Plant Biology: A Mixture of Life Forms

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 1 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 a and 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 a and c; carotenoids, especially fucoxanthin	none or 2 flagella		scales (coccoliths) of organic material;	mostly marine; cause toxic “blooms”
Diatoms	Bacillariophyta	diatoms	none or chlorophylls a and c; carotenoids mainly fucoxanthin	none or 2 flagella; apical	chrysolaminarin	silica	marine and fresha water
Chrysophytes	Chrysophyta	chrysophytes, yellow-green algae	none or chlorophylls a and 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 a and 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 a and b; carotenoids	none, 2 or more flagella	starch	proteins, noncellulose carbohydrates, cellulose; some with plasmodesmata	marine and freshwater many symbionts