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 sea-weeds), 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, 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.

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 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 has led to a great deal of confusion regarding these organisms. Another reason for ignorance is that the group of eukaryotic microbes is large, with much diversity in tropical regions, whereas protozoologists are scarce and concentrated in the north temperate zones. Furthermore, distinguishing species of free-living protoctists often requires time-consuming genetic 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 make 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, now 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. Adapted from The Five Kingdoms by Lynn Margulis. W.H. Freeman and Company, NY, 1988.

For inquiries contact Aaron Haselton