Chapter 28 Protists Overview A World in a Drop of Water In the past taxonomists classified all protists in a single kingdom Protista However it is now clear that Protista is in fact paraphyletic some protists are more closely related to plants fungi or animals than they are to other protists As a result the kingdom Protista has been abandoned Various lineages are recognized as kingdoms in their own right Scientists still use the convenient term protist but only to refer to eukaryotes that are not plants animals or fungi Concept 28 1 Protists are an extremely diverse assortment of eukaryotes Protists exhibit more structural and functional diversity than any other group of organisms Most protists are unicellular although there are some colonial and multicellular species Unicellular protists are considered the simplest eukaryotes At the cellular level many protists are very complex the most elaborate of all cell This is to be expected of a single cell that must carry out the basic functions performed by all the specialized cells in a multicellular organism Protists are the most nutritionally diverse of all eukaryotes Some are photoautotrophs containing chloroplasts Some are heterotrophs absorbing organic molecules or ingesting food particles Some are mixotrophs combining photosynthesis and heterotrophic nutrition These have all arise independently in many protest lineages Protists can be divided into three groups based on their roles in biological communities photosynthetic protists algae ingestive protists protozoans and absorptive protists Protist habitats are also very diverse These groups are not monophyletic Reproduction and life cycles of protists vary greatly Some are exclusively asexual while other reproduce sexually or at least employ the processes of meiosis and syngamy Endosymbiosis in Eukaryotic Evolution Much of protist diversity is the result of endosymbiosis a process in which unicellular organisms engulfed other cells which became endsymbionts that and ultimately organelles in the host cell The earliest eukaryotes acquired mitochondria by engulfing alpha proteobacteria The early origin of mitochondria is supported by the fact that all eukaryotes studied so far either have mitochondria or had them in the past Later in eukaryotic history one lineage of heterotrophic eukaryotes acquired an additional endosymbiont a photosynthetic cyanobacterium that evolved into plastids This lineage gave rise to red and green algae This hypothesis is supported by the observation that the DNA of plastids in red and green algae closely resembles the DNA of cyanobacteria Plastids in red and green algae are surrounded by two membranes which correspond to the inner and outer of the gram negative cyanobacterial endosymbionts On several occasions during eukaryotic evolution red and green algae underwent secondary endosymbiosis They were ingested in the food vacuole of a heterotrophic eukaryote and became endosymbionts themselves For example algae known as chlorarachniophytes evolved when a heterotrophic eukaryote engulfed a green alga This process likely occurred comparatively early in evolutionary time because the engulfed alga still carries out photosynthesis with its plastids and contains a tiny vestigial nucleus called a nucleomorph Concept 28 2 Diplomads and parabasalids have modified mitochondria Most diplomonads and parabasalids are found in anaerobic environments These protists lack plastids and their mitochondria lack DNA an electron transport chain and the enzymes needed for the citric acid cycle In some species the mitochondria are very small and produce cofactors for enzymes involved in ATP production in the cytosol Diplomonads have two equal sized nuclei and multiple flagella Giardia intestinalis is an infamous diplomonad parasite that lives in the intestines of mammals The most common method of acquiring Giardia is by drinking water contaminated with feces containing the parasite in a dormant cyst stage Diplomonads Parabasalids Parabasalids include the protists called trichomonads The best known species Trichomonas vaginalis inhabits the vagina of human females If the normal acidity of the vagina is disturbed T vaginalis can outcompete beneficial bacteria and infect the vaginal lining The male urethra may also be infected but without symptoms The infection is sexually transmitted Genetic studies of T vaginalis suggest that the species became pathogenic after some individuals acquired a particular gene through horizontal gene transfer from other vaginal bacteria The gene allows T vaginalis to feed on epithelial cells Concept 28 3 Euglenozoans have flagella with a unique internal structure Euglenozoa the euglenozoans is a diverse clade that includes predatory heterotrophs photosynthetic autotrophs and pathogenic parasites The main feature that distinguishes protists in this clade is the presence of a spiral or crystalline rod of unknown function inside their flagella Most euglenozoans have disc shaped mitochondrial cristae The best studied groups of euglenozoans are the kinetoplastids and euglenids Kinetoplastids Kinetoplastids have a single large mitochondrion that contains an organized mass of DNA called a kinetoplast These protists include free living consumers of prokaryotes in freshwater marine and moist terrestrial ecosysyems as well as species that parasitize animals plants and other protists For example Trypanosoma causes African sleeping sickness a disease spread by the African tsetse fly and Chagas disease which is transmitted by bloodsucking bugs Trypanosomes evade immune detection by switching surface proteins from generation to generation preventing the host from developing immunity One third of Trypanosoma s genome codes for these surface proteins Euglenids Euglenids hace a pocket at one end of the cell from which one or two flagella emerge They also have a unique glucose polymer paramylon which functions as a storage molecule Many species of the euglenid Euglena are autotrophic but can become heterotrophic in the dark Other euglenids can engulf pre by phagocytosis Members of the clade Alveolata are characterized by membrane bound sacs alveoli just under the Concept 28 4 Alveolates have sacs beneath the plasma membrane The function of the alveoli is unknowm but they may help stabilize the cell surface or regulate water and ion content Alveolata include three groups flagellates dinoflagellates parasites apicomplexans and ciliates plasma membrane move by means of