Protists

What is a Protist?

A protist is any organism that is not a plant, an animal, a fungus, or a prokaryote. The 1st eukaryotic organisms on Earth,

appearing nearly 1.5 billion years ago, were protists.

Protists are a diverse group that may include more than 200,000 species.

Most protists are unicellular, aquatic organisms, including both freshwater and marine species.

Protists can reproduce sexually or asexually, depending on the species.

A Sample of Protistan Diversity

Most protists are classified by their method of obtaining nutrients: Animal-like protists

are heterotrophs – absorb or ingest food Plant-like protists

Photosynthesize – contain chloroplasts for photosynthesis Fungus-like protists

are parasites or decomposers

The 4 phyla of animal-like protists (called PROTOZOA) are distinguished from one another by their means of movement:

1. Zooflagellates swim with flagella2. Sarcodines move by extensions of cytoplasm3. Ciliates move by means of cilia4. Sporozoans do not move on their own

The Protozoans: Animal-Like Protists

Zooflagellates

Animal-like protists that swim using flagella are referred to as zooflagellates.

Most have either 1 or 2 flagella (although some species have many).

Absorb food through their cell membranes Most live in lakes and streams, although some live in

the bodies of other organisms Most reproduce asexually by means of binary fission,

although some have a sexual life cycle

Sarcodines

Sarcodines are animal-like protists that use pseudopods for feeding and movement. Pseudopods (false feet)

are temporary projections of cytoplasm.

The best known sarcodines are amoebas. Amoeboid movement

involves the cytoplasm of the cell streaming into the pseudopod allowing the rest of the cell to follow.

CiliatesAnimal-like protists

that move and feed by means of cilia are known as ciliates. Cilia are hair-like

projections similar to flagella.

Most ciliates are free living (non parasitic).

Some of the best known ciliates belong to the genus Paramecium.

Sporozoans

Members of the group Sporozoa are animal-like protists that do not move on their own and are parasitic.

The sporozoan Plasmodium, which causes malaria, is carried by the female Anapheles mosquito.

As many as 2 million people die from malaria each year.

Other diseases caused by animal-like protists include:

African Sleeping Sickness – trypanosoma parasite is transmitted by tsetse fly.

Giardia - diarrheal illness caused by a microscopic parasite, Giardia intestinalis – a sporozoan.

Figure 28.11

Inside mosquito Inside human

Sporozoites(n)

Oocyst

MEIOSIS

Liver

Liver cell

Merozoite(n)

Red bloodcells

Gametocytes(n)

FERTILIZATION

Gametes

Zygote(2n)

Key

Haploid (n)Diploid (2n)

Merozoite

Red blood cell

Apex

0.5 µm

Life Cycle of Plasmodium

An infected Anopheles mosquito bites a person, injecting Plasmodium sporozoites in its saliva.

1 The sporozoites enter the person’s liver cells. After several days, the sporozoites undergo multiple divisions and become merozoites, which use their apical complex to penetrate red blood cells (see TEM below).

2

The merozoites divide asexually inside the red blood cells. At intervals of 48 or 72 hours (depending on the species), large numbers of merozoites break out of the blood cells, causing periodic chills and fever. Some of the merozoites infect new red blood cells.

3

Some merozoites form gametocytes.4

Another Anopheles mosquitobites the infected person and picksup Plasmodium gametocytes alongwith blood.

5 Gametes form from gametocytes.Fertilization occurs in the mosquito’sdigestive tract, and a zygote forms.The zygote is the only diploid stagein the life cycle.

6

An oocyst developsfrom the zygote in the wall of the mosquito’s gut. Theoocyst releases thousands

of sporozoites, whichmigrate to the mosquito’s

salivary gland.

7

Ecology of Animal Like Protists

Not so Good: Can be parasitic/cause disease Malaria, African Sleeping Sickness, Cryptosporidium

Good: Symbiosis Termites have beneficial animal-like protists called

Trichonympha in their stomachs Break down cellulose in wood so termites can use it as

food

Main Characteristic: Chlorophyll Green pigment that traps light, carries out

photosynthesis Evolved from symbiosis of photosynthetic bacteria and

larger, heterotrophic bacteria

Plant-like protists are commonly called “Algae”

Can contain cell wall like plants Unicellular (4 types)

Euglenophytes, Dinoflagellates, Chrysophytes, Diatoms

Multi-cellular (3 types) Red Algae, Brown Algae, Green Algae

Plant-Like Protists

The 4 phyla of plant-like protists that are grouped together as unicellular algae are:

1. Euglenophytes2. Dinoflagellates3. Chrysophytes4. Bacillariophytes

Unicellular Algae

Euglenophytes

Algal Blooms

DinoflagellatesDinoflagellates are plant-like organisms that can

be photosynthetic or heterotrophic. Abundant components of both marine and freshwater

phytoplankton Many species are luminescent.

Red TidesGreat blooms of

the dinoflagellates have occurred in recent years on the east coast. These blooms are

known as “red tides”. These species release

a potentially harmful toxin that infect certain shellfish.

Eating these infected shellfish can be harmful or fatal.

Dinoflagellates & Bioluminescence

Chrysophytes

Members of the phylum Chrysophyta are a diverse group of plantlike protists that have gold-colored chloroplasts.

includes yellow-green algae and golden-brown algae reproduction can be sexual or asexual

Diatoms

Diatoms are plant-like protists that produce thin, delicate cell walls rich in silicon (the main ingredient in glass). Diatoms are among the most abundant organisms on

Earth! Diatoms are beautiful!

Ecology of Plant-Like Protists

Most unicellular species beneficialAct as producers in the marine food chain

Form Phytoplankton for consumer organisms to eat Approx. ½ of the photosynthesis on earth!!!

Produce large amount of oxygen

Symbiosis: Coral Reefs, Clams Provide food via photosynthesis, receive a home

The 3 phyla of algae that are largely multicellular are commonly known as: red algae brown algae green algae

A major difference among these phyla are their photosynthetic pigments.

Multicellular Algae

Red Algae

(a) Bonnemaisonia hamifera. This red alga has a filamentous form.

Dulse (Palmaria palmata). This edible species has a “leafy” form.

(b)

A coralline alga. The cell walls ofcoralline algae are hardened by calcium carbonate. Some coralline algae aremembers of the biological communities around coral reefs.

(c)

Brown Algae

Giant kelp

Green Algae

Chlamhydomonas

unicellular

green algae

Volvox

colonial

green algaeUlva

multicellular

green algae

Spirogyra

Multicellular green algae

Food for humans Food for invertebrates and fishes in mariculture Animal feed Soil fertilizers and conditioners in agriculture Treatment of waste water Diatomaceous earth (= diatoms) Chalk deposits Phycocolloids (agar, carrageenan from red algae;

alginates from brown algae) Drugs Model system for research Phycobiliproteins for fluorescence microscopy

Beneficial Aspects of Algae

Blooms of freshwater algaeRed tides and marine bloomsToxins accumulated in food chainsDamage to cave paintings, frescoes, and other

works of artFouling of ships and other submerged surfacesFouling of the shells of commercially important

bivalves

Detrimental Aspects of Algae

Fungus-Like Protists

Fungus-like protists are heterotrophs that absorb nutrients from dead or decaying organic matter. Unlike true fungi, however, funguslike protists

have centrioles and lack chitin in their cell wall.The fungus-like protists include:

Cellular slime molds Acellular slime molds Water molds

Slime Molds

Slime molds are fungus-like protists that play key roles in recycling organic material. They are found in damp places that are rich in organic

matter, such as forest floors.

The 2 groups of slime molds are: Cellular slime molds: individual cells remain distinct –

BUT – when food becomes scarce, cells join with others to reproduce in a multicellular unit with a distinct cell membrane

Acellular (Plasmodial) slime molds: cells fuse to form larger cells called plasmodium during some life cycle phases – this is the feeding stage of the organism

CELLULAR SLIME MOLDS

ACELLULAR SLIME MOLDS

Cellular v/s Acellular Slime Molds

What is the difference between the large slug-like colony formed by cellular slime molds and the plamodium formed by acellular slime molds? The slug-like colony is a mass of cells that is

multicellular (made up of many individual cells) EACH WITH A DISTINCT CELL MEMBRANE.

The plamodium is a mass of cytoplasm that contains many nuclei but NO CELL WALLS OR MEMBRANES

Oomycetes (Water Molds and Their Relatives)

Oomycetes Include water molds, white rusts,

and downy mildews Were once considered fungi based

on morphological studiesMost oomycetes

Are decomposers or parasites Have filaments (hyphae) that

facilitate nutrient uptake

Ecology of Fungus-Like Protists

The ecological impact of oomycetes can be significant Phytophthora infestans causes late blight of

potatoes – Irish Potato Famine Overgrowth of water mold caused by wet and cool

conditionsSlime molds and water molds are the MOST

important recyclers of organic materialWhy is the earth not littered with dead

organisms? Tissues broken down by Fungi Like Protists and

other decomposers