Biology 202- protists

Alternation of Generations

A variety of life cycles have evolved among the multicellular algae. The most complex life cycles include an alternation of generations, the alternation of multicellular haploid and diploid forms. Although haploid and diploid conditions alternate in all sexual life cycles—human gametes, for exam- ple, are haploid—the term alternation of generations applies only to life cycles in which both haploid and diploid stages are multicellular. As you will read in Chapter 29, alternation of generations also evolved in plants. Some species have heteromorphic, structurally different, generations; others have isomorphic, structurally similar, generations

classification of protists -what kind of group are protist? -which classification is it not?

Advances in eukaryotic systematics have caused the classification of protists to change significantly Protists constitute a polyphyletic group, and Protista is no longer a kingdom

amoebozoans

Amoebozoans are amoebas that have lobe- or tube-shaped, rather than threadlike, pseudopodia ● They include slime molds, tubulinids, and entamoebas

Brown algae

Brown algae are the largest and most complex algae All are multicellular, and most are marine● Brown algae include many species commonly called "seaweeds" Brown algal seaweeds have plantlike structures: the rootlike holdfast, which anchors the alga, and a stemlike stipe, which supports the leaflike blades Some have gas-filled, bubble-shaped floats to keep their photosynthetic structures near the water surface ● However, unlike plants, brown algae lack true tissues and organs Brown algae include species that have the most complex multicellular anatomy of all algae; some even have specialized tissues and organs that resemble those in plants. But morphological and DNA evidence indi- cates that the similarities evolved independently in the algal and plant lineages and are thus analogous, not homologous

Ciliates

Ciliates are a large, varied group of protists named for their use of cilia to move and feed the cilia may completely cover the cell surface or may be clustered in a few rows or tufts. In certain species, rows of tightly packed cilia function collectively in locomotion. Other ciliates scurry about on leg-like structures constructed from many cilia bonded together. A distinctive feature of ciliates is the presence of two types of nuclei: tiny micronuclei and large macronuclei. A cell has one or more nuclei of each type.

Diatoms

Diatoms are unicellular algae with a unique two- part, glass-like wall of silicon dioxide Diatoms are a major component of phytoplankton and are highly diverse After a diatom bloom, many dead individuals fall to the ocean floor, where decomposition is slow The carbon they took up from the atmosphere and incorporated into their biomass is sequestered on the ocean floor for decades to centuries Some scientists advocate fertilizing the ocean with iron to promote diatom blooms and facilitate movement of CO2 to the bottom of the ocean The abundance of diatoms in the past is also evident in the fossil record, where massive accumulations of fossilized diatom walls are major constituents of sediments known as diatomaceous earth. These sediments are mined for their qual- ity as a filtering medium and for many other uses. extreme strength

diploid, haploid

Diploid is a cell or organism that has paired chromosomes, one from each parent Haploid is the quality of a cell or organism having a single set of chromosomes.

Euglenids (euglozoans)

Euglenids have one or two flagella that emerge from a pocket at one end of the cell ● Some species are mixotrophs; they can be autotrophic or heterotrophic depending on the environmental conditions

Euglenozoans (excavata) -main characteristic

Euglenozoa is a diverse clade that includes predatory heterotrophs, photosynthetic autotrophs, mixotrophs, and parasites ● The main feature distinguishing them as a clade is a spiral or crystalline rod inside their flagella ● This clade includes the kinetoplastids and euglenids

secondary endosymbiosis examples

For example, protists known as chlorarachniophytes likely evolved when a heterotrophic eukaryote engulfed a green alga The engulfed cell contains a vestigial nucleus called a nucleomorph

conjugation of cilia

Genetic variation results from conjugation, a sexual process in which two individu- als exchange haploid micronuclei but do not reproduce (Figure 28.11b). Ciliates generally reproduce asexually by bi- nary fission, during which the existing macronucleus disinte- grates and a new one is formed from the cell's micronuclei. Each macronucleus typically contains multiple copies of the ciliate's genome. Genes in the macronucleus control the everyday functions of the cell, such as feeding, waste re- moval, and maintaining water balance

heteromorphic, isomorphic,

In Laminaria, the two generations are heteromorphic, meaning that the sporophytes and gametophytes are struc- turally different. Other algal life cycles have an alternation of isomorphic generations, in which the sporophytes and ga- metophytes look similar to each other, although they differ in chromosome number.

red algae

Many of the 6,000 known species of red algae (rhodophytes, from the Greek rhodos, red) are reddish, owing to a photosyn- thetic accessory pigment called phycoerythrin, which masks the green of chlorophyll (Figure 28.20). However, species adapted to more shallow water have less phycoerythrin. As a result, red algal species may be greenish red in very shallow species of red alga has been discovered near the Bahamas at a depth of more than 260 m. There are also a small number of freshwater and terrestrial species. Most red algae are multicellular. Although none are as big as the giant brown kelps, the largest multicellular red algae are included in the informal designation "seaweeds." You may have eaten one of these multicellular red algae, Porphyra (Japanese "nori"), as crispy sheets or as a wrap for sushi (see Figure 28.20). Red algae have especially diverse life cycles, and alternation of generations is common. But unlike other algae, they have no flagellated stages in their life cycle and depend on water currents to bring gametes to- gether for fertilization.

plasmodial slime molds

Many plasmodial slime molds are brightly colored, often yellow or orange (Figure 28.24). At one stage in their life cycle, they form a mass called a plasmodium, which may grow to a diameter of many centimeters. Despite its size, the plasmodium is not multicellular; it is a sin- gle mass of cytoplasm that is undivided by plasma membranes and that contains many nuclei. This "supercell" is the product of mitotic nuclear divisions that are not followed by cytokinesis. Within the plasmodium, cytoplasm streams first one way, then the other, in pulsing flows that are beautiful to watch through a microscope. This cytoplasmic streaming apparently helps distribute nutrients and oxygen. The plasmodium ex- tends pseudopodia through moist soil, leaf mulch, or rotting logs, engulfing food particles by phagocytosis as it grows. If the habitat begins to dry up or there is no food left, the plas- modium stops growing and differentiates into fruiting bodies, which function in sexual reproduction.

mitochondria and plastids

Mitochondria arose first through descent from a bacterium that was engulfed by a cell from an archaeal lineage he plastid lineage evolved later from a photosynthetic cyanobacterium that was engulfed by a heterotrophic eukaryote

Apicomplexans

Most apicomplexans are parasites of animals; some cause serious human diseases ● They spread through their host as infectious cells called sporozoites ● One end of a sporozoite, the apex, contains a complex of organelles specialized for penetrating host cells and tissues ● The life cycles of most apicomplexans have sexual and asexual stages and require two or more different hosts

The life cycle of Chlamydomonas, a unicellular chlorophyte.

Most chlorophytes have complex life cycles with both sexual and asexual reproductive stages Alternation of generations has evolved in some chlorophytes, including Ulva

Opisthokonts

Opisthokonts are an extremely diverse group of eukaryotes that includes animals, fungi, and several groups of protists. We will discuss the evolutionary history of fungi and animals in Chapters 31-34. Of the opisthokont protists, we will dis- cuss the nucleariids in Chapter 31 because they are more closely related to fungi than they are to other protists. Simi- larly, we will discuss choanoflagellates in Chapter 32, since they are more closely related to animals than they are to other protists. The nucleariids and choanoflagellates illustrate why scientists have abandoned the former kingdom Protista: A monophyletic group that included these single-celled eukary- otes would also have to include the multicellular animals and fungi that are closely related to them.

Protists are more nutritionally diverse than other eukary- ote groups

Photoautotrophs, which contain chloroplasts Heterotrophs, which absorb organic molecules or ingest larger food particles Mixotrophs, which combine photosynthesis and heterotrophic nutrition

●Some protists are parasitic

Plasmodium causes malaria ● Pfiesteria shumwayae is a dinoflagellate that causes fish kills ● Phytophthora ramorum causes sudden oak death ● P. infestans causes potato late blight, which contributed to the Irish famine of the 19th century

Protists play key roles in ecological communities

Protists are found in diverse aquatic and moist terrestrial environments ● Protists play two key roles in their habitats: that of symbiont and that of producer Many protists are important producers that obtain energy from the sun to convert CO2 to organic compounds ● In aquatic communities, photosynthetic protists and prokaryotes are the main producers ● Photosynthetic protists are limited by nutrients; populations can explode when limiting nutrients are added

radiolarians

Radiolarians, mostly marine protists, have delicate, symmetrical internal skeletons generally made of silica ● Pseudopodia reinforced by microtubules radiate from the central body of radiolarians ● Cytoplasm covering the microtubules engulf prey that become attached to the pseudopodia

SAR

SAR is a monophyletic supergroup named for the first letters of its three major clades: stramenopiles, alveolates, and rhizarians This group is one of the most controversial of the four supergroups

Single-celled protists can be very complex

Single-celled protists are justifi- ably considered the simplest eukaryotes, but at the cellular level, many protists are very complex—the most elaborate of all cells. In multicellular organisms, essential biological func- tions are carried out by organs. Unicellular protists carry out the same essential functions, but they do so using subcellu- lar organelles, not multicellular organs. contractile vac- uoles that pump excess water from the protistan cell

Slime Molds

Slime molds, or mycetozoans, were once thought to be fungi due to their spore-producing fruiting bodies ● This resemblance between slime molds and fungi is a result of convergent evolution ● Slime molds include two lineages, plasmodial slime molds and cellular slime molds Slime molds, or mycetozoans (from the Latin, meaning "fun- gus animals"), were once thought to be fungi because, like fungi, they produce fruiting bodies that aid in spore dispersal. However, the resemblance between slime molds and fungi ap- pears to be another example of evolutionary convergence. Molecular systematics places slime molds in Amoebozoa and suggests that they descended from unicellular ancestors. Slime molds have diverged into two main branches, plasmodial slime molds and cellular slime molds, distinguished in part by their unique life cycles.

Reproduction and life cycles also are highly varied among protists

Some protists are only known to reproduce asexu- ally; others can also reproduce sexually or at least employ the sexual processes of meiosis and fertilization. A

malaria

The apicomplexan Plasmodium is the parasite that causes malaria Plasmodium requires both mosquitoes and humans to complete its life cycle Approximately 200 million people in the tropics are infected, and 600,000 die each year from malaria The first malarial vaccine was approved in Europe in 2015, but it provides only partial protection Efforts are ongoing to develop vaccines that target this pathogen

Five Supergroups of Eukaryotes - Excavata

The clade Excavata is characterized by its cytoskeleton Some members have an "excavated" feeding groove on one side of the body three monophyletic groups: the diplomonads, parabasalids, and euglenozoans

life cycle of multicellular algae

The diploid generation is called a sporophyte; the haploid generation is called a gametophyte ● The sporophyte produces haploid spores, called zoospores in the brown algae Laminaria ● Spores develop into multicellular haploid male and female gametophytes, which produce gametes ● Fertilization of gametes results in a diploid zygote, which grows into a new sporophyte

green algae

The grass-green chloroplasts of green algae have a structure and pigment composition much like the chloroplasts of land plants. Molecular systematics and cellular morphology leave little doubt that green algae and land plants are closely re- lated. In fact, some systematists now advocate including green algae in an expanded "plant" kingdom, Viridiplantae (from the Latin viridis, green). Phylogenetically, this change makes sense, since otherwise the green algae are a para- phyletic group. Green algae are divided into two main groups, the charo- phytes and the chlorophytes. The charophytes are the algae most closely related to land plants, and so we will discuss them along with plants in Chapter 29. The second group, the chlorophytes (from the Greek chloros, green), includes more than 7,000 species. Most live in fresh water, but there are also many marine and some terres- trial species. The simplest chlorophytes are unicellular organ- isms such as Chlamydomonas, which resemble gametes or zoospores of more complex chlorophytes. Various species of unicellular chlorophytes live in aquatic habitats as phyto- plankton or inhabit damp soil. Some live symbiotically within other eukaryotes, contributing part of their photosyn- thetic output to the food supply of their hosts. Some chloro- phytes have even adapted to one of the last habitats you might expect to find them: snow. These chlorophytes carry out photosynthesis despite subfreezing temperatures and in- tense visible and ultraviolet radiation. They are protected by the snow itself, which acts as a shield, and by radiation- blocking compounds in their cytoplasm. Other chlorophytes contain similar protective compounds in their cell wall or in a durable coat that surrounds the zygote. Larger size and greater complexity evolved in chloro- phytes by three different mechanisms: 1. The formation of colonies of individual cells, as seen in Volvox (see Figure 28.3) and in filamentous forms that con- tribute to the stringy masses known as pond scum 2. The formation of true multicellular bodies by cell divi- sion and differentiation, as in Ulva (Figure 28.21a) 3. The repeated division of nuclei with no cytoplasmic divi- sion, as in Caulerpa (Figure 28.21b) Most chlorophytes have complex life cycles, with both sexual and asexual reproductive stages. Nearly all species of chlorophytes reproduce sexually by means of biflagellated ga- metes that have cup-shaped chloroplasts (Figure 28.22). Alter- nation of generations has evolved in some chlorophytes, including Ulva.

Cellular Slime Molds

The life cycle of the protists called cellular slime molds can prompt us to question what it means to be an individual organism. The feeding stage of these organisms consists of solitary cells that function individ- ually, but when food is depleted, the cells form an aggregate that functions as a unit (Figure 28.25). Although this mass of cells superficially resembles a plasmodial slime mold, the cells remain separated by their individual plasma membranes. Cel- lular slime molds also differ from plasmodial slime molds in being haploid organisms (only the zygote is diploid) and in having fruiting bodies that function in asexual rather than sexual reproduction. Dictyostelium discoideum, a cellular slime mold commonly found on forest floors, has become a model organism for studying the evolution of multicellularity. One line of research has focused on the slime mold's fruiting body stage. During this stage, the cells that form the stalk die as they dry out, while the spore cells at the top survive and have the potential to reproduce. Scientists have found that mutations in a single gene can turn individual Dictyostelium cells into "cheaters" that never become part of the stalk. Because these mutants gain a strong reproductive advantage over noncheaters, why don't all Dictyostelium cells cheat? Recent discoveries suggest an answer to this question. Cheating mutants lack a protein on their cell surface, and noncheating cells can recognize this difference. Noncheaters preferentially aggregate with other noncheaters, thus depriv- ing cheaters of the opportunity to exploit them. Such a recognition system may have been important in the evolu- tion of multicellular eukaryotes such as animals and plants.

chromalveolata

The supergroup Chromalveolata (the chromalveolates), a large, extremely diverse clade of protists, has recently been proposed based on two lines of evidence. First, some (though not all) DNA sequence data suggest that the chromalveolates form a monophyletic group. Second, some data support the hypothesis that the chromalveolates originated more than a billion years ago, when a common ancestor of the group en- gulfed a single-celled, photosynthetic red alga. Because red algae are thought to have originated by primary endosym- biosis (see Figure 28.2), such an origin for the chromalveo- lates is referred to as secondary endosymbiosis. How strong is the evidence that the chromalveolates origi- nated by secondary endosymbiosis? Many species in the clade have plastids whose structure and DNA indicate that they are of red algal origin. Others have reduced plastids that seem to be derived from a red algal endosymbiont. Still other species lack plastids altogether, yet some of these species have plastid genes in their nuclear DNA. Such data have led researchers to suggest that the common ancestor of the chromalveolates had plastids of red algal origin, but that later, some evolutionary lineages within the group lost the plastids. Others question this idea, based on the absence of plastid genes in the genomes of several chromalveolates that lack plastids. Overall, the endosymbiotic origin of the chromalveolates is an interesting idea, but like any scientific hypothesis, new data may show it to be incorrect. The chromalveolates are perhaps the most controversial of the five supergroups we describe in this chapter. Even so, for many scientists, this supergroup represents the best current hypothesis for the phylogeny of the two large protist clades to which we now turn: the alveolates and the stramenopiles

Unikonts include protists that are closely related to

The supergroup Unikonta includes animals, fungi, and some protists ● This group includes two clades: the amoebozoans and the opisthokonts (animals, fungi, and related protists) ● The root of the eukaryotic tree remains controversial ● It is unclear whether unikonts separated from other eukaryotes relatively early or late

What gave rise to the enormous diversity of protists that exist today?

There is abundant evidence that much of protist di- versity has its origins in endosymbiosis, the process in which certain unicellular organisms engulf other cells, which become endosymbionts and ultimately organelles in the host cell. DNA sequence data indicate that the first eukaryotes acquired mitochondria by engulfing an aerobic prokaryote (specifically, an alpha proteobacterium). The early origin of mitochondria is supported by the fact that all eu- karyotes studied so far have either mitochondria or modified versions of them.

Excavata - diplomonads and Parabasalids -main chatacteristic

These two groups lack plastids and have modified mitochondria, and most live in anaerobic environments

On several occasions during eukaryotic evolution, red algae and green algae underwent secondary endosymbiosis

They were ingested in the food vacuoles of heterotrophic eukaryotes and became endosymbionts themselves.

Trypanosoma

Trypanosomes evade host immune responses by producing cell-surface proteins with different molecular structures in each generation ● These frequent changes prevent the host from developing immunity

Entamoebas

Whereas most amoebozoans are free-living, those that be- long to the genus Entamoeba are parasites. They infect all classes of vertebrate animals as well as some invertebrates. Humans are host to at least six species of Entamoeba, but only one, E. histolytica, is known to be pathogenic. E. histolytica causes amebic dysentery and is spread via contaminated drinking water, food, or eating utensils. Responsible for up to 100,000 deaths worldwide every year, the disease is the third- leading cause of death due to eukaryotic parasites, after malaria (see Figure 28.10) and schistosomiasis (see Figure 33.11).

Golden algae

are named for their color, which results from their yellow and brown carotenoids The cells of golden algae are typically biflagellated, with both flagella near one end ● All golden algae are photosynthetic; some are mixotrophs ● Most are unicellular, but some are colonial

Antoni van Leeuwenhoek

discovered, viewing a drop of pond water under a light microscope can reveal a fas- cinating world of unicellular protists and prokaryotes.

protists are

eukaryotes- have organelles and are more complex than prokaryotic cells the organisms in most eukaryotic lineages are protists most protists are unicellular

Plastids

evolved later by endosymbiosis of a photosynthetic cyanobacterium The ancestral host cell may have been a lokiarchaeote, a newly discovered group of archea

Kinetoplastids (euglozoans)

have a single mitochondrion with an organized mass of DNA called a kinetoplast Some species parasitize animals, plants, and other protists- For example, kinetoplastids in the genus Trypanosoma cause sleeping sickness in humans ● Another pathogenic trypanosome causes Chagas' disease

Parabasalids

have reduced mitochondria called hydrogenosomes that generate some energy anaerobically ● include Trichomonas vaginalis, a sexually transmitted parasite

Diplomonads

have reduced mitochondria called mitosomes ● derive energy from anaerobic biochemical pathways ● have two equal-sized nuclei and multiple flagella● are often parasites, for example, Giardia intestinalis

Plastids arose when

heterotrophic protist acquired a cyanobacterial endosymbiont The photosynthetic descendants of this ancient protist evolved into red algae and green algae ● Plants are descended from the green algae● Archaeplastida is the supergroup that includes red algae, green algae, and plants

(Stramenopiles)

includes some of the most important photosynthetic organisms on Earth Most have a "hairy" flagellum paired with a "smooth" flagellum Stramenopiles include diatoms, golden algae, and brown algae

polyphyletic group

is a set of organisms, or other evolving elements, that have been grouped together but do not share an immediate common ancestor.

Protist

is the informal name of the group of mostly unicellular eukaryotes

Rhizaria

lthough its members vary greatly in morphology, DNA evidence suggests that rhizarians are a monophyletic group. According to some recent phyloge- netic studies, Many species in Rhizaria are among the organisms re- ferred to as amoebas. Amoebas were formerly defined as protists that move and feed by means of pseudopodia, ex- tensions that may bulge from almost anywhere on the cell sur- face. An amoeba moves by extending a pseudopodium and anchoring the tip; more cytoplasm then streams into the pseudopodium. However, based on molecular systematics, it is now clear that amoebas do not constitute a monophyletic group but are dispersed across many distantly related eukaryotic taxa. Most of those that belong to the clade Rhizaria are dis- tinguished morphologically from other amoebas by having threadlike pseudopodia. Rhizarians include three groups that we'll examine here: radiolarians, forams, and cercozoans.

Some protist symbionts benefit their hosts

or example, photosynthetic dinoflagellates are food-providing sym- biotic partners of the coral polyps that build coral reefs. Coral reefs are highly diverse ecological communities. That diversity ultimately depends on corals—and on the mutualistic protist symbionts that nourish them. Corals support reef diversity by providing food to some species and habitat to many others. Another example is the wood-digesting protists that inhabit the gut of many termite species (Figure 28.26). Unaided, ter- mites cannot digest wood, and they rely on protistan or prokaryotic symbionts to do so. Termites cause over $3.5 billion in damage annually to wooden homes in the United States.

producers

organisms that use energy from light (or inorganic chemicals) to convert carbon dioxide to organic compounds. Producers form the base of ecological food webs.

Cercozoans

orm a large group that contains most of the amoeboid and flagellated protists that feed with threadlike pseudopodia. Cercozoan protists are common in marine, freshwater, and soil ecosystems. Most cercozoans are heterotrophs. Many are parasites of plants, animals, or other protists; many others are predators. The predators include the most important consumers of bac- teria in aquatic and soil ecosystems, along with species that eat other protists, fungi, and even small animals. One small group of cercozoans, the chlorarachniophytes (mentioned earlier in the discussion of secondary endosymbiosis), are mixotrophic: These organisms ingest smaller protists and bacteria as well as perform photosynthesis. At least one other cercozoan, Paulinella chromatophora, is an autotroph, deriving its energy from light and its carbon from carbon dioxide. This species has a distinctive sausage-shaped internal structure where photosynthesis is performed (Figure 28.19). Genetic and morphological analyses indicate that these structures were derived from a cyanobacterium, although not the same cyanobacterium from which all other plastids were derived. As such, Paulinella appears to represent an intriguing additional evolutionary example of a eukaryotic lineage that obtained its photosynthetic apparatus directly from a cyanobacterium.

plastid-bearing lineage gave rise to two lineages of photosynthetic protists,

red and green algae This hypothesis is supported by the observation that the DNA of plastid genes in red algae and green algae closely resembles the DNA of cyanobacteria plas- tids in red algae and green algae are surrounded by two mem- branes. Transport proteins in these membranes are homologous to proteins in the inner and outer membranes of cyanobacterial endosymbionts,

Red algae and green algae are the closest

relatives of plants

Protists exhibit more

structural and functional diversity than any other group of eukaryotes

Though most protists are unicellular

there are some colonial and multicellular species

Alveolates

● Alveolates have membrane-enclosed sacs (alveoli) just under the plasma membrane ● The alveolates include ●Dinoflagellates ●Apicomplexans ●Ciliates

temp and protists

● Biomass of photosynthetic protists has declined as sea surface temperature has increased ● Growth of phytoplankton communities relies on nutrients delivered from the ocean bottom through the process of upwelling ● Warm surface water acts as a barrier to upwelling If sea surface temperature continues to warm due to global warming, this could have large effects on ● marine ecosystems● fishery yields● the global carbon cycle

Dinoflagellates

● Dinoflagellates have two flagella, and each cell is reinforced by cellulose plates ● They are abundant components of both marine and freshwater phytoplankton ● They are a diverse group of aquatic phototrophs, mixotrophs, and heterotrophs ● Toxic "red tides" are caused by dinoflagellate blooms

forams,

● Foraminiferans, or forams, are named for their porous shells, called tests ● Pseudopodia extend through the pores in the test and are used for swimming, test formation, and feeding ● Some are also nourished by the photosynthetic activity of symbiotic algae living within their tests ● Both freshwater and marine forms are known Foram tests in marine sediments form an extensive fossil record ● Researchers can use measures of the magnesium content in fossilized forams to estimate changes in ocean temperature over time

TUbulinids

● Tubulinids are a diverse group of amoebozoans with lobe- or tube-shaped pseudopodia ● They are common unicellular protists in soil as well as freshwater and marine environments ● Most tubulinids are heterotrophic and actively seek and consume bacteria and other protists