Protists
Protists are an incredibly diverse group of Eukayotes (most are unicellular, some are multicellular).
The kingdom Protista has largely been abandoned, but the term protist is still used to describe eukaryotes
that are not plants, animals or fungi.
Protists are the most nutritionally diverse of all eukaryotes, they include
 Photoautotrophs, which contain chloroplasts
 Heterotrophs, which absorb organic molecules or ingest larger food particles
 Mixotrophs, which combine photosynthesis and heterotrophic nutrition
Protist habitats are also diverse and including freshwater and marine environments; reproduction and life
cycles are also highly varied with both sexual and asexual species.
AP Biology 2014
Examples of Protists and their Key Adaptations (italics)
1. Diplomonads and Parabasalids
These lack plastids and have mitochondria that lack DNA, electron transport chains, and citric-acid cycle
enzymes. Consequently they are adapted to anaerobic environments.
 Diplomonads have multiple flagella—so they can purposely move.
2. Euglenozoa is a diverse clade that includes predatory heterotrophs, photosynthetic autotrophs, and
pathogenic parasites. They are characterized by having flagella.
 The parasitic Trypanosoma which causes sleeping sickness in humans is a protist that belongs to
this group.
3. Alveolates
Members of this group have membrane-bounded sacs (alveoli) just under the plasma membrane. These
sacs may help stabilize the cell surface or be used in osmoregulation. This group includes:
(i) Dinoflagellates: a diverse group of aquatic photoautotrophs and heterotrophs that make up marine and
freshwater phytoplankton. The rapid growth of some dinoflagellates is responsible for causing “red tides,”
which can be toxic to humans.
(ii) Apicomplexans are parasites of animals and some cause serious human diseases.
(iii) Ciliates are a large varied group of protists and are named for their use of cilia to move and feed.
Living in a Fresh Water Environment:
This presents a problem for many fresh water protists as they are constantly taking on water via
osmosis. Paramecium, a species of protist belonging to the ciliate clade possess an adaptation that deals
with this problem, it is the CONTRACTILE VACUOLE. This is an organelle that collects water and
periodically expels the water (by contracting) and this is a key adaptation in many freshwater protist
species.
4. Stramenopiles
The clade Stramenopila includes several groups of heterotrophs as well as certain groups of algae.
(i) Oomycetes: Include water molds, white rusts, and downy mildews. These were once considered fungi
based on morphological studies. Most oomycetes are decomposers or parasites and have filaments
(hyphae) that facilitate nutrient uptake.
(ii) Diatoms: Unicellular algae with a unique two-part, glass-like wall of hydrated silica. Diatoms are a
major component of phytoplankton and are highly diverse. Accumulations of fossilized diatom walls
compose much of marine sediments.
(iii) Golden Algae: Named for their color, which results from yellow and brown carotenoid pigments,
which are adaptations allowing them to absorb specific wavelengths of light.
(iv) Brown Algae: The largest and most complex algae. They are all multicellular, and most live in the
sea. Brown algae include many of the species commonly called seaweeds. Seaweeds have the most
complex multicellular anatomy of all algae. Kelps, or giant seaweeds, live in deep parts of the ocean.
 Seaweeds have a plant-like body and a root like structure called a HOLDFAST which anchors
the seaweed to rocks etc.
 Some have floats which keep the photosynthesizing leaves near the water surface. Some brown
algae have alternation of generations.
Alternation of Generations
A variety of life cycles have evolved among the multicellular algae, and the most complex life cycles
include an alternation of generations. This is the alternation of multicellular haploid and diploid forms.
5. Cercozoans, Foraminiferans (Forams) and Radiolarians
The key adaptation in members of these clades is the threadlike “PSEUDOPODIUM” (false foot) which
is used by the organism to move and feed. These clades are distinguished from amoebozoan clades (see
below) in that their pseudopodia are threadlike.
6. Amoebozoans
The key adaptation here is lobe-shaped pseudopodia which increases feeding efficiency and movement
over the threadlike pseudopodia seen in the previous clades. Ex. Amoeba
7. Red Algae and Green Algae
These are the closest relatives of land plants. Over a billion years ago, a heterotrophic protist engulfed a
cyanobacteria (which became an endosymbiont) and the photosynthetic descendants of this ancient protist
evolved into red algae and green algae.
(i) Red Algae:
Reddish in color due to an accessory pigment call phycoerythrin, which masks the green of chlorophyll.
This pigment is a key adaptation because it allows these algae to absorb wavelengths of light that
penetrate into deep water, thus allowing these algae to inhabit otherwise uninhabitable environments.
 Red algae are usually multicellular; the largest are seaweeds and they are the most abundant large
algae in coastal waters of the tropics.
(ii) Green Algae
Named for their grass-green chloroplasts. They are closely related to land plants.

They include unicellular, colonial, and multicellular forms. Most live in fresh water, although
some are marine. Others live in damp soil, as symbionts in other eukaryotes or as mutualistic
partners with fungi (lichens).

Most have complex life cycles with both sexual and asexual reproductive stages.