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Chapter 5 : The Diversity of Free-Living Protists Seen and Unseen, Cultured and Uncultured
Category: Genomics and Bioinformatics; Environmental Microbiology
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The development of improved preservation and staining techniques and particularly the application of electron microscopy provided a wealth of morphological information to improve the taxonomic criteria used for the description and identification of protists. This chapter concerns the "seen and unseen," "cultured and uncultured" protists, with a brief overview of these three categories. Ecological studies of cultured protists have included representatives from most of the major lineages of protists and have detailed the nutritional aspects of these species, their elemental stoichiometries, feeding behaviors and rates, growth rates, and growth efficiencies. The understanding of the biogeochemical significance and activities of these species has been ascertained largely through the manipulation and experimental examination of cultured protists. Using this information, ecologists have generated biogeochemical and food web models that significantly improved our understanding of the activities of photosynthetic and heterotrophic protists in natural communities and thus lend better insight into how aquatic communities function. The alveolates contain three well-known groups of protists: the ciliates, the dinoflagellates, and the apicomplexans. An example of the extensive genetic diversity detected in these very small protists can be found in the picoprasinophyte genus Micromonas. Success in this work obviously requires some general knowledge of the nutrition of the target cells, so these attempts can improve dramatically as the "needs" of the protistan taxa are characterized.
One recent proposition for the evolutionary relationships among living eukaryotic organisms. Note that protistan taxa dominate the many eukaryotic lineages of organisms with respect to diversity and evolutionary breadth. From S. L. Baldauf, D. Bhattacharya, J. Cockrill, P. Hugenholtz, J. Pawlowski, and A. G. B. Simpson. The tree of life: an overview, p. 43–75. In J. Cracraft and M. J. Donoghue (ed.), Assembling the Tree of Life, chapter 4. Oxford University Press, Oxford, United Kingdom, 2004.
Examples of morphologically described and commonly cultured protists (“seen and cultured”) and morphologically described but uncultured protists (“seen but uncultured”). (A) Differential interference contrast micrograph of Euglena gracilis, a facultatively phototrophic/heterotrophic protist (from D. J. Patterson). Euglena species have been cultured for many years. (B) A phase-contrast micrograph of a symbiont bearing antarctic acantharian. Neither the host nor the symbionts have been cultured. (C) Dark-field photomicrograph of the planktonic foraminiferan, Globigerinoides sacculifer, and its intracellular dinoflagellate symbionts. Planktonic foraminifera have never been cultured, the symbionts of G. sacculifer (Gymnodinium beii) have been. (D) An antarctic tintinnid ciliate photographed using differential interference contrast microscopy. Tintinnids have been isolated and cultured from numerous marine ecosystems. (E) Scanning electron micrograph of Lingulodinium polyedrum, a cultured, red-tide dinoflagellate. (F) Light micrograph of an antarctic species of Dinophysis. Species of this genus have only recently been brought into laboratory culture (see text). (G) Phase-contrast micrograph of a lobose amoeba maintained in enrichment cultures. (H) Dark-field photomicrograph of several species of symbiont-bearing colonial radiolaria. Central capsules of the radiolaria are visible as small dots within the pseudopodial networks. Dinoflagellate symbionts give the capsules a yellowish green color. The dinoflagellate symbiont, Scrippsiella nutricula, has been cultured, but the hosts have never been. (I) Phase-contrast micrograph of Phaeocystis antarctica, a cultured prymnesiophyte alga that forms mucilaginous colonies. Individual cells are the small dots embedded in the colony matrix. (J) Ornithocercus sp., a heterotrophic dinoflagellate with episymbiotic cyanobacteria, visualized using phase-contrast microscopy. The host has not been cultured, but the cyanobacterium has. (K) Negatively stained, transmission electron micrograph of Pyramimonas sp., a cultured antarctic prasinophyte. (L) Phase micrograph of Cylindrotheca sp., an antarctic diatom. (M) Phaselight micrograph of an unknown, uncultured antarctic diatom. (N) Light micrograph of the heterotrophic ebriid, Hermesinum sp. Ebriids have not been cultured. (O) The commonly cultured ciliate, Uronema marina, with ingested prey (the pelagophyte alga, Aureococcus anophagefferens), photographed using light microscopy. The pelagophyte has also been cultured and can be used to maintain cultures of this ciliate. Marker bars are 20 μm (A, B, F, L), 300 μm (C, H), 40 μm (D, E, I, M), 10 μm (G, J, N, O), and 2 μm (K).
(A) Rank-abundance curve of microbial taxa in an idealized microbial (bacterial) community and (B) an actual rank-abundance curve for a natural protistan assemblage. The inset in panel B shows an enlargement of the rank-abundance curve for the most commonly encountered phylotypes. Panel A is from Pedrós-Alió (2006) ; panel B is from Countway et al., 2007 .
A proposed phylogenetic tree indicating the placement of major clades of “undescribed, uncultured” protistan taxa within the alveolates. Taken from Groisellier et al., 2006.
A proposed phylogenetic tree noting the association of several novel protistan clades within parasitic protistan lineages. Taken from Moreira et al., 2003 .
A proposed phylogenetic tree showing the placement of numerous novel marine stramenopile (MAST) phylotypes. Taken from Massana et al., 2004 .