Hogg, 1861 (sensu H.F. Copeland, 1956)
(To complete all classifications ETI has added the Kingdom and the Phyla of all the different taxa treated on this DVD-ROM without higher classification descriptions. Texts from Lynn Margulis and Karlene V. Schwartz, Five Kingdoms. CD-ROM. Copyright 2002 ETI / Freeman & Co Publishers).
[Editor's note: the concerned authors in South Atlantic Zooplankton used the Kingdom Protista Haeckel, 1886 instead of the Protoctista to accomodate the Foraminifera, the Acantharia, the Radiolaria, and the Ciliophora].
The Kingdom Protoctista comprises the eukaryotic microorganisms and their immediate descendants: all algae, including the seaweeds, undulipodiated (flagellated) water molds, the slime molds and slime nets, the traditional protozoa, and other even more obscure aquatic organisms. Its members are not animals (which develop from a blastula), plants (which develop from an embryo), or fungi (which lack undulipodia and develop from spores). Nor are protoctists prokaryotes. All protoctist cells have nuclei and other characteristically eukaryotic features. Many photosynthesize (have plastids), most are aerobes (have mitochondria), and most have [9(2)+2] undulipodia with their kinetosome bases at some stage of the life cycle. All protoctists evolved from symbioses between at least two different kinds of bacteria — in some cases, between many more than two. As the symbionts integrated, a new level of individuality appeared.
Many different combinations of ancient bacteria into symbiotic consortia did not pass the test of natural selection. But those that survived gave rise to modern-day protoctist lineages, which may be grouped according to their organelle structure. In the mitochondrion, for example, the most essential (and therefore slowly evolving) membranous structures are the cristae. These structures may be flat (as in the stramenopiles, chytrids and zoomastigotes), tubular (as in the alveolates), (as in the amebas [...]); or altogether absent (as in the archaeprotists and microsporans). Photosynthetic pigment profiles, essential to chloroplast function, are major criteria similarly employed by taxonomists to resolve the bewildering diversity of Kingdom Protoctista.
Undulipodia and their insertions, the kinetosomes always embedded in kinetids, are crucial to an understanding of protoctists. Undulipodia were present in common ancestors to all the phyla even before mitochondria, given that the anaerobic archaeprotists bear them. Their behavior as they move and reproduce is related to mitotic cell division. In some phyla, all members bear undulipodia; in other phyla, they are absent; but most protoctists produce and retract them as a function of their life histories. Although the importance of the undulipodia that develop from kinetosomes is emphasized by all who study protoctists — algologists, invertebrate zoologists, microbiologists, mycologists, parasitologists, protozoologists, and others — some feel that the use of the term “flagella” should be retained. But “flagella” are entirely unrelated rotary structures of bacteria, and hence the word, when applied to cilia, sperm tails, and other undulipodia, is confusing.
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 past two decades, however, the basis for classifying single-celled organisms separately from multicellular ones has weakened. Multicellularity evolved many times in unicellular organisms—many multicellular beings are far more closely related to certain unicells than they are to other multicellular organisms. For example, the ciliates (Phylum Ciliophora), most of which are unicellular microbes, include at least one species that forms a sorocarp, a multicellular cyst-bearing structure. Euglenids (Phylum Discomitochondriates), chrysomonads (planktonic), and diatoms also evolved multicellular descendants.
Here we adopt the concept of protoctist propounded in modern times by American botanist Herbert F. Copeland in 1956. The word had been introduced by English naturalist 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 recognized, as had several scholars in the nineteenth century, the absurdity of referring to giant kelp by the word “protist,” a term 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 cryptomonad alga Nephroselmis. The protoctist kingdom thus defined also solves the problem of blurred boundaries that arises if the unicellular organisms are assigned to the intrinsically multicellular kingdoms.
Attempting to reconcile ultrastructural and genetic information with newly acquired molecular data, we here propose 30 protoctist phyla. This number is more a matter of taste than tradition, because no rules for defining protoctist phyla are enforced. Our groupings are debatable; for example, some argue that the cellular and plasmodial slime molds (Phyla Paramyxa and Myxomycota, respectively) should be united. Some believe that the Phyla Oomycota, Hyphochytriomycota, and Chytridiomycota, respectively, are really fungi and that the Phylum Chlorophyta are plants. Some insist that chaetophorales and prasinophytes, which here are within the Chlorophyta, ought to be raised to phylum status. Most would reunite conjugating green algae (Phylum Gamophyta) with the others in the Phylum Chlorophyta. There are arguments for and against these views (references to alternative kingdoms and phyla are listed in the bibliography). Our system has the advantage of limiting the number of highest taxa and precisely defining the three kingdoms of large organisms. Although it has the disadvantage that these eukaryotes have little in common with one another, grouping together xenophyophores, cercomonads, water molds, and the others as the single kingdom Protoctista is superior to the tradition of ignoring them entirely.
Protoctists are aquatic: some marine, some freshwater, some terrestrial in moist soil, and some parasitic or symbiotic in moist tissues of others. Nearly all animals, fungi, and plants — perhaps all — have protoctist associates. Phyla such as the Phyla Microspora and Apicomplexa include myriad species, all of which live in the tissues of others.
No one knows the number of protoctist species. Although 40,000 extinct foraminifera alone are documented in the paleontological literature and more than 10,000 live protoctists are described in the biological literature, Georges Merinfeld (Dalhousie University, Halifax, Nova Scotia) estimates that there are more than 65,000 extant species and John Corliss (University of Maryland) suggests that there are more than 250,000. Water molds and plant parasites are described in the literature on fungi, parasitic protozoa in the medical literature, algae by botanists, and free-living protozoa by zoologists. Contradictory practices of describing and naming species have led to confusion that Five Kingdoms attempts to dispel. Another problem is that much protoctist diversity is in tropical regions, where scientists are scarce. Furthermore, the documentation of new species of protoctists often requires time-consuming life-cycle and ultrastructural study. Most funding is limited to those temperate-zone protoctists that are sources of food, industrial products, or disease.
Remarkable variation in cell organization, patterns of cell division, and life cycle is evident in this diverse group of eukaryotic microbes and their relatives. Whereas the algae are oxygenic phototrophs, the others are heterotrophs that ingest or absorb their food. In many, the type of nutrition varies with condition: they photosynthesize when light is plentiful and feed in the dark. Although protoctists are more diverse in life style and nutrition than are animals, fungi, or plants, metabolically they are far less diverse than bacteria.
Increasing knowledge about the ultrastructure, genetics, life cycle, developmental patterns, chromosomal organization, physiology, metabolism, fossil history, and especially the molecular systematics of protoctists has revealed many differences between them and animals, fungi, and plants. The major protoctist groups, described here [= Five Kingdoms] as phyla or groups of phyla, are so distinct as to deserve kingdom status in the minds of some authors, as explained in the Handbook of Protoctista (Margulis, Corliss, Melkonian, and Chapman, editors) and the Illustrated Glossary of Protoctista (Margulis, McKhann, and Olendzenski, editors). [...]. New molecular biological data relating the protoctist taxa are described by Mitchell Sogin. Because no single person or group can master all the biological details of the protoctists, we expect years of animated discussion about their optimal taxonomy. With awe for protoctist diversity, a recognition of their common eukaryotic heritage, and a sense of humility toward both their complexity and our ignorance, we present our 30 protoctist phyla [in Five Kingdoms].