A Mixture of Life Forms

The Kingdom Protista (Protoctista) is a hodgepodge of organisms with little relationship to one another. Most members of this kingdom have features of one or more of the other kingdoms, but not enough to place them legitimately into any one of these as currently defined. Cladists (those who classify organisms based on cladistics) suggest rearranging the tree of life to accommodate the protista and redefining its branches based on gene sequencing and other new data. Three new kingdoms—each equal in rank to the present animals, fungi, and plants—would be added and the plant and animal kingdoms expanded to include the rest of the protista. The proposal is still in the pro versus con heated debate stage. This could be simply another esoteric controversy among biologists if not for the fact that the organisms under discussion are some of the most important in the world. Scores of protists are the single‐celled, photosynthetic, primary producers of marine and freshwater food chains—and have been for the past billion years. In addition, all of the protists provide basic clues to how life evolved on the early Earth; the kingdom is a collection of experiments on how to manage energy and conduct life. Within the protista are found the basic plans that were adopted, adapted, and modified into the life we see around us today.

With classifications in such a state of flux, which organisms one includes in the Kingdom Protista, and how to group them, are matters of choice. Table follows the lead of several popular textbooks. Animal‐like protists have been omitted from the table. They include such organisms as Sarcodina: amoebas, forams, radiolarians; Flagellates: dinoflagellates, euglenoids, zooflagellates (trypanosomes, Giardia); parasitic Sporozoans; and Ciliates like Paramecium. Those protists of particular interest to plant biologists usually include the fungi‐like and plant‐like organisms. As always, follow your instructor's views on the systematics of the protista.

TABLE 1 Major Phyla of Protists and Some of Their Characteristics

Group

Phylum

Organisms

Chlorophylls

Locomotion

Reserve Carbohydrate

Cell Wall Composition

Habitat

FUNGUS-LIKE

Water molds

Oomycota

oomycetes, Saprolegnia, Phytophthora, Plasmopara, Pythium

none

2 flagella in zoospores & male gametes only

glycogen

cellulose cell walls

marine, freshwater & terrestrial; plant pathogens

Slime molds

plasmodial

Myxomycota

myxomycete

none

2 flagella, in gametes; amoeboid

glycogen

none on plasmodium

terrestrial

cellular

Dictyosteliomycota (Acrasiomycota)

dictyostelids

none

amoeboid

glycogen

cellulose

terrestrial

PLANT-LIKE (ALGAE)

Cryptomonads

Cryptophyta

cryptomonads, cryptophytes

none, or chlorophylls aand c; phycobilins; carotenoids

flagella, unequal, subapical

starch

no cell wall; protein plates

marine and fresh-water; cold water

Red algae

Rhodophyta

red algae, coralline algae

chlorophyll a; phycobilins; carotenoids

none

floridean starch

cellulose embedded in gelatinous matrix

mostly marine, warm waters;

Haptophytes

Haptophyta

haptophytes, coccoliths

chlorophylls aand c; carotenoids, especially fucoxanthin

none or 2 flagella

scales (coccoliths) of organic material;

mostly marine; cause toxic “blooms”

Diatoms

Bacillariophyta

diatoms

none or chlorophylls aand c; carotenoids mainly fucoxanthin

none or 2 flagella; apical

chrysolaminarin

silica

marine and fresha water

Chrysophytes

Chrysophyta

chrysophytes, yellow-green algae

none or chlorophylls aand c; carotenoids mainly fucoxanthin

none or 2 flagella; apical

chrysolaminarin

none or silica scales; some cellulose scales

mainly freshalgae water, a few marine

Brown algae

Phaeophyta

brown algae, kelps, rock-weed, Fucus

chlorophylls aand c; carotenoids mainly fucoxanthin

2 flagella only in reproductive cells

laminarin transported as mannitol

cellulose in algin; some algin; some with plasmodesmata

almost all marine, in cold waters

Green algae

Chlorophyta

green algae

chlorophylls aand b; carotenoids

none, 2 or more flagella

starch

proteins, noncellulose carbohydrates, cellulose; some with plasmodesmata

marine and freshwater many symbionts