lecture 5 microbial eukaryotes: the diverse world of protists dr. angelika stollewerk chapter 27...

Lecture 5

Microbial eukaryotes: The diverse world of Protists

Dr. Angelika Stollewerk

Chapter 27

Leishmania major

Protists

Aims:

• To understand the diversity of protists

• To appreciate the impact of protists on the world around them

• To look closely at three examples in order to illustrate the basic biology of protists

Protists

Aims:

• To understand the diversity of protists

• To appreciate the impact of protists on the world around them

• To look closely at three examples in order to illustrate the basic biology of protists

These lecture aims form part of the knowledge required for learning outcome 2:

Describe basic organism structure and diversity (LOC2).

Protist

Essential reading

•pages 582-594

Recommended reading

Pages 74-75: 4.3 What are the characteristics of eukaryotic cells (This will be covered in depth in SEF032 Molecules to Cells, but it is useful background here)

27 The Origin and Diversification of the Eukaryotes

• 27.1 How Do Microbial Eukaryotes Affect the World Around Them?

• 27.2 How Did the Eukaryotic Cell Arise?

• 27.3 How Did the Microbial Eukaryotes Diversify?

• 27.4 How Do Microbial Eukaryotes Reproduce? (start of section)

27.1 How Do Microbial Eukaryotes Affect the World Around Them?

Eukaryotes that are neither plants, animals, or fungi are called protists, or microbial eukaryotes (though not all are microbial).

They do not constitute a clade, they are paraphyletic.

Their true phylogeny is the subject of research and debate.

Table 27.1 Major Eukaryote Clades (Part 1)


There is great diversity of microbial eukaryotes. Most are microscopic, but some are large (e.g., giant kelp).

Many are constituents of plankton—free floating, microscopic, aquatic organisms. Plankton that are photosynthetic are called phytoplankton.


In marine food webs, phytoplankton are the primary producers.

Diatoms (a clade) are dominant in the phytoplankton. They do one-fifth of the carbon fixation on Earth.

The primary producers are consumed by heterotrophs.


Endosymbiosis, in which one organism lives inside another, is common in microbial eukaryotes.

Dinoflagellates are common endosymbionts in animals and other microbial eukaryotes; some are photosynthetic.


Many radiolarians have photosynthetic endosymbionts. Often, both organisms benefit from the relationship.

Some dinoflagellates live as endosymbionts in corals.


Pathogens:

Plasmodium—cause of malaria. Part of its life cycle is spent as a parasite in red blood cells.

Female Anopheles mosquito is the vector; takes up Plasmodium gametes with the blood, zygotes form in mosquito gut. Plasmodium is passed to another human.

Figure 27.3 The Life Cycle of the Malarial Parasite


Plasmodium’s complex life cycle makes it difficult to control. Best strategy—remove stagnant water where mosquitoes breed. Insecticides are also used.

The genomes of Plasmodium falciparum, and Anopheles gambiae have been sequenced.


Trypanosomes (kinetoplastids) are some of the most deadly organisms on Earth, causing sleeping sickness, leishmaniasis, and Chagas’ disease.


Some chromalveolates, including diatoms, dinoflagellates, and haptophytes, can form “red tides.”

Color is from pigments in dinoflagellates. Cell concentrations are extremely high.

Some produce neurotoxins that kill fish. Gonyaulax produces a toxin that accumulates in shellfish.


Coccolithophores (haptophytes) can also form immense blooms in the ocean. Blooms can reduce the amount of sunlight that penetrates deeper waters.

Emiliania huxleyi—one of smallest unicellular eukaryotes. May contribute to global warming through its metabolism.


Diatoms store oil as an energy reserve.

Over millions of years, diatoms have died and sunk to the ocean floor, and through chemical and physical changes form petroleum and natural gas deposits.


Foraminiferans secrete shells of calcium carbonate.

Discarded shells make up extensive deposits of limestone. Some beach sands are made of fragments of foram shells.

Foram shells are also used to date and characterize sedimentary rocks, and are used to infer temperatures from the past.

27.2 How Did the Eukaryotic Cell Arise?

Eukaryotic cells arose as the environment was changing dramatically—from anaerobic to aerobic.

Major events that occurred in the evolution of eukaryote cells are still conjectural—a framework for thinking about this challenging problem.


The main events:

• Origin of a flexible cell surface

• Origin of a cytoskeleton

• Origin of a nuclear envelope

• Appearance of digestive vesicles or vacuoles

• Endosymbiotic acquisition of some organelles


Flexible cell surface: Prokaryotic cell wall was lost; cells can grow larger.

As cell size increases, surface area-to-volume ratio decreases, but with a flexible surface, infolding can occur, creating more surface area.

A flexible cell surface also allowed endocytosis to develop.

Figure 27.6 Membrane Infolding


A cytoskeleton provided cell support, allowed cells to change shape, and move materials around the cell, including daughter chromosomes.

In some cells microtubules gave rise to flagella.

The nuclear envelope may have developed from the plasma membrane.

The DNA of a prokaryote is attached to the plasma membrane; infolding of the membrane could have been the first step in development of the nucleus.

Figure 27.7 From Prokaryotic Cell to Eukaryotic Cell


The next step was probably phagocytosis—the ability to engulf and digest other cells.

The first true eukaryotes had a cytoskeleton and nuclear envelope; they probably had ER, Golgi apparatus, and perhaps flagella.


Cyanobacteria were producing oxygen; at some point, some Eukarya incorporated proteobacteria that evolved into mitochondria—the endosymbiotic theory.

The function of mitochondria initially might have been to detoxify O2 by reducing it to water. Later this became associated with ATP production.


Some eukaryotes incorporated a prokaryote related to today’s cyanobacteria, which developed into chloroplasts.

Evolution of chloroplasts probably occurred in a series of endosymbiotic events. Evidence comes from nucleic acid sequencing and electron microscopy.


Primary endosymbiosis: All chloroplasts descended from a gram-negative cyanobacterium with an inner and outer membrane.

A small amount of peptidoglycan from the bacterial cell wall is found today in the glaucophytes—the first group to branch off.


Primary endosymbiosis gave rise to chloroplasts of green algae (chlorophytes and charophytes) and the red algae.

Photosynthetic land plants arose from a green algal ancestor.

Red algal chloroplasts retain some pigments that were present in the original cyanobacterium.


Secondary and tertiary endosymbiosis gave rise to chloroplasts in the other microbial eukaryote groups.

The euglenid ancestor engulfed a chlorophyte, retaining the chloroplasts.

Euglenid chloroplasts have the same pigments as green algae and land plants, and has a third membrane.

Figure 27.8 Endosymbiotic Events in the Family Tree of Chloroplasts


The cryptophytes (a clade of chromalveolates) engulfed a red algal cell that became the chloroplast.

These chloroplasts contain reduced red algal nuclei, and appear to be a sister clade to all other chromalveolate chloroplasts.


Dinoflagellates engaged in tertiary endosymbiosis:

Karenia brevis lost its chloroplast and took up a haptophyte (a result of secondary endosymbiosis).

One case of sequential secondary endosymbiosis—a dinoflagellate lost its red algal chloroplast and took up a chlorophyte.


Uncertainties remain about the origins of eukaryotic cells.

Lateral gene transfer complicates the study of relationships.

Endosymbiosis does not account for all bacterial genes in eukaryotes.

A recent suggestion is that Eukarya arose from the fusion of a gram-negative bacterium and an archaean.

27.3 How Did the Microbial Eukaryotes Diversify?

Microbial eukaryotes have evolved a diversity of lifestyles.

Most are aquatic, marine and freshwater; but also damp soils and decaying organic matter.

Some are photosynthetic, some are heterotrophs, some can switch between modes.


Some used to be considered animals, and are called protozoans. But this term lumps phylogenetically unrelated groups. Most protozoans are ingestive heterotrophs.

The term algae also lumps many groups of photosynthetic microbial eukaryotes and does not reflect phylogeny.


Locomotion

Amoeboid motion—cells form pseudopods that are extensions of the cell. A network of cytoskeletal microfilaments squeezes the cytoplasm forward.


Cilia and flagella developed from microtubules.

Cilia beat in a coordinated fashion; move cell forward or backward.

Flagella have whip-like movement. Some pull, some push the cell forward.

Flagella have a 9 + 2 arrangement of microtubules.

Figure 4.22 Sliding Microtubules Cause Cilia to Bend

Movement in the euglenid Eutreptia


Vacuoles increase effective surface area in large cells.

Contractile vacuoles in freshwater microbial eukaryotes such as Paramecium are used to excrete excess water.

Figure 27.10 Contractile Vacuoles Bail Out Excess Water


Food vacuoles are formed by Paramecium and others when solid food particles are ingested by endocytosis. The food is digested in the vacuole. Smaller vesicles pinch off—increasing surface area for products of digestion to be absorbed by the rest of the cell.

A Paramecium uses cilia for feeding

Figure 27.11 Food Vacuoles Handle Digestion and Excretion


Cell surfaces

Many microbial eukaryotes have diverse means of strengthening their surfaces.

Paramecium has a covering of surface proteins called a pellicle, making it flexible but resilient.

Other groups secrete a “shell,” such as foraminiferans.


Some amoebas make a “shell” or test from bits of sand beneath the plasma membrane.

Diatoms form glassy cell walls of silica. These walls are exceptionally strong, and perhaps enhanced defense against predators.

27.4 How Do Microbial Eukaryotes Reproduce?

Most microbial eukaryotes have both sexual and asexual reproduction.

Asexual processes:

• Binary fission—equal splitting

• Multiple fission—splitting into more than two cells.


• Budding—outgrowth of a new cell from the surface of an old cell.

• Spores—specialized cells that are capable of growing into a new individual.


The ciliate clade (such as Paramecium) have a single macronucleus and one to several micronuclei.

The macronucleus contains many copies of the genetic information, packaged into units; regulates the life of the cell.

Micronuclei are essential for genetic recombination.


In conjugation, two Paramecia line up together, the oral groove regions fuse, and nuclear material is exchanged and reorganized.

Each cell gets two haploid nuclei, one from each cell. These fuse to form a new diploid micronucleus.

Conjugation is a sexual process, but it is not reproductive. Asexual clones must periodically conjugate.

Figure 27.13 Paramecia Achieve Genetic Recombination by Conjugating


In alternation of generations, a diploid spore-forming organism gives rise to a haploid gamete-forming organism.

When haploid gametes fuse (fertilization or syngamy) a diploid individual is formed.

The haploid or diploid organism, or both, may reproduce asexually.

Figure 27.14 Alternation of Generations


In multicellular organisms, the sporophyte is the multicellular diploid generation; the gametophyte is the multicellular haploid generation.

The generations may be different morphologically—heteromorphic.

Isomorphic—the generations have similar morphology.


Specialized cells in the sporophyte (sporocytes) divide meiotically to produce haploid spores.

The spores germinate and divide mitotically to produce the haploid gametophyte generation.

Gametes produced by the gametophyte generation must fuse to form a new sporophyte generation.

Check out

27.2 Recap, page 590

27.3 Recap, page 592

27.1 Chapter summary, page 608

27.2 Chapter summary, page 608 and WEB/CD Activity 27.1

27.3 Chapter summary, page 608 and WEB/CD Activity 27.2

27.4 Chapter summary, page 608 and WEB/CD Activity 27.1 and 27.2

For discussion

Page 609: Chapter 27, questions 2 and 4

Protists

Protists

Key terms:

amoeba, asexual, cell wall, chromosome, cilium (pl. cilia), contractile vacuole, cytoskeleton, diploid, endocytosis, endocytosis, endoplasmic reticulum, endosymbiont, euglena, gamete, Golgi apparatus, haploid, heteromorphic, heterotrophic, hypertonic, invagination, isomorphic, mitochondrion (pl. Mitochondria), monophyletic, morphological, multicellular, nuclear envelope, organelle, osmosis, Paramecium, peroxisome, phagocyte, photosynthetic, plasma membrane, primary producer, pseudopod (pl. pseudopodia), sexual, spore, unicellular, vesicle

lecture 5 microbial eukaryotes: the diverse world of protists dr. angelika stollewerk chapter 27...

Documents

diversity of protists

diverse world of protists

leishmania major slide

malarial parasite slide

impact of protists

basic biology of protists

major eukaryote clades

photosynthetic endosymbionts