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THE “KINGDOMLESS” PROTISTS. Characteristics. The first eukaryotes arose somewhere around two billion years ago perhaps about the same time the atmosphere & oceans became Oxygen rich.
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The first eukaryotes arose somewhere around two billion years ago perhaps about the same time the atmosphere & oceans became Oxygen rich. Protists are extremely diverse. They are found in most aquatic environments as well as in moist soils, and leaf litter. Many protists are unicellular but others are multicellular (think kelp) and may be up to 60 m. long. They are aerobic and have mitochondria (sites of ATP production)
They also have diverse metabolic mechanisms: Photoautotrophs (the plant-like protists, and algae) Heterotrophs (animal-like protists, protozoans) Mixotrophs combine photosynthesis and heterotrophic nutrition (fungus-like protists)
Most have flagella or cilia. These locomotor appendages are different than those of bacteria in that they are extensions of the cytoplasm and have a 9+2 arrangement of the microtubules.
Mitosis occurs in most protists Reproduction varies: asexual, sexual, syngamy (where two gametes unite, shuffle genes, and then reproduce asexually). Diverse sexual life histories can be found among the different species.
The protists play diverse ecological roles and are found in almost all aquatic environments, many species are important components of plankton (zoo- and phytoplankton)
How did these first eukaryotes evolve? Autogenous model - eukaryotes arose from prokaryotes that showed a specialization of membranes (derived from the infoldings of the plasma membrane).
Endosymbiotic model - small prokaryotes lived symbiotically within larger prokaryotes. Evidence in support of this includes: the existence of endosymbiots, similarities between bacteria and mitochondria and chloroplasts (similar cell sizes between mitochondria, chloroplasts, and average bacterium, all reproduce by binary fission, similar ribosomal components, one of the algal-like protists, Cyanophora, looks like a chloroplast, and circular DNA
The origin of eukaryotes may actually have involved a combination of the two.
Diplomonoadida and Parabasala:Giardia and other diplomonads are unique (they have modified mitochondria called mitosomes , and two haploid nuclei), Trichomonas vaginalis is the most well known parabasalid. Both of these are parasites with Giardia causing hiker’s diarrhea and Trichomonas causing a vaginitis.
Euglenozoans are divided into Eugelnids and Kinetoplastids and include autotrophic, hetertrophic (including parasites), and mixotrophic protists that utilize flagella for movement Euglenids . These protists produce polysaccharide, paramylon, for glucose storage that is characteristic of this taxon. Some of these organisms are mixotrophs (see above). Most of the euglenoids are found in freshwater.
Kinetoplastids possess one large mitochondrion that contains a mass of DNA called a kinetoplast. They are heterotrophic. Some are free-living, some symbiotic with other organisms (e.g., living in the guts of termites) and some are parasitic [e.g., Trypanosoma (African Sleeping Sickness and Chaga’s disease).
Alveolates: Unicellular protists with subsurface sacs or cavities (alveoli). The function of the alveoli is unknown but they may have a role in stabilizing the cell membrane and regulating the cell’s water and ion content. These protists are ecologically diverse. They are divided into three subgroups
The apicomplexans are about 3,900 species which are heterotrophic, all parasitic (of animals), nonmotile, and have very complex life cycles with sexual and asexual stages and multiple hosts. Examples: Plasmodium species (malaria and Toxoplasma (flu-like symptoms, birth defects)
The ciliates. These protists have cilia for movement and food acquisition. They are also heterotrophic. They have tough but flexible pellicles. They are found in freshwater environments and are unique in that they posses several nuclei (a macronucleus and several micronuclei). An example is Paramecium.
The dinoflagellates. Many are part of the phytoplankton but some are heterotrophic. Most are marine but some live in freshwater. Most are unicellular. Blooms of these organisms cause problems (e.g., ride tides) and one particularly nasty species, Pfisteria piscicida is a carnivore that causes fish kills, lesions on humans who come into contact with it and even neurological problems. Other species are responsible for toxic alga blooms called red tides.
Stramenopiles at some stage all of these protists have flagella with distinctive hair-like projections. They are important as primary producers
Diatoms. They are yellow to brown in color. They have unique glass-like walls of silica and are also components of phytoplankton. They reproduce asexually. An example of dead diatoms is diatomaceous earth. Dead diatoms also contain Carbon that is “pumped” to the benthos
Brown algae. They are the largest and most complex protists (e.g., true multicellularity). Sea weeds are members of the brown, red and green algae. Most are marine. An example is kelp
Golden algae. There are freshwater and marine species. They are also components of the plankton. Some are photosynthetic but many are mixotroph
Oomycetes. These are the water molds, white rusts, and downy mildews. They were previously considered to be fungi and some are very destructive plant pathogens.
Group Rhizaria. These are amoebas that move and feed by pseudopodia. Note that there are other amoebas in another group
Forams. 90% are fossils. They are heterotrophic, marine and have porous shells (with pseudopodia extending through pores of the shell) with CaCO3 (calcium carbonate). The shells are called tests
Radiolarians have delecate skeletons often made of silica and are components of plankton that form slender pseudopodia (called axopodia). Including heliozoans (sun animals; freshwater) and radiozoans (primarily marine species)
