Chapter 26

parasitism

where one partner (the parasite) lives on or in another (the host) and metabolically depends on it Some para- sitic protists are important pathogens (disease-causing agents) of plants or animals.

terrestrial protists

Even parasitic protists are aquatic because they live in the watery environments of other organisms' body fluids. Terrestrial protists are restricted to damp places such as soil, cracks in bark, and leaf litter.

coenocytes

some are coenocytes, consisting of a multinucleate mass of cytoplasm;

colonies

Some protists form colonies, loosely connected groups of cells;

mutualism

These intimate associations range from mutualism, a more or less equal part- nership where both partners benefit

Chromalveolates

Diverse protists that may have originated as a result of secondary endosymbiosis in which an ancestral cell engulfed a red alga; bikonts Alveolates: Dinoflagellates, ciliates, apicomplexans Alveoli (flattened vesicles) just inside the plasma membrane Stramenopiles: Water molds, diatoms, brown algae, golden algae Most have two flagella, one with hairs; no flagella in some

excavates info

Excavates are a diverse group of unicellular protists with fla- gella. These protists are so named because many have a deep, or excavated, oral groove. Unlike other protists, excavates have atypical, greatly modified mitochondria. Many excavates are endosymbionts and live in anoxic (without oxygen) environ- ments. These excavates do not carry out aerobic respiration; they obtain energy by the anaerobic pathway of glycolysis (presumably by fermentation). Include: diplomonads, parabasalids, euglenoids, trypanosomes. Additional studies are needed to determine if the excavates as currently presented are a monophyletic group.

paraphyletic group

Given the diversity in protist ultrastructure and molecular data, biologists regard the protists as a paraphyletic group; that is, protists contain some, but not all, of the descendants of a common eukaryote ancestor. Molecular and ultrastructural analyses con- tinue to help biologists clarify relationships among the various pro- tist phyla and among protists and the other eukaryotic kingdoms.

supergroups

Many biologists currently classify eukaryotes into five major "supergroups." 1. Excavates 2. Chromalveolates 3. Rhizarians 4. Archaeplastids 5. Unikonts At node A (root of this supergroup), all five supergroups branch from the same point, indicat- ing that the branching order is unresolved at this time. The same is true at node B for the diatoms, brown algae, and golden algae.

plankton

Most protists are aquatic and live in the ocean or in freshwater streams, lakes, and ponds. They make up most of the plankton, the floating, often microscopic organisms that inhabit surface waters and are the base of the food web in aquatic ecosystems.

cilia

Other protists move by flexing individual cells; by gliding over surfaces; by waving cilia, short, hairlike organelles;

pseudopodia

Protists, most of which are motile at some point in their life cycle, have various means of locomotion. Some move by pushing out cytoplasmic extensions called pseudopodia along the leading edge and retracting the cytoplasm that trails behind, as an amoeba does.

Rhizarians more specific info

Rhizarians are a diverse supergroup of amoeboid cells that often have hard outer shells, called TESTS, through which cyto- plasmic projections extend. The threadlike cytoplasmic pro- jections suggest the name rhizarian, from the Greek rhiza, meaning "root." Forams and actinopods are rhizarians, as are certain shell-less amoebas. Not all amoebas are rhizarians, however, and many amoeba species are more closely related to other eukaryotic clades. Current molecular evidence indi- cates that the rhizarian supergroup is monophyletic.

multicellular

and some are multicellular, composed of many cells. Unlike animals, plants, and many fungi, most multicellular protists have rela- tively simple body forms without specialized tissues.

flagella

or by lash- ing flagella, long, whiplike organelles. Some protists have two or more means of locomotion, such as both flagella and pseudopodia.

commensalism

to commensalism, where one partner benefits and the other is unaffected;

diatoms

Diatoms are stramenopiles with shells composed of two parts Most diatoms are unicellular, although a few exist as colonies. The cell wall of each diatom consists of two shells that overlap where they fit together, much like a petri dish. There are two basic groups of diatoms: those with radial symmetry (wheel shaped) and those with bilateral symmetry (boat shaped or needle shaped). Although some diatoms are part of the floating plankton, others live on rocks and sediments, where they move by gliding. This gliding movement is facilitated by the secretion of a slimy material from a small groove along the shell. Diatoms most often reproduce asexually by mitosis. When a diatom divides, the two halves of its shell separate, and each becomes the larger half of a new diatom shell. some get progressively smaller every time this happens, so ever succeeding generation. When a diatom reaches a fraction of its original size, sexual reproduction occurs, with the production of shell- less gametes. Sexual reproduction restores the dia- tom to its original size because the resulting zygote, a 2n cell that results from the fusion of n gametes, grows substantially before producing a new shell. Diatoms are common in fresh water but are especially abundant in relatively cool ocean water. -major producers in aquatic ecosystems. At least one species is toxic and linked to shellfish poisonings, marine mammal strandings, and the deaths of sea lions When diatoms die, their shells trickle to the ocean floor and accumulate in layers that eventually become sedimentary rock. Called diatomaceous earth, these deposits are mined and used as filtering, insulating, and soundproofing materials. As a filtering agent, diatomaceous earth is used to refine raw sugar and process vegetable oils. Because of its abrasive properties, diatomaceous earth is a common ingredient in scouring powders and metal polishes. The intricately detailed dia- tom shells are often used to test microscope resolution down to 1 μm. After cell division, each new cell retains half of the original shell. The newly synthesized half of the shell always fits inside the original half. As a result, one of the new cells is slightly smaller than the other.

primary endosymbiosis

Molecular evi- dence supports the view that incorporation of an ancient cyano- bacterium within a host cell, known as primary endosymbiosis, resulted in the chloroplasts in today's red algae, green algae, and land plants.(The host cell was eukaryotic because mitochon- dria almost certainly evolved before chloroplasts.)theee chloroplasts that are encased by 2 membranes later provided other eukaryotes with their chloroplasts during secondary endosymbiosis In primary endosymbiosis, an ancient eukaryotic cell engulfed a cyanobacterium, which survived and evolved into a chloroplast. The ancient cell is depicted as a eukaryotic cell because mitochondria almost certainly evolved before chloroplasts.

Archaeplastids

Plastids bounded by outer and inner mem- branes; include land plants; bikonts Red algae: Chloroplast pigments include phycoerythrin (red pigment) and phycocyanin (blue pigment) Green algae: Chloroplast pigments identical to those in land plants

ultrastructure

Ultrastructure is the fine details of cell structure revealed by electron microscopy. In many cases, ultrastructure data complement molecular data. Electron microscopy reveals simi- lar structural patterns among those protist taxa that compara- tive molecular evidence suggests are MONOPHYLETIC; that is, they evolved from a common ancestor example, molecular and ultrastructure data suggest that water molds, dia- toms, golden algae, and brown algae—protist taxa that at first glance seem to share few characteristics—are a monophyletic group.

serial endosymbiosis

According to the hypoth- esis of serial endosymbiosis, certain eukaryotic organelles, par- ticularly mitochondria and chloroplasts, arose from symbiotic relationships between larger cells and smaller bacteria that were incorporated and lived within them. -Cell biologists hypoth- esize that mitochondria originated from aerobic bacteria. DNA suggest that it is a remnant from the mitochondrion's past, when it was an independent organism. Ribosomal RNA (rRNA) sequences from mitochondria closely match rRNAs found in purple bacteria, suggesting that ancient purple bacteria were the ancestors of mitochondria. -Chloroplast evolution is more complex given that there were probably several endosymbiotic events

amoebas

Amoebas move by forming pseudopodia Amoebas are unicellular amoebozoa found in soil, fresh water, the ocean, and other organisms (as parasites). Because of the extreme flexibility of their outer plasma membrane, many members of this group have an asymmetrical body form and continually change shape as they move. (The word amoeba derives from a Greek word meaning "change.") An amoeba moves by pushing out lobose pseudopodia from the surface of the cell. More cytoplasm flows into the pseudopodia, enlarging them until all the cytoplasm has entered and the organism as a whole has moved. Pseudo- podia also capture and engulf food by surrounding and form- ing a vacuole around it -digestion occurs when the food vacuole fuses with a lysosome containing digestive enzymes. -Amoebas repro- duce asexually, splitting into two equal parts after mitotic divi- sion of the nucleus; sexual reproduction has not been observed. Parasitic amoebas include Entamoeba histolytica, which causes amoebic dysentery, a serious human intestinal disease characterized by severe diarrhea, bloody stools, and ulcers in the intestinal wall. Entamoeba histolytica is transmitted as cysts in contam- inated drinking water. Acanthamoeba, are usually free-living but produce opportunis- tic infections such as eye infections in wearers of contact lenses. Chaos amoebas are generally scavengers that feed on debris in freshwater habitats, but they ingest living organisms when the opportunity arises.

Rhizarians

Amoeboid cells that often have tests (shells); bikonts Forams: Porous tests (hard shells) through which cytoplasmic projections (pseudopods) extend Actinopods: Endoskeletons (internal shells) through which axo- pods (filamentous pseudopods) extend

Amoebozoa

Amoebozoa are unikonts with lobose pseudopodia Most amoebozoa produce temporary cytoplasmic projec- tions called pseudopodia (meaning "false foot") at some point in their life cycle. The pseudopo- dia of amoebozoa are lobose—that is, rounded and wide—as opposed to the slender cytoplasmic projections characteristic of rhizarians. Many biologists currently classify amoebas, plas- modial slime molds, and cellular slime molds as amoebozoa.

Apicomplexans

Apicomplexans are spore-forming parasites of animals Malaria is caused by an apicomplexan mode of transmission by means of mosquitoes. Plasmodium, the apicomplexan that causes malaria. chemotherapies that include the drug artemisinin in combination with other drugs have emerged as the standard of care, but there are signs that resistance of the parasite to artemisinin is beginning to evolve in some countries. Advanced sequencing of the genomes of P. falciparum and the Anopheles mosquito may lead to new diagnostics, drugs, and vaccines. Apicomplexans are a large group of parasitic, spore- forming alveolates, some of which cause serious diseases in humans. they con- tain the unpigmented remnant of a chloroplast derived from a red alga. Apicomplexans lack specific structures for locomotion (cilia, flagella, or pseudopodia) and move by flexing. have an apical complex of microtubules that attaches the parasite to its host cell; the apical complex (can be seen through electron microscopy) They also have the ability to form a structure known as a MOVING JUNCTION, which enables them to form a vacuole that encloses and pro- tects them as they invade a host cell. At some stage in their life cycle, apicomplexans produce sporozoites, small infec- tive agents transmitted to the next host. Many apicomplexans spend part of their complex life cycle in one host species and part in a different host species.

Brown algae

Brown algae are multicellular stramenopiles Brown algae are the largest and most complex of all algae com- monly called seaweeds. All brown algae are multicellular and range in size from a few centimeters (about an inch) to 75 m (about 260 ft). Their body forms are branched filaments; tufts; fleshy "ropes"; or thick, flattened branches. The largest brown algae, called kelps, are tough and leathery in appearance. Many kelps have leaflike BLADES in which most photo- synthesis occurs, stemlike STIPES, and rootlike anchoring HOLDFASTS. They often have gas-filled bladders that provide buoyancy. (The blades, stipes, and holdfasts of brown algae are not homologous to the leaves, stems, and roots of plants. Brown algae and plants arose from different unicellular ancestors, complicated form of reproduction Their reproductive cells, both asexual zoospores and sexual gametes, are usually biflagellate. Most have a life cycle that exhibits ALTERNATION OF GENERATIONS, in which they spend part of their life as multicellular haploid organisms and part as multi- cellular diploid organisms a polysaccharide called algin that is harvested from kelps such as Macrocystis and used as a thickening and stabilizing agent in ice cream, toothpaste, shaving cream, hair spray, and hand lotion. important human food rich sources of certain vitamins and minerals such as iodine. Brown algae are common in cooler marine waters, espe- cially along rocky coastlines, where they live mainly in the intertidal zone or relatively shallow offshore waters. Kelps form extensive underwater "forests," or kelp beds. they are important food producers, provide habitats for some marine life.

unikonts

Cells that have a single flagellum or are amoebas with no flagella; have a triple-gene fusion that is lacking in other eukaryotes; include animals and fungi Amoebozoa: Amoebas, plasmodial slime molds, cellular slime molds Naked amoebas (no tests) with lobelike pseudopods Opisthokonts: Choanoflagellates No flagella or single posterior flagellum on motile cells

Cellular slime molds

Cellular slime molds feed as individual amoeboid cells The cellular slime molds are amoebozoa with close affinities to amoebas and plasmodial slime molds. During its feeding stage, each cellular slime mold is an individual amoeboid cell that behaves as a separate, solitary organism Each cell creeps over rotting logs and soil or swims in fresh water, ingesting bacteria and other particles of food as it goes. Each amoeboid cell has a haploid nucleus and reproduces by mitosis, as a true amoeba does. -When moisture or food becomes inadequate, certain cells send out a chemical signal, cyclic adenosine monophosphate that causes them to aggregate by the hundreds or thousands. during this the cells creep about for short distances as a single multicellular aggregate or slug. -Each cell of the slug retains its plasma membrane and indi- vidual identity. Eventually, the slug settles and reorganizes, forming a stalked fruiting body containing spores. After being released each spore opens, and a single haploid amoe- boid cell—the feeding stage—emerges. The spore-forming reproductive cycle is asexual, although sexual reproduction is observed occasionally. The life cycles of most cellular slime molds lack a flagellate stage. -The cellular slime mold Dictyostelium discoideum is a model organism for the study of cell differentiation, cell com- munication, and cell motility and adhesion. Relates to CELL SIGNALING molecules, such as cAMP 1. slime mold amoebas 2. aggregation 3. multicellular slug forms 4. slug differentiates 5. fruiting body 6. spores Slime mold amoebas ingest food, grow, and reproduce by cell division.The fruiting body releases spores, each of which opens in a favorable environment to liberate an amoeboid cell.

Choanoflagellates

Choanoflagellates are opisthokonts closely related to animals Choanoflagellates are collared flagellates in the opisthokont clade, which also includes fungi and animals. These small, inconspicuous unikonts are found globally in both fresh- water and marine environments. Choanoflagellates include both free-swimming and SESSILE species that are permanently attached by a thin stalk to bacteria-rich debris. Their single flagellum is surrounded at its base by a delicate collar of microvilli that trap food. -Choanoflagellates are of special interest because of their striking resemblance to collar cells in sponges Other animal phyla, such as cnidarians, flat- worms, and echinoderms, also contain choanoflagellate-like cells, but no other group of protists has been observed to pos- sess these cells. Given the similarities in structure and molecular genom- ics that has accumulated in recent years, many biologists hypothesize that choanoflagellates are the closest living non- animal relative of animals. Thus, living choanoflagellates and animals probably share a common choanoflagellate-like ancestor. -obtain food by wav- ing their flagella, causing water currents to carry bacteria and other small particles of food into the collar of microvilli. A colonial form is shown. Each cell is 5 to 10 μm long, not including the flagellum.

Ciliates

Ciliates use cilia for locomotion Ciliates are among the most complex of eukaryotic cells. These unicellular alveolates have a pellicle that gives them a definite but changeable shape. In Paramecium the surface of the cell is covered with several thousand fine, short, hair- like cilia that extend through pores in the pellicle to facilitate movement. The cilia beat with such precise coordination Not all ciliates are motile. Some sessile forms have stalks, and others, although capable of some swimming, are more likely to remain attached to a rock or other surface at one spot. Ciliates differ from other protists in having two kinds of nuclei: one or more small, diploid MICRONUCLEI that function in reproduction; and a larger, polyploid MACRONUCLEUS that controls cell metabolism and growth. Most ciliates are capable of a sexual process called CONJUGATION, in which two individuals come together and exchange genetic material. two identical new cells but different from what they were before conjugation. Ciliates usually divide perpen- dicularly to their longitudinal axis. so we don't need mitosis and cell division.

Diplomonads

Diplomonads are small, mostly parasitic flagellates Diplomonads are excavates that have one or two nuclei, no functional mitochondria, no Golgi complex, and up to eight flagella. -Interestingly, Giardia has two haploid nuclei, each of which contains a complete copy of Giardia's genome. Giardia lacks a functional mitochondria, although it contains certain genes that code for proteins associated with mitochondria in other organisms. has reduced structures that somewhat resemble mitochondria. an early eukaryotic ancestor of Giardia may have possessed mitochondria, which were somehow lost or reduced at a later time during its evolutionary history. -Giardia intestinalis is a major cause of water-borne diar- rhea throughout the world. Giardia is eliminated as a resistant cyst in the feces of many vertebrate animals.In a heavy infection, much of the wall of the small intestine is coated with these flagellates, which interfere with the absorption of digested nutrients and cause weight loss, abdominal cramps, and diarrhea. -a parasitic diplomonad, reveals two nuclei.

euglenoids

Euglenoids and trypanosomes include both free-living species and parasites Euglenoids and trypanosomes are characterized by an unusual flagellum: in addition to the 9 + 2 arrangement of micro- tubules characteristic of all eukaryotic flagella, these excavates have a crystalline rod in their flagella; the function of this rod is unknown. euglenoids and trypano- somes also have atypical mitochondria. Most euglenoids are unicellular flagellates, and about one-third of them are photosynthetic They generally have two flagella: one long and whiplike and one that is often so short that it does not extend outside the cell. Some euglenoids, such as Euglena, change shape continually as they move through the water because their PELLICLE, or outer covering, is flexible. Euglena's pellicle is flexible and changes shape easily. The eyespot may shield a light detector at the base of the long flagellum, thereby helping Euglena move to light of an appropriate intensity. -Autotrophic euglenoids have chloroplasts with the same photosynthetic pigments that green algae and plants have. chloroplasts were acquired by secondary endosymbiosis; euglenoids are not closely related to either group -Some photosynthetic eugle- noids lose their chlorophyll when grown in the dark, and they obtain their nutrients heterotrophically, by ingesting organic matter. Other euglenoids are always colorless and heterotro- phic. Some heterotrophic species absorb organic compounds from the surrounding water, whereas others engulf bacteria and protists by phagocytosis; they digest the prey within food vacuoles.

forams

Forams extend cytoplasmic projections that form a threadlike, interconnected net Almost all foraminiferans (forams) are marine rhizarians that produce elaborate tests The ocean contains enormous numbers of forams, which secrete chalky, many- chambered tests with pores through which cytoplasmic projec- tions are extended. The cytoplasmic projections form a sticky, interconnected net that entangles prey contain unicellular algal endosymbionts (green algae, red algae, or dia- toms) that provide food by photosynthesis. many live on the ocean floor but other are part of the plankton. Dead forams settle on the bottom of the ocean, where their tests form a gray mud that is gradually transformed into chalk. With geologic uplifting, these chalk formations become part of the land, as in the White Cliffs of Dover in England. Forams are well preserved in the fossil record, and biologists use some as INDEX FOSSILS, markers to help identify ancient sedimentary rock layers

Green algae

Green algae share many similarities with land plants Green algae have pigments, energy reserve products, and cell walls that are chemically identical to those of land plants. Green algae are photosynthetic, with chloroplasts of a wide variety of shapes. Most green algae have cell walls with cellulose, although some lack walls. Because of these and other similarities, biologists generally accept that land plants arose from ancestral green algae. some biologists classify this diverse group in the plant kingdom. Green algae exhibit a variety of body types, from single cells to colonial forms, to coenocytic algae (multinucleate), to multicellular filaments and sheets The multi- cellular forms do not have cells differentiated into tissues, a characteristic that separates them from land plants. Most green algae have, or produce, flagellate cells during their life cycle, although a few are totally nonmotile. Reproduction in the green algae is as varied as their body forms, with both sexual and asexual reproduction. Many green algae have life cycles with an alternation of multicellular haploid and multicellular diploid generations. Asexual repro- duction is by mitosis and cell division in single cells or by fragmentation in multicellular forms. Many green algae pro- duce spores asexually by mitosis; if these spores have flagella and are motile, they are called ZOOSPORES sexual reproduction in the green algae involves gamete formation in unicellular GAMETANGIA (sing., gametangium), reproductive structures in which gametes are produced. Green algae are found in both aquatic and terrestrial environments. Aquatic green algae primarily inhabit fresh water, Terrestrial green algae are restricted to damp soil, cracks in tree bark, and other moist places. some live as endosymbionts in body cells of invertebrates, and a few grow together with fungi as "compound organisms" called lichens important producers Chlamydomonas is a unicellular, haploid green alga with two mating types, (+) and (−). The only diploid cell in the life cycle is the zygote. Chara, a green alga commonly called a stonewort, is closely related to land plants. Chara is widely distributed in fresh water.

archaeplastids

In the classification scheme adopted in this text, the mono- phyletic group of archaeplastids includes red algae and green algae, which are discussed here, and land plants, which are in a separate kingdom these are classified together based on molecular data and that presence of chloroplasts bounded by outer and inner membranes suggesting that they developed directly from a Cyanobacteria endosymbiont. All photosynthetic protists other than archaeaplastids have plastids surrounded by 3 or 4 membranes.

dinoflagellates

Most dinoflagellates are a part of marine plankton Dinoflagellates are generally unicellular, although a few are colonial. Their alveoli contain interlocking cellulose plates impregnated with silicates. The typical dinoflagellate has two flagella. One flagellum wraps around a transverse groove in the center of the cell like a belt, and the other lies in a longi- tudinal groove (perpendicular to the transverse groove), pro- jecting behind the cell this propeller the dinoflagellate through the water like a spinning top. Many marine dinoflagellates are bioluminescent. Many dinoflagellates are photosynthetic, but others are heterotrophic and ingest other microorganisms for food. Some dinoflagellates are endosymbionts that live in the bodies of marine invertebrates such as mollusks, jellyfish, and corals (see Fig. 54-12). These symbiotic dinoflagellates, called ZOOXANTHELLAE, photosynthesize and provide carbohydrates for their invertebrate partners. Zooxanthellae contribute sub- stantially to the productivity of coral reefs. Other dinoflagel- lates that are endosymbionts lack pigments and are parasites that live off their hosts. A few dinoflagellates are known to have occasional population explosions, or blooms. These blooms, known as RED TIDES, frequently color coastal waters orange, red, or brown. Dinoflagellate blooms are par- ticularly common in warm, nutrient-enriched water. Some dinoflagellate species that form red tides produce a toxin that attacks the nervous systems of fishes, leading to fish kills birds, and dolphin deaths. dinoflagellates produce the orange cloudiness

diatoms and forams

Other than a few protists with hard shells, such as diatoms and forams, most ancient protists did not leave many fossils because their bodies were too soft to leave perma- nent traces. Evolutionary studies of protists focus primarily on molecular and structural comparisons of present-day organ- isms, which contain many clues about their evolutionary history.

Plasmodial slime molds

Plasmodial slime molds feed as multinucleate plasmodia The feeding stage of a plasmodial slime mold is a plasmodium, a multinucleate mass of cytoplasm that can grow up to 30 cm (1 ft) in diameter The slimy plasmodium streams over damp, decaying logs and leaf litter, often forming a network of channels that covers a large sur- face area. As it creeps along, it ingests bacteria, yeasts, spores, and decaying organic matter. -Within these structures, called SPORANGIA, meiosis produces haploid spores that are extremely resistant to adverse environmental conditions. -When conditions become favorable, the spores germinate, and a haploid reproductive cell emerges from each. This haploid cell is either a biflagellate swarm cell or an amoeboid myxamoeba, depending on available moisture; flagellate cells form in wet con- ditions. Swarm cells and myxamoebas act as gametes, which fuse to form a zygote with a diploid nucleus. The resultant diploid nucleus divides many times by mitosis, but the cytoplasm does not divide, so the result is a multinucleate plasmodium. -The plasmodial slime mold Physarum polycephalum is a model organism used as a mode for such as growth, cytoplasmic streaming, and the function of the cytoskeleton.

protists

Protists are an informal group of primarily aquatic eukaryotic organisms with diverse body forms, types of reproduction, modes of nutrition, and lifestyles. Protists, which include algae, water molds, slime molds, and protozoa, are unicellular, colonial, or simple multicellular organisms that have a eukaryotic cell organization protists were the first eukaryotes to evolve. -most protests are unicellular, with each cell forming a complete organisms capable of performing all the functions characteristic of life. -most algae are autotrophic and photosynthesize as plants do. -some heterotrophic protist obrtain their nutrients by absorption, as fungi do whereas others resemble animals in that they ingest food. Some protists switch their modes of nutrition and are autotrophic at certain times and heterotrophic at others. -Reproduction is varied among protists. Almost all protists reproduce asexually, and many also reproduce sexually. However, most protists do not develop multicellular reproductive organs, nor do they form embryos the way more complex organisms do.

red algae

Red algae do not produce motile cells The vast majority of red algae are multicellular organisms, although there are a few unicellular species. The multicellular body form of red algae commonly consists of complex, inter- woven filaments that are delicate and feathery a few red algae are flattened sheets of cells. -Most multicellular red algae attach to rocks or other substrates by a basal hold- fast. -Reproduction in the red algae is remarkably complex, with an alternation of sexual and asexual stages. No flagellate cells develop during the life cycle. -Some unicellular red algae on the genome of a unicellular extremophilic red alga adapted to toxic hot sulfur springs. 75 genes acquired from archaea and bacteria needed for this extreme lifestyle, providing a striking example of horizon- tal gene transfer involving a eukaryote. -primarily live in warm tropical ocean waters, although a few species occur in fresh water and in soil. Some red algae, known as coralline algae, incorporate calcium carbonate in their cell walls from the ocean water The hard calcium carbonate may protect coralline algae from the rigors of wave action. These coralline red algae build "coral" reefs and are perhaps as crucial as coral animals in this process. The cell walls of red algae often contain thick, sticky poly- saccharides that have commercial value. For example, agar is a polysaccharide extracted from certain red algae used as a food thickener and culture medium, a substrate on which to grow microorganisms and propagate some plants, such as orchids. Another polysaccharide extracted from red algae, carrageenan, is a food additive used to stabilize chocolate milk; it also provides a thick, creamy texture to ice cream and other soft processed foods. stabilized paints and cosmetics Red algae are a source of vitamins (par- ticularly A and C) and minerals, Bossiella is a coralline red alga encrusted with calcium carbonate. It lives in the Pacific Ocean.

secondary endosymbiosis

Secondary endosymbiosis occurred frequently in eukaryote evolution, as evidenced by the presence of additional chloroplast membranes. For example, three membranes envelop the chloro- plasts of euglenoids and dinoflagellates, and four membranes surround the chloroplasts of diatoms, golden algae, and brown algae. Even non-photosynthetic protists may contain chloroplast relics from secondary endosymbiotic events. Apicomplexans— protists such as Plasmodium, which causes malaria—have a non-photosynthetic chloroplast derived from a red alga, sur- rounded by four external membranes. Because this plastid carries out certain functions essential to the survival of the parasite, it has become a target of ongoing research focused on the development of antimalarial drugs. In a secondary endosymbiotic event, a heterotrophic eukaryotic cell (with mitochondria) engulfed a eukaryotic cell with chloroplasts (a red alga is depicted). The red alga survived and evolved into a chloroplast surrounded by three membranes (a dinoflagellate chloroplast is depicted). Other secondary endosymbiotic events resulted in more complex chloroplast membrane structures.

chromalveolates

The chromalveolates are a super- group composed of extremely diverse protists with few shared characters. Chromalveolates prob- ably originated as a result of sec- ondary endosymbiosis in which an ancestral cell engulfed a red alga (which itself was the result of pri- mary endosymbiosis). Most chrom- alveolates are photosynthetic, and evidence suggests that heterotro- phic chromalveolates, such as the water molds and ciliates, descended from autotrophic ancestors. Clas- sification of the chromalveolates as a monophyletic supergroup is controversial. The chromalveolates are divided into two main groups: alveolates and stramenopiles.

stremenopiles

The stramenopiles include water molds, diatoms, golden algae, and brown algae. At first glance strameno- piles appear too diverse to classify together. However, most stramenopiles have motile cells with two flagella, one of which has tiny hairlike projections extending from the shaft. The word stramenopile comes from Latin words referring to "straw" (i.e., the shaft of the flagellum) and "hairs."

alveolates

The unifying features of pro- tists classified as alveolates include similar ribosomal DNA sequences and alveoli (sing., alveolus), flat- tened vesicles located just inside the plasma membrane. In some alveolates the vesicles contain plates of cellulose. Alveolates include the dinoflagellates, api- complexans, and ciliates.

kinetoplastid

Trypanosomes are excavates with a single mitochondrion that has an organized deposit of DNA called a kinetoplastid. Trypanosomes are colorless, and many are parasitic and cause disease. In vertebrates, including humans, trypanosomes live in the blood. For example, Trypanosoma brucei is a human parasite that causes African sleeping sickness. It is transmitted by the bite of infected tsetse flies. trypanosoma brucei causes African sleeping sickness in humans.

excavates

Unicellular protists with atypical, greatly modi- fied mitochondria; bikonts Diplomonads and parabasalids: Two or more flagella; ventral oral (feeding) groove Euglenoids and trypanosomes: Some with plastids; crystalline rod in flagella

unikonts

Unikonts are a supergroup composed of certain amoebas, plasmodial slime molds, cellular slime molds, choanoflagel- lates, fungi, and animals Unikonts share a single posterior flagellum in flagellate cells such as sperm and motile spores, although some extant organisms in this supergroup have lost the flagellum. Many unikonts also have a single centriole. unikonts are divided into two clades. One clade is the OPISTHOKONTS, which consist of fungi, choanoflagellates, and animals. Unikonts also have a TRIPLE-GENE FUSION THAT has major evolutionary significance. In a triple-gene fusion, three separate genes fused into a single unit early in the course of eukaryote evolution; the fused gene codes for a multi- enzyme protein.triple-gene fusion provides evidence of a bifurcation, or branch of the eukaryotes into two main clades, close to the root of the com- mon ancestor of all eukaryotes The two branches consist of 1. unikonts, which had a common ancestor with a single posterior flagellum and 2. all other eukaryotes, collectively called BIKONTS, which had a common ancestor with two flagella. The double-gene fusion is present in all eukaryotes examined except for unikonts; bacteria and archaea also lack the double-gene fusion. The unikonts have the triple-gene fusion that is lacking in all other eukaryotes, bacteria, and archaea. These gene fusions suggest a fundamental division of eukaryotes into two main clades, the unikonts and all other eukaryotes, which are collectively called bikonts.