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Kingdom Protoctista

Greek protos, very first; ktistos, to establish

Kingdom Protoctista is defined by exclusion: its members are neither animals (which develop from a blastula), plants (which develop from an embryo), fungi (which lack undulipodia and develop from spores), nor prokaryotes. They comprise the eukaryotic microorganisms and their immediate descendants: all nucleated algae (including the seaweeds), undulipodiated (flagellated) water molds, the slime molds and slime nets, and the protozoa. Protoctist cells have nuclei and other characteristically eukaryotic properties; most have aerobiosis and respiration in mitochondria and 9 + 2 undulipodia at some stage of the life cycle.

Why "protoctist" rather than "protist"? Since the nineteenth century, the word protist, whether used informally or formally, has come to connote a single-celled organism. In the last two decades, however, the basis for classifying single-celled organisms separately from multicellular ones has weakened. It has become evident that multicellularity evolved many times from unicellular forms-many multicellular organisms are far more closely related to certain unicells than they are to any other multicellular organisms. For example, the ciliates (Ciliophora), which are unicellular microbes, include at least one species that forms a sorocarp, a multicellular spore-bearing structure; euglenoids, chrysophytes and diatoms also have multicellular derivatives.

We have adopted the concept of protoctist propounded in modern times by H. F. Copeland in 1956 (see the bibliography at the end of this chapter). The word had been introduced by John Hogg in 1861 to designate "all the lower creatures, or the primary organic beings;-both Protophyta, . . . having more the nature of plants; and Protozoa . . . having rather the nature of animals." Copeland appreciated, as had several scholars in the nineteenth century, the absurdity of calling giant kelp by a word "protist," that had come to imply unicellularity and, thus, smallness. He proposed an amply defined Kingdom Protoctista to accommodate certain multicellular organisms as well as the unicells that may resemble their ancestors-for example, kelp as well as the tiny brownish cryptophyte alga Nephroselmis. The kingdom thus defined also solves the problem of blurred boundaries that arises if the unicellular organisms are assigned to the various multicellular kingdoms.

We propose 27 protoctist phyla. This number is a matter of taste rather than tradition because there are no established rules for defining protoctist phyla. Our groupings are debatable; for example, some argue that the Caryoblastea, comprising a single species of nonmitotic amoeba, ought to be placed in Phylum Rhizopoda with the other amoebas, or that the cellular and plasmodial slime molds should be placed together. Some believe that the chytrids, hyphochytrids, and oomycotes belong to the fungi and that the chlorophytes belong to the plants. Some insist that chaetophorales and prasinophytes, which here are considered chlorophytes, ought to be raised to phylum status. There are arguments for and against these views.* Our system has the advantage of defining the three multicellular kingdoms precisely, but the disadvantage of grouping together as protoctists amoebae, kelps, water molds, and other eukaryotes that have little in common with one another.

Protoctists are aquatic: some primarily marine, some primarily freshwater, and some in watery tissues of other organisms. Nearly every animal, fungus, and plant, perhaps every species, has protoctist associates. Some protoctist phyla, such as Apicomplexa and Cnidosporidia, include hundreds of species, all of which are parasitic on other organisms.

No one knows how many species of protoctists there really are; thousands have been described in the biological literature. Water molds and plant parasites have traditionally been dealt with by the mycological literature, parasitic protozoa by the medical literature, algae by the botanical literature, free-living protozoa by the zoological literature, and so forth. Inconsistent practices of describing, naming, and defining species have led to a confusion that this book attempts to dispel. Another reason for ignorance is that the group of eukaryotic microbes is large, with much diversity in tropical regions, whereas protozoologists and phycologists are scarce and concentrated in the north temperate zones. Furthermore, distinguishing species of free living protoctists often requires time-consuming genetic, morphologic and ultrastructural studies. Funding for such studies is limited because most protoctists are not sources of food and cause no diseases; thus, they are of no direct economic importance.

The protoctists show remarkable variation in cell organization, patterns of cell division, and life cycle. Some are photoautotrophs, which eliminate oxygen; others are ingesting or absorbing heterotrophs (such as phagotrophs or osmotrophs). In many species, the type of nutrition depends on conditions: when light is plentiful, they photosynthesize; in the dark, they feed. However, although protoctists are far more diverse in life style and nutrition than are animals, fungi, or plants, they are far less diverse metabolically than the bacteria.

Increasing knowledge about the ultrastructure, genetics, life cycle, developmental patterns, chromosomal organization, physiology, metabolism, and protein amino-acid sequences of the protoctists has revealed many differences between them and the animals, fungi, and plants. It has even been suggested that the major protoctist groups, here called phyla, are so distinct from each other as to deserve kingdom status, and that nearly 20 kingdoms ought to be created to accommodate them. In a recent comprehensive compilation of information about protoctists, nearly 50 phyla are recognized. We can anticipate many years of animated discussion about the optimal taxonomy of this amazing group of organisms. With due respect for their differences, a recognition of their common eukaryotic heritage, and a sense of humility toward both their complexity and our ignorance, we present our 27 protoctist phyla.

Kingdom Protoctista