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8.15: Introduction to Groups of Protists - Biology

8.15: Introduction to Groups of Protists - Biology


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Classify protists into unique categories

In the span of several decades, the Kingdom Protista has been disassembled because sequence analyses have revealed new genetic (and therefore evolutionary) relationships among these eukaryotes. There is still evidence lacking for the monophyly of some groups.

The classification of eukaryotes is still in flux, and the six supergroups may be modified or replaced by a more appropriate hierarchy as genetic, morphological, and ecological data accumulate. Keep in mind that the classification scheme presented here is just one of several hypotheses, and the true evolutionary relationships are still to be determined. When learning about protists, it is helpful to focus less on the nomenclature and more on the commonalities and differences that define the groups themselves.

What You’ll Learn to Do

  • Identify characteristics and examples of protists in the supergroup Excavata
  • Identify characteristics and examples of protists in the supergroup Chromalveolata
  • Identify characteristics and examples of protists in the supergroup Rhizaria
  • Identify characteristics and examples of protists in the supergroup Archaeplastida
  • Identify characteristics and examples of protists in the supergroup Amoebozoa
  • Identify characteristics and examples of protists in the supergroup Opisthokonta

Learning Activities

The learning activities for this section include the following:

  • Excavata
  • Chromalveolata
  • Rhizaria
  • Archaeplastida
  • Amoebozoa
  • Opisthokonta
  • Self Check: Groups of Protists

Features unique to protists

Protists vary greatly in organization. Some are single-celled others are syncytial (coenocytic essentially a mass of cytoplasm) and still others are multicellular. (While protists may show multicellularity, they are never multitissued.) They may manifest as filaments, colonies, or coenobia (a type of colony with a fixed number of interconnected cells embedded in a common matrix before release from the parental colony). Not all protists are microscopic. Some groups have large species indeed for example, among the brown algal protists some forms may reach a length of 60 metres (197 feet) or more. A common range in body length, however, is 5 μm (0.0002 inch) to 2 or 3 mm (0.08 or 0.1 inch) some parasitic forms (e.g., the malarial organisms) and a few free-living algal protists may have a diameter, or length, of only 1 μm.

While many protists are capable of motility, primarily by means of flagella, cilia, or pseudopodia, others may be nonmotile for most or part of the life cycle. Resting stages (spores or cysts) are common among many species, and modes of nutrition include photosynthesis, absorption, and ingestion. Some species exhibit both autotrophic and heterotrophic nutrition. The great diversity of protist characteristics supports theories about the antiquity of the protists and of the ancestral role they play with respect to other eukaryotes.


8.15: Introduction to Groups of Protists - Biology

Kingdom Protista Phyla Groups

Introduction to Kingdom Protista

The Kingdom Protista includes an incredible diversity of different types of organisms, including algae, protozoans, and slime molds. No one even knows how many species there are, though estimates range between 65,000 to 200,000. All protists are eukaryotes, complex cells with nuclear membranes and organelles like mitochondria and chloroplasts. They can be either unicellular or multicellular, and in this group we find the first inkling of what is to come in evolutionary history, the union of eukaryotic cells into a colonial organism, where various cell types perform certain tasks, communicate with one another, and together function like a multicellular organism.

Two Major Groups of Protists:

1. Some protists are autotrophs, a photosynthetic group of phyla referred to as the algae. Autotrophs manufacture their own energy by photosynthesis or chemosynthesis. Algae use various combinations of the major chlorophyll pigments, chlorophyll a, b, and c, mixed with a wide array of other pigments that give some of them very distinctive colors.

2. Some protists are heterotrophs, a group of phyla called the protozoa. Heterotrophs get their energy by consuming other organisms. Protists reproduce asexually by binary fission, and a few species are capable of sexual reproduction. Many have very complex life cycles.

Protist Characteristics:

Protists are so small that they do not need any special organs to exchange gases or excrete wastes. They rely on simple diffusion, the passive movement of materials from an area of high concentration to an area of low concentration, to move gases and waste materials in and out of the cell. Diffusion results from the random motion of molecules (black and white marble analogy). This is a two-edged sword. They don't need to invest energy in complex respiratory or excretory tissue. On the other hand, diffusion only works if you're really small, so most protists are limited to being small single cells. Their small size is also due to the inability of cilia or flagella to provide enough energy to move a large cell through the water.

Protists eat by phagocytosis - they engulf their food in their cell membrane, and pinch off a section of membrane to form a hollow space inside the cell. This hollow space, now enclosed by membranes, is called a vacuole. Vacuoles are handy little structures. Protists also use them to store water, enzymes, and waste products. Paramecium and many other protists have a complex type called a contractile vacuole, which drains the cell of waste products and squirts them outside the cell.

All protists are aquatic. Many protists can move through the water by means of flagella, or cilia, or pseudopodia (= false feet). Cilia and flagella are tiny movable hairs. Motile cells usually have one or two long flagella, or numerous shorter cilia. The internal structure of cilia and flagella is basically the same. All of the characteristics that this group shares are primitive traits, a perilous thing to base any classification on, because convergent evolution may be responsible for these superficial similarities. So the concept of the Kingdom Protista has been justly criticized as a "taxonomic grab bag" for a whole bunch of primitive organisms only distantly related to one another.

Protists are mainly defined by what they are not - they are not bacteria or fungi, they are not plants or animals. Protists gave rise to all other plants and animals. But where did protists themselves come from? The earliest protists we can recognize in the fossil record date back to about 1.2 billion years ago. We do not know how the various groups of protists are related to one another. We assume they arose from certain groups of bacteria, but which groups and when are still investigating. Different phyla of protists are so unlike one another, many probably evolved independently from completely different groups of bacteria. Lynn Margulis recognizes nearly 50 different phyla of protists, or Protoctista, as this kingdom is sometimes called. We will take a more conservative approach, and focus on nine important phyla of protists.

Taxonomy Kingdom Protista (Protoctista)

Phylum Sporozoa - (Plasmodium - "the protist that causes malaria"u)

Phylum Chlorophyta - green algae (Spirogyra, Volvox, Chlamydomonas)

Characteristics of Phyla The protozoa:

    These ciliates move by means of numerous small cilia. Ciliates are covered with these tiny little hairs called cilia. This is their defining characteristic that sets them apart from the other protists. They are complex little critters, with lots of organelles and specialized structures. Many of them, like Paramecium, even have little toxic threads or darts that they can discharge to defend themselves. Typical ciliates you may see in lab include Paramecium and Blepharisma.

    These ciliates have a most unusual way of getting about. They extend part their body in a certain direction, forming a pseudopod or false foot, and then flow into that extension (cytoplasmic streaming). Many forms have a tiny shell made from organic or inorganic material. They eat other protozoans, algae, and even tiny critters like rotifers. Amoeba is a typical member of this phylum. Many sarcodines are parasites, such as the species Entamoeba histolytica, which causes amoebic dysentery. 10 million Americans are infected at any one time with some form of parasitic amoeba, and up to half of the population in tropical countries. Somewhat more unusual sarcodines are the Foraminiferans. These "forams" can have fantastically sculptured shells, with prominent spines. They extend cytoplasmic "podia" out along these spines, which function in feeding and in swimming. Forams are so abundant in the fossil record, and have such distinctive shapes, that they are widely used by geologists as markers to identify different layers of rock. The famous white cliffs of Dover are made up of billions of foraminiferan shells.


The apical complex is the defining feature of the sporozoans.

Plasmodium, the parasite that causes malaria, is typical of this group. In more general terms, spores are haploid reproductive cells that can develop directly into adults.


The apical complex is the defining feature of the sporozoans.

Plasmodium, the parasite that causes malaria, is typical of this group. In more general terms, spores are haploid reproductive cells that can develop directly into adults.

Common symptoms of malaria

In the early stages, malaria symptoms are sometimes similar to those of many other infections caused by bacteria, viruses, or parasites. Symptoms may include:

Symptoms may appear in cycles and may come and go at different intensities and for different lengths of time. But, especially at the beginning of the illness, the symptoms may not follow this typical pattern.

The cyclic pattern of malaria symptoms is due to the life cycle of malaria parasites as they develop, reproduce, and are released from the red blood cells and liver cells in the human body. This cycle of symptoms is also one of the major indicators that you are infected with malaria.

Other common symptoms of malaria

Other common symptoms of malaria include:

In rare cases, malaria can lead to impaired function of the brain or spinal cord, seizures, or loss of consciousness.

Infection with the Plasmodium falciparum parasite is usually more serious and may become life-threatening.


Modes of Locomotion in Protists: 5 Modes

The following points highlight the five modes of locomotion in Protists. The modes are: 1. Pseudopodial Locomotion 2. Flagellar Locomotion 3. Ciliary Locomotion 4. Wriggling Locomotion 5. Locomotion by Mucilage Propulsion.

Mode # 1. Pseudopodial Locomotion:

It is slow creeping type of locomotion which is per­formed with the help of protoplasmic outgrowths called pseudopodia. Pseudopodial locomotion occurs in sarcodines and slime moulds.

Pseudopodia are of four types:

(i) Lobo-podia:

These pseudopodia are lobe- like with broad and blunt ends. These are present in Amoeba,

(ii) Filo-podia:

These pseudopodia are fine, thread-like, tapering, and are composed of ecto­plasm. These are found in Euglypha.

(iii) Axo-podia:

These are long and stiff, with hard axial filament. These pseudopodia are present in Actinophrys.

(iv) Reticulopodia:

These are long and branching. The branches of adjacent pseudopodia may form network. These are found in Globigerina.

Mode # 2. Flagellar Locomotion:

Flagella show whip-like movement. They usually beat in­dependently. This type of locomotion occurs in dinoflagellates (e.g., Gonyaulax), euglenoids (e.g., Euglena) and zoo-flagellates (e.g., Leishmania).

Mode # 3. Ciliary Locomotion:

Cilia show oar-like movement. All the cilia of a cell show coordi­nated movements which are of two types, isochronic and metachronic rhythms. In isoch­ronic or synchronous rhythm, all the cilia of a cell beat simulta­neously. They do so in rapid succession one after the other in case of metachronic rhythm. It occurs in ciliates (e.g., Parame­cium).

Basically flagella and cilia are identical in structure. However, they differ in some respects as mentioned below.

Mode # 4. Wriggling Locomotion:

It is slow worm-like movement which is performed with the help of a wave of contraction and expansion in the body, e.g., sporozoans, no flagellates, euglenoids.

Mode # 5. Locomotion by Mucilage Propulsion:

Some protists like diatoms do not have any organelles of locomotion. They can, however, move from one place to another through secretion of mucilage. This type of locomotion occurs in the direction opposite to that of mucilage secretion.


Introduction to the Alveolates

The Alveolates are a very recently recognized group. Detailed studies of the internal structure of these protists demonstrates that they all share a system of sacs underneath their cell membranes. These closely packed sacs are called alveoli.

Alveoplates include some of the most familiar and numerous protist groups, including the Ciliata, or ciliates, such as Paramecium and Stentor. The ciliates are the most diverse group of heterotrophic protists, with 7000 species.

Also very diverse, though not as familiar, are the Foraminifera. While the average person may not be familiar with this group, they are very well known to paleontologists, who use their fossils to date rocks, determine paleoclimate, and search for oil deposits.

A third group is the dinoflagellates. These alveolates are photosynthetic, able to manufacture their own food from sunlight, carbon dioxide, and sufficient dissolved nutrients. The dinoflagellates are best known for their periodic population booms that result in "red tides" that may kill fish and shellfish.

A final group of Alveolates are the Apicomplexa, a group of parasitic and disease-causing protists. They are known for having some of the most complex life cycles among single-celled creatures.

For additional information:
Visit the Alveolates page on the Tree of Life for current information on systematics of this group.


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Ecology

The distribution of protists is worldwide as a group, these organisms are both cosmopolitan and ubiquitous. Every individual species, however, has preferred niches and microhabitats, and all protists are to some degree sensitive to changes in their surroundings. The availability of sufficient nutrients and water, as well as sunlight for photosynthetic forms, is, however, the only major factor restraining successful and heavy protist colonization of practically any habitat on Earth.

Free-living forms are particularly abundant in natural aquatic systems, such as ponds, streams, rivers, lakes, bays, seas, and oceans. Certain of these forms may occur at specific levels in the water column, or they may be bottom-dwellers (benthic). More specialized, sometimes human-made, habitats are also often well populated by both pigmented and nonpigmented protists. Such sites include thermal springs, briny pools, cave waters, snow and ice, beach sands and intertidal mud flats, bogs and marshes, swimming pools, and sewage treatment plants. Many are commonly found in various terrestrial habitats, such as soils, forest litter, desert sands, and the bark and leaves of trees. Cysts and spores may be recovered from considerable heights in the atmosphere.

Fossilized forms are plentiful in the geologic record. Fossils of unicellular organisms have been found in strata dated to about 1.9 billion years ago, during the Precambrian. Many lineages of protists have left no record of their now extinct forms, however, making speculation about early phylogenetic and evolutionary relationships with other eukaryotes difficult to verify.

Symbiotic protists are as widespread as free-living forms, since they occur everywhere their hosts are to be found. Hundreds or even thousands of kinds of protists live as ectosymbionts or episymbionts, finding suitable niches with plants, fungi, vertebrate and invertebrate animals, or even other protists. Seldom are the hosts harmed in fact, these often mobile substrates are actually used as a means of dispersal.

Endosymbionts include commensals, facultative parasites, and obligate parasites the latter category embraces forms that have effects on their hosts ranging from mild discomfort to death. Protozoan and certainly nonphotosynthetic protists are implicated far more often in such associations than are algal forms. In a few protists, both cytoplasm and nuclei can be invaded by other protists, and intimate, mutually beneficial relationships between protistan hosts and protistan symbionts have been seen, such as foraminiferans or ciliates that nourish symbiotic algae in their cytoplasm. When higher eukaryotes are hosts to protists, all body cavities and organ systems are susceptible to invasion, although terrestrial plants bear relatively few such parasites. In animal hosts, the three principal areas serving as sites for endosymbiotic species are the coelom, the digestive tract and its associated organs, and the circulatory system.

The numbers of individuals in populations of many protists reach staggering figures. There are, on the average, tens of thousands of protists in a gram of arable soil, hundreds of thousands in the gut of a termite, millions in the rumen of a bovine mammal, billions in a tiny patch of floating plankton in the sea, and trillions in the bloodstream of a person infected with severe malaria. Some severe diseases of humans are caused by protists, primarily blood parasites. Malaria, trypanosomiasis (e.g., African sleeping sickness), leishmaniasis, toxoplasmosis, and amoebic dysentery are debilitating or fatal afflictions.

Protist parasites infecting domesticated livestock, poultry, hatchery fishes, and other such food sources deplete supplies or render them unpalatable. The economic losses can be considerable. Certain free-living marine dinoflagellates are the causative agents of the so-called red tide outbreaks that occur periodically along coasts throughout the world a toxin released by the blooming protists kills fishes in the affected area. Other dinoflagellates produce a toxin that may be taken up by certain shellfish (bivalve mollusks) and that causes shellfish poisoning, characterized in severe cases by respiratory paralysis and death, when the mollusk is eaten by humans. Some of the “lower” fungal protists have had significant effects on human history. One species was responsible for the great Irish potato famine of the mid-19th century, and later, another nearly ruined the entire French wine industry before a fungicide was developed to destroy it.

Many protists provide humans with benefits, some more obvious than others. Because protists are located near the bottom of the food chain in nature (just above the bacteria), they serve a crucial role in sustaining the higher eukaryotes in fresh and marine waters. In addition to directly and indirectly supplying organic molecules (such as sugars) for other organisms, the pigmented (chlorophyll-containing) algal protists produce oxygen as a by-product of photosynthesis. Algae may supply up to half of the net global oxygen. Deposits of natural gas and crude oil are derived from fossilized populations of algal protists. Much of the nutrient turnover and mineral recycling in the oceans and seas comes from the activities of the heterotrophic (nonpigmented) flagellates and the ciliates living there, species that feed on the bacteria and other primary producers present in the same milieu. Seaweeds (e.g., brown algae) have long been used as fertilizers.

The calcareous test, or shell, of the foraminiferans is preservable and constitutes a major component of limestone rocks. Assemblages of certain of these protists, which are abundant and usually easily recognized, are known to have been deposited during various specific periods in Earth’s geologic history. Geologists in the petroleum industry study foraminiferan species present in samples of drilled cores in order to determine the age of different strata in Earth’s crust, thus making possible the identification of rich oil deposits. Before synthetic substitutes, blackboard chalk consisted mostly of calcium carbonate derived from the scales (coccoliths) of certain algal protists and from the tests of foraminiferans. Diatoms and some ciliate species are useful as indicators of water quality and therefore of the amount of pollution in natural aquatic systems and in sewage purification plants. Selected species of parasitic protozoans may play a significant role as biological control organisms against certain insect predators of food plants.


Section Summary

Fungi are important to everyday human life. Fungi are important decomposers in most ecosystems. Mycorrhizal fungi are essential for the growth of most plants. Fungi, as food, play a role in human nutrition in the form of mushrooms, and also as agents of fermentation in the production of bread, cheeses, alcoholic beverages, and numerous other food preparations. Secondary metabolites of fungi are used as medicines, such as antibiotics and anticoagulants. Fungi are model organisms for the study of eukaryotic genetics and metabolism.


Respiration and nutrition

At the cellular level, the metabolic pathways known for protists are essentially no different from those found among cells and tissues of other eukaryotes. Thus, the plastids of algal protists function like the chloroplasts of plants with respect to photosynthesis, and, when present, the mitochondria function as the site where molecules are broken down to release chemical energy, carbon dioxide, and water. The basic difference between the unicellular protists and the tissue- and organ-dependent cells of other eukaryotes lies in the fact that the former are simultaneously cells and complete organisms. Such microorganisms, then, must carry out the life-sustaining functions that are generally served by organ systems within the complex multicellular or multitissued bodies of the other eukaryotes. Many such functions in the protists are dependent on relatively elaborate architectural adaptations in the cell. Phagotrophic feeding, for example, requires more complicated processes at the protist’s cellular level, where no combination of tissues and cells is available to carry out the ingestion, digestion, and egestion of particulate food matter. On the other hand, obtaining oxygen in the case of free-living, free-swimming protozoan protists is simpler than for multicellular eukaryotes because the process requires only the direct diffusion of oxygen from the surrounding medium.

Although most protists require oxygen (obligate aerobes), there are some that may or must rely on anaerobic metabolism—for example, parasitic forms inhabiting sites without free oxygen and some bottom-dwelling (benthic) ciliates that live in the sulfide zone of certain marine and freshwater sediments. Mitochondria typically are not found in the cytoplasm of these anaerobes rather, microbodies called hydrogenosomes or specialized symbiotic bacteria act as respiratory organelles.

The major modes of nutrition among protists are autotrophy (involving plastids, photosynthesis, and the organism’s manufacture of its own nutrients from the milieu) and heterotrophy (the taking in of nutrients). Obligate autotrophy, which requires only a few inorganic materials and light energy for survival and growth, is characteristic of algal protists (e.g., Chlamydomonas). Heterotrophy may occur as one of at least two types: phagotrophy, which is essentially the engulfment of particulate food, and osmotrophy, the taking in of dissolved nutrients from the medium, often by the method of pinocytosis. Phagotrophic heterotrophy is seen in many ciliates that seem to require live prey as organic sources of energy, carbon, nitrogen, vitamins, and growth factors. The food of free-living phagotrophic protists ranges from other protists to bacteria to plant and animal material, living or dead. Scavengers are numerous, especially among the ciliated protozoans indeed, species of some groups prefer moribund prey. Organisms that can utilize either or both autotrophy and heterotrophy are said to exhibit mixotrophy. Many dinoflagellates, for example, exhibit mixotrophy.

Feeding mechanisms and their use are diverse among protists. They include the capture of living prey by the use of encircling pseudopodial extensions (in certain amoeboids), the trapping of particles of food in water currents by filters formed of specialized compound buccal organelles (in ciliates), and the simple diffusion of dissolved organic material through the cell membrane, as well as the sucking out of the cytoplasm of certain host cells (as in many parasitic protists). In the case of many symbiotic protists, methods for survival, such as the invasion of the host and transfer to fresh hosts, have developed through long associations and often the coevolution of both partners.


Bacteria

Bacteria are well-known decomposers of dead animal flesh and are efficient at converting animal tissues to simpler organic compounds. A number of saprotrophic bacteria, including Escherichia coli, are associated with food-borne illnesses, since meat and other food products are also the kinds of resources they would consume in nature.

Some bacteria, such as Spirochaeta cytophaga, have the capacity to decompose cellulose through absorptive nutrition. Symbiotic cellulose-degrading bacteria are found in the rumen of cows and aid digestion through the fermentation of the cellulose in grass. Similar to fungi, these organisms also have the capacity to partially break down cellulose into intermediate molecules and facilitate the process of decomposition.


Watch the video: Understanding Group Dynamics (July 2022).


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