Lectures on Protists
“Protists”
1)  Basic traits of the protists
2)  Evolutionary origin and diversification
of the eukaryotes via endosymbiosis
3)  Modern diversity of protists, Part 1:
Plant-like protists
Figure 27.2 The three domains of life
•  Generalizations about protist ecology
•  For each group, pay attention to:
–  Mode of nutrition
–  Life cycle
–  Distinguishing characteristics
•  Serial endosymbiosis, primary vs.
seconday endosymbiosis
•  Eukaryotic Cell Advantages
•  Know how the different groups we study
are related
Figure 26.1 Some major episodes in the history of life
Figure 28.2 “Protista” is NOT a monophyletic group.
Diversity in traits of protists:
•  Obviously, all are eukaryotes
•  Varied Nutrition:
photoautotrophs (“algae”),
ingestive heterotrophs
(“protozoa”), absorptive
heterotrophs (fungus-like),
and “mixotrophs” (e.g.,
Euglena)
•  Most have at least one stage
that is motile (via flagella)
•  Much variation in life cycles
(pay attention to diploidy vs.
haploidy)
•  Most are found in water
(damp soil, oceans, lakes,
streams, animal bodies)
“Protists”
1)  Basic traits of the protists
2)  Evolutionary origin and diversification
of the eukaryotes via endosymbiosis
3)  Modern diversity of protists, Part 1:
Plant-like protists
1
Evidence supporting the serial
endosymbiosis theory
Figure 26.1 Some major episodes in the history of life. Note: the evolution of
the eukaryotic cell resulted in a burst of evolutionary diversification on earth.
Why did this happen?
•  Existence of endosymbioses today
(wolbachia)
•  Similarity between bacteria and
mitochondria/chlorplasts
–  Similar size
–  inner membrane enzymes & transport systems
–  replication by binary fission
–  circular DNA molecule, with similar sequences
–  similar ribosomes
Figure 28.8 A tentative phylogeny of eukaryotes.
“Protists”
1)  Basic traits of the protists
2)  Evolutionary origin and diversification of
the eukaryotes via endosymbiosis
3)  Modern diversity of protists, Part 1:
Plant-like protists
Figure 28.3 Euglena: an example of a single–celled protist. The first
eukaryotes were similar single-celled ancestors of the protists. How did the
first eukaryote evolve from a prokaryote ancestor?
Key features of Euglenids
•  Eye spot, light detector, phototaxis
•  Unicellular
•  Motile
•  Mixotrophy
•  Asexual reproduction only
•  No cell walls (protein bands for
strength)
•  Chlorophyll a and b and carotenoids
2
Figure 28.25 A hypothetical history of plastids in the photosynthetic
eukaryotes
Table 27.1 Classifying organisms by how they obtain carbon (to build cells and
organic molecules) and energy (to power metabolism and molecular
construction).
Table 27.1 Classifying organisms by how they obtain carbon (to build cells and
organic molecules) and energy (to power metabolism and molecular
construction).
Mixotrophy
•  Euglena have chloroplasts and carry out
photosynthesis, acquiring energy from
sunlight (autotrophic)
•  When light availability is inadequate,
Euglena can absorb organic nutrients from
the environment or engulf prey
(heterotrophic)
•  This ability to switch between autotrophy
and heterotrophy is called mixotrophy
Figure 28.4 A tentative phylogeny of eukaryotes.
Protist Diversity cont.
•  The golden browns: Chrysophyta
•  The Dinoflagellates: Dinophyta
•  The Diatoms: Bacillariophyta
• 
• 
• 
• 
Evolutionary notes
General Characteristics
Reproduction
Ecology/human impact
How many
membranes do you
think the golden
algae’s chlorplasts
have?
3
Golden Algae: Chrysophyta
Figure 28.9 Alveolates are characterized by membrane-bound sacs (alveoli)
beneath the plasma membrane.
•  Photosynthetic
•  Chlorophyll a & c and
carotenoids (Fucoxanthin)
•  Biflagellated
•  Autotrophs or mixotrophs
•  Can form cysts
•  Unicellular or colonial
•  Mostly fresh water
•  Cellulose or silica cell walls
Figure 28.10 Dinoflagellates spin due to the beating of a pair of spiral flagella
lying in a groove encircling the cell.
Dinoflagellates (Dinophyta)
•  Mostly phosynthetic autotrophs, some are
heterotrophic
•  Unicellular
•  2 flagella (many)
•  Chlorophyll a & c, carotenoids (peridinin)
•  Cellulose cell wall (or none)
•  Many are bioluminescent
•  Some are mutualistic symbionts in marine
invertebrates
•  Some species are responsible for red tides
(toxins)
Sexual (2N)
Asexual
(binary fission)
1N
4
Figure 32.1 A coral reef. Corals are colonial animals, with photsynthetic
dinoflagellate symbionts.
Zooxanthellae - keys to coral reef productivity
Figure 28.8 A tentative phylogeny of eukaryotes.
Stramenopila = hairy
flagellum
Figure 28.17 Diatoms (Phylum Bacillariophyta): one of the heterokont algae.
Diatoms have unique glass-like cell walls made of silica. They are VERY
abundant as “plankton” in the surface waters of lakes, rivers, and oceans.
They reproduce sexually only rarely.
Alga = photosynthetic
protist
“Heterokont” algae are
the algae in
Stramenopila (browns,
goldens, and diatoms)
The plastids of the
heterokont algae
evolved by secondary
endosymbiosis, and
thus have triple
membranes.
The colors of algae are
due to accessory
pigments in their plastids
Diatoms: Bacillariophyta
Figure 28.17x Diatom shell. Note: diatoms have a two-part cell wall, one of
which fits inside the other like the parts of a shoe box.
•  Photoautotrophs
•  Solitary or colonial
•  Make up phytoplankton in oceans, lakes,
streams - extremely important contributors to
global Oxygen!
•  Silica cell walls
•  Primarily asexual reproduction, diploid - some
sexual reproduction
•  Form auxospores - resting stage
•  Chlorophyll a and c and fucoxanthin (a
carotenoid)
5
Diatom Life Cycle
asexual Reproduction
pennate vs. centric shapes
A diatom frustule
They get smaller with successive generations!
Diatom Life Cycle 2.
Occasional Sexual Reproduction
n
meiosis
2n
n
Diatom Life Cycle 2.
Occasional Sexual Reproduction
Oogamous in centric
diatoms
Isogamous in pennate
(with no flagellated
male gametes)
n
meiosis
2n
n
fertilization
2n
When diatoms do reproduce sexually, their life
cycle is like an animal’s, except never multicellular
(and some algae, like diatoms)
6