Algae
Algae (singular: alga) are photosynthetic protists, usually aquatic
- not a phylogenetically meaningful term, but widely used
- despite their complexity, algae are NOT PLANTS
Algae are responsible for most photosynthesis in the ocean
(which covers 70% of the planet’s surface)
Shallow water dominated by macroalgae with large, multicellular
bodies; critical as food for near-shore animals
- giant kelp Macrocystis pyrifera, sea lettuce Ulva lactuca
Open ocean dominated by phytoplankton: single-celled
photosynthesizers that fix CO2
- form the basis of the food web; they are eaten by little critters,
which are eaten by fish, which are eaten by bigger fish...
Gametic life cycle
No gametophyte (haploid body) -- Gametes are only haploid cells
sperm +
eggs
Sporic life cycle
common among multicellular protists like algae; one stage built
out of haploid cells, another made of diploid cells
Alternation of Generations:
haploid & diploid stages may look different
meiosis only occurs in the
sporophyte stage
Alternation of Generations
Many protists exhibit alternation of generations, in which a
multicellular haploid stage alternates w/ multicellular diploid stage
1) Meiosis produces spores (N, haploid), which grow into a whole
haploid body called the gametophyte
2) The gametophyte produces gametes (sperm + eggs) by mitosis
3) Syngamy produces the zygote, “the baby” (2N, diploid)
4) The zygote grows into the sporophyte stage (2N, diploid),
which undergoes meiosis...
Macroalgae: 3 lineages
Phylum Chlorophyta: Green Algae
Chloroplasts very similar to those in true plants
- chlorophyll a and b
Green algae share a common ancestor with land plants, and are
included in the “Kingdom Plantae” by some
Some Chlorophytes live symbiotically with fungi as lichens,
super-organisms that grow on rocks in moist terrestrial
environments
Phylum Rhodophyta: Red Algae
Red from an accessory pigment, phycoerythrin
- allows absorption of blue + green light, the wavelengths that
penetrate the deepest in ocean water (red is reflected)
Some have lost their plastids,
become parasites on other algae
- may explain why endosymbiosis
occurred repeatedly in this group
Nori, used in sushi rolls,
is the red alga Porphyra
Phylum Stramenopiles - Brown algae
Largest, most complex multicellular colonies of any protist
- convergent evolution has resulted in many plant-like features
- blades look like leaves; stipes look like stems;
holdfasts look like roots
However, brown algae lack true tissues which exist in land plants
Blade
Stipe
Holdfast
Phylum Stramenopiles - Brown algae
Giant kelp Macrocystis can grow 1 meter per day, 60 m in a year!
- fastest growth rate of any organism on the planet
Grows here in CA because
coastal upwelling brings nutrientrich bottom water to the surface
- supports a diverse community of
fish + other animals that depend
on the kelp to support the food web
Phylum Stramenopiles - Brown algae
Diploid (2N)
sporophyte
Haploid (1N)
spores
gametophytes
(male + female)
Microscopic photosynthesizers: 3 lineages
Diatoms (Phylum Stramenopiles)
Diatoms contribute half of the primary production in marine
ecosystems
- secrete 2-valve shell of silicate (glass) over plasma membrane
Phylum Dinoflagellata
Possess 2 flagella that twirl as they swim (Greek dinos, whirling)
Can be autotrophic (photosynthetic), heterotrophic, or some
can even switch back and forth (mixotrophic)
Include the photosynthetic endosymbionts of hard corals
Phylum Dinoflagellata
Can cause red tides, up to 100 million cells per liter
- use up all O2, leading to
mass kills of sea life
- locally, create intense
bioluminescence in
breaking waves at night
Phylum Dinoflagellata
Include photosynthetic endosymbionts of hard corals
Can cause red tides, up to 100 million cells per liter
- increasing in frequency, due to run-off from agriculture
- produce potent toxins, which concentrate in filter feeders
(clams, mussels) and are very dangerous to humans:
- saxitoxins: Paralytic Shellfish Poisoning
- other toxins: Diarrhetic or Amnesiac poisoning
Economic development can occur at the expense of
environmental quality
Increasing incidence
of red tides in Chinese
seas …
…coincident with
increased economic
Development (GDP)
Liu & Diamond (2005)
Nature 435:1179-1186
Animal-like protists
Diverse heterotrophic lineages of eukaryotes
Alveolates -- ciliates, Plasmodium (malaria parasite)
Euglenids -- Euglena, Trypanosoma (sleeping sickness parasite)
Amoebazoans -- Amoeba
Diplomonads -- Giardia
Alveolata: 3 related phyla
Phylum Dinoflagellata – dinoflagellates
Phylum Ciliophora– ciliates
Phylum Apicomplexa– includes malaria parasite
Note the diversity of metabolic processes and lifestyles found
in this one group of protists –
- parasites & endosymbionts
- free-living heterotrophs
- autotrophs & mixotrophs
Phylum Ciliophora– Paramecium
Bacteria are pushed along
food groove into mouth
by cilia
Food is engulfed
by phagocytosis,
digested in food
vacuoles
Phylum Ciliophora
Ciliated protozoans, eg. Paramecium
- tiny hairs called cilia beat together, making water flow
past the body
propels cell forward
Phylum Apicomplexa
Plasmodium causes malaria, affects 500 million people and
kills 2 million per year
Spread by female Anopheles mosquito;
can only complete its life cycle when
syngamy happens inside a mosquito
- stays inside our liver + blood cells;
hard for our immune system to attack it
- eats hemoglobin
- contain a non-functioning plastid,
formerly a green alga endosymbiont
Phylum Kinetoplastida
Trypanosomes – parasitic
- have both an insect and a vertebrate host
- Trypanosoma causes African Sleeping Sickness (tsetse fly)
- other species cause leishmaniasis, Chagas’ disease
(infect >20 million in Central and South America)
Evade host immune response by constantly changing proteins
that coat their cell surface, which would otherwise be recognized
by antibodies produced by our immune system
- rotate among 1,000 genes for surface proteins
Single, elongated mitochondrion with entwined circles of DNA
called the kinetoplast
Phylum Euglenida
Some are photosynthetic
- like plants, plastid was formerly a cyanobacterium
Move using a whiplike flagellum
May only be asexual (very rare)
Amoebae
Named for pseudopods, temporary extensions of cytoplasm used in
feeding and locomotion
Look for action of water
vacuoles -- organelles that
collect water as it flows into
the cell, then expel it so the
cell doesn’t pop
Phagocytosis: food particles are
engulfed in vesicles, followed by
intracellular digestion
Foraminifera
Related to amoebae, but with a shell of
calcium carbonate covering
plasma membrane
Pseudopods extrude through holes in test, trap prey
- empty shells contribute to carbonate depositions on sea floor
(chalk layers, great pyramid building blocks)

shell
(calcium
carbonate)
Foraminifera
Isotope composition of calcium carbonate shells = a record
of past temperatures, critical for studies of past climate change
Diplomonads
Includes Giardia, the worst most
awful parasite in the world
(because it came after me)
Weird cells: 2 nuclei, 8 flagella
- lack mitochondria, Golgi, smooth Endoplasmic Reticulum
 once thought to be a transitional stage between prokaryotes
and eukaryotes
 actually just very simplified cells that lost most features,
which they no longer need because of their parasitic lifestyle