. Indeed, there is evidence that they, by feeding on oil-degrading bacteria, increase bacterial growth in much the same way that they enhancerates of decomposition in soils, thereby speeding up the breakdown of oil spillages.

Some radiolarians and foraminiferans harbour symbiotic algae that provide their protozoan hosts with a portion of the products of photosynthesis. The protozoans reciprocate by providing shelter and carbon and essential phytonutrients. Many ciliates contain endosymbiotic algae, and one species, Mesodinium rubrum, has formed such a successful relationship with its red-pigmented algal symbiontthat it has lost the ability to feed and relies entirely on symbiosis for its livelihood. Mesodinium often forms dense red blooms, or red tides, when it reaches high densities in water. Among the ciliates with endosymbionts, Mesodinium is the only completely photosynthetic species. Other ciliates achieve photosynthesis in another way. Although they do not have symbiotic algae, they consume plantlike flagellates, sequester the organelles that contain the plant pigments, and use them for photosynthesis. These organelles are known as plastids. Because the isolated plastids eventually age and die, they must be replaced continuously.

The impact of protozoan grazing on phytoplankton can be considerable. It has been estimated that at least half of the phytoplankton production in marine waters is consumed by protozoans. Like the soil protozoans, these planktonic protozoans excrete nitrogen and phosphorus at high rates. The protozoans are a fundamental component in recycling essential nutrients (nitrogen and phosphorus) to the phytoplankton.