Protists WORLD OF MICROBIOLOGY AND IMMUNOLOGY460•
plex life cycles, as they usually live in more than one host in
their lifetimes.
The plant-like protists, or algae, are all photosynthetic
autotrophs. These organisms form the base of many food
chains. Other creatures depend on these protists either directly
for food or indirectly for the oxygen they produce. Algae are
responsible for over half of the oxygen produced by photo-
synthesizing organisms. Many forms of algae look like plants,
but they differ in many ways. Algae do not have roots, stems,
or leaves. They do not have the waxy cuticle plants have to
prevent water loss. As a result, algae must live in areas where
water is readily available. Algae do not have multicellular
gametangia as the plants do. They contain chlorophyll, but also
contain other photosynthetic pigments. These pigments give
the algae characteristic colors and are used to classify algae
into various phyla. Other characteristics used to classify algae
are energy reserve storage and cell wall composition.
Members of the phylum Euglenophyta are known as
euglenoids. These organisms are both autotrophic as well as
heterotrophic. There are hundreds of species of euglenoids.
Euglenoids are unicellular and share properties of both plants
and animals. They are plant-like in that they contain chloro-
phyll and are capable of photosynthesis. They do not have a
cell wall of cellulose, as do plants; instead, they have a pelli-
cle made of protein. Euglenoids are like animals in that they
are motile and responsive to outside stimuli. One particular
species, Euglena, has a structure called an eyespot. This area
of red pigments is sensitive to light. An Euglena can respond
to its environment by moving towards areas of bright light,
where photosynthesis best occurs. In conditions where light is
not available for photosynthesis, euglenoids can be het-
erotrophic and ingest their food. Euglenoids store their energy
as paramylon, a type of polysaccharide.
Members of the phylum Bacillariophyta are called
diatoms. Diatoms are unicellular organisms with silica shells.
They are autotrophs and can live in marine or freshwater envi-
ronments. They contain chlorophyll as well as pigments called
carotenoids, which give them an orange-yellow color. Their
shells resemble small boxes with lids. These shells are covered
with grooves and pores, giving them a decorated appearance.
Diatoms can be either radially or bilaterally symmetrical.
Diatoms reproduce asexually in an unique manner. The two
halves of the shell separate, each producing a new shell that
fits inside the original half. Each new generation, therefore,
produces offspring that are smaller than the parent. As each
generation gets smaller and smaller, a lower limit is reached,
approximately one quarter the original size. At this point, the
diatom produces gametes that fuse with gametes from other
diatoms to produce zygotes. The zygotes develop into full
sized diatoms that can begin asexual reproduction once more.
When diatoms die, their shells fall to the bottom of the ocean
and form deposits called diatomaceous earth. These deposits
can be collected and used as abrasives, or used as an additive
to give certain paints their sparkle. Diatoms store their energy
as oils or carbohydrates.
The dinoflagellates are members of the phylum
Dinoflagellata. These organisms are unicellular autotrophs.
Their cell walls contain cellulose, creating thick, protectiveplates. These plates contain two grooves at right angles to each
other, each groove containing one flagellum. When the two
flagella beat together, they cause the organism to spin through
the water. Most dinoflagellates are marine organisms,
although some have been found in freshwater environments.
Dinoflagellates contain chlorophyll as well as carotenoids and
red pigments. They can be free-living, or live in symbiotic
relationships with jellyfish or corals. Some of the free-living
dinoflagellates are bioluminescent. Many dinoflagellates pro-
duce strong toxins. One species in particular, Gonyaulax
catanella, produces a lethal nerve toxin. These organisms
sometimes reproduce in huge amounts in the summertime,
causing a red tide. There are so many of these organisms pres-
ent during a red tide that the ocean actually appears red. When
this occurs, the toxins that are released reach such high con-
centrations in the ocean that many fish are killed.
Dinoflagellates store their energy as oils or polysaccharides.
The phylum Rhodophytaconsists of the red algae. All
of the 4,000 species in this phylum are multicellular (with the
exception of a few unicellular species) and live in marine envi-
ronments. Red algae are typically found in tropical waters and
sometimes along the coasts in cooler areas. They live attached
to rocks by a structure called a holdfast. Their cell walls con-
tain thick polysaccharides. Some species incorporate calcium
carbonate from the ocean into their cell walls as well. Red
algae contain chlorophyll as well as phycobilins, red and blue
pigments involved in photosynthesis. The red pigment is
called phycoerythrin and the blue pigment is called phyco-
cyanin. Phycobilins absorb the green, violet, and blue light
waves that can penetrate deep water. These pigments allow the
red algae to photosynthesize in deep water with little light
available. Reproduction in these organisms is a complex alter-
nation between sexual and asexual phases. Red algae store
their energy as floridean starch.
The 1,500 species of brown algae are the members of
the phylum Phaeophyta. The majority of the brown algae live
in marine environments, on rocks in cool waters. They contain
chlorophyll as well as a yellow-brown carotenoid called
fucoxanthin. The largest of the brown algae are the kelp. The
kelp use holdfasts to attach to rocks. The body of a kelp is
called a thallus, which can grow as long as 180 ft (60 m). The
thallus is composed of three sections, the holdfast, the stipe,
and the blade. Some species of brown algae have an air blad-
der to keep the thallus floating at the surface of the water,
where more light is available for photosynthesis. Brown algae
store their energy as laminarin, a carbohydrate.
The phylum Chlorophytais known as the green algae.
This phylum is the most diverse of all the algae, with greater
than 7,000 species. The green algae contain chlorophyll as
their main pigment. Most live in fresh water, although some
marine species exist. Their cell walls are composed of cellu-
lose, which indicates the green algae may be the ancestors of
modern plants. Green algae can be unicellular, colonial, or
multicellular. An example of a unicellular green alga is
Chlamydomonas. An example of a colonial algae is Volvox. A
Volvox colonyis a hollow sphere of thousands of individual
cells. Each cell has a single flagellum that faces the exterior of
the sphere. The individual cells beat their flagella in a coordi-womi_P 5/7/03 11:11 AM Page 460