Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Protists

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nated fashion, allowing the colony to move. Daughter colonies

form inside the sphere, growing until they reach a certain size

and are released when the parent colony breaks open.

Spirogyra and Ulva are both examples of multicellular green

algae. Reproduction in the green algae can be both sexual and

asexual. Green algae store their energy as starch.

The fungus-like protists resemble the fungi during some

part of their life cycle. These organisms exhibit properties of

both fungi and protists. The slime molds and the water molds

are members of this group. They all obtain energy by decom-

posing organic materials, and as a result, are important for

recycling nutrients. They can be brightly colored and live in

cool, moist, dark habitats. The slime molds are classified as

either plasmodial or cellular by their modes of reproduction.

The plasmodial slime molds belong to the phylum

Myxomycota, and the cellular slime molds belong to the phy-

lum Acrasiomycota.

The plasmodial slime molds form a structure called a

plasmodium, a mass of cytoplasmthat contains many nuclei

but has no cell walls or membranes to separate individual

cells. The plasmodium is the feeding stage of the slime mold.

It moves much like an amoeba, slowly sneaking along decay-

ing organic material. It moves at a rate of 1 in (2.5 cm) per

hour, engulfing microorganisms. The reproductive structure of

plasmodial slime molds occurs when the plasmodium forms a

stalked structure during unfavorable conditions. This structure

produces spores that can be released and travel large distances.

The spores land and produce a zygote that grows into a new

plasmodium.

The cellular slime molds exist as individual cells during

the feeding stage. These cells can move like an amoeba as

well, engulfing food along the way. The feeding cells repro-

duce asexually through cell division. When conditions become

unfavorable, the cells come together to form a large mass of

cells resembling a plasmodium. This mass of cells can move

as one organism and looks much like a garden slug. The mass

eventually develops into a stalked structure capable of sexual

reproduction.

The water molds and downy mildews belong to the phy-

lum Oomycota. They grow on the surface of dead organisms or

plants, decomposing the organic material and absorbing nutri-

ents. Most live in water or in moist areas. Water molds grow as

a mass of fuzzy white threads on dead material. The difference

between these organisms and true fungi is the water molds

form flagellated reproductive cells during their life cycles.

Many protists can cause serious illness and disease.

Malaria, for example, is caused by the protist Plasmodium.

Plasmodia are sporozoans and are transferred from person to

Diatoms, an example of protists.

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