Chapter 28 Protists

Overview: A World in a Drop of Water Even a low-power microscope Can reveal an astonishing menagerie of organisms in a drop of pond water Figure 28.1 50 m

These amazing organisms Belong to the diverse kingdoms of mostly single-celled eukaryotes informally known as protists Advances in eukaryotic systematics Have caused the classification of protists to change significantly

Protists are more diverse than all other eukaryotes Concept 28.1: Protists are an extremely diverse assortment of eukaryotes Protists are more diverse than all other eukaryotes And are no longer classified in a single kingdom Most protists are unicellular And some are colonial or multicellular

Protists, the most nutritionally diverse of all eukaryotes, include Photoautotrophs, which contain chloroplasts Heterotrophs, which absorb organic molecules or ingest larger food particles Mixotrophs, which combine photosynthesis and heterotrophic nutrition

Protist habitats are also diverse in habitat And including freshwater and marine species Figure 28.2a–d 100 m 4 cm 500 m The freshwater ciliate Stentor, a unicellular protozoan (LM) Ceratium tripos, a unicellular marine dinoflagellate (LM) Delesseria sanguinea, a multicellular marine red alga Spirogyra, a filamentous freshwater green alga (inset LM) (a) (b) (c) (d)

Reproduction and life cycles Are also highly varied among protists, with both sexual and asexual species

A sample of protist diversity Table 28.1

Endosymbiosis in Eukaryotic Evolution There is now considerable evidence That much of protist diversity has its origins in endosymbiosis

The plastid-bearing lineage of protists Evolved into red algae and green algae On several occasions during eukaryotic evolution Red algae and green algae underwent secondary endosymbiosis, in which they themselves were ingested

Diversity of plastids produced by secondary endosymbiosis Cyanobacterium Heterotrophic eukaryote Primary endosymbiosis Red algae Green algae Secondary Plastid Dinoflagellates Apicomplexans Ciliates Stramenopiles Euglenids Chlorarachniophytes Alveolates Figure 28.3

Plasmodial slime molds Concept 28.2: Diplomonads and parabasalids have modified mitochondria A tentative phylogeny of eukaryotes Divides eukaryotes into many clades Rhodophyta Chlorophyta Diplomonadida Euglenozoa Animalia Parabasala Plantae Cercozoa Radiolaria Fungi Alveolata Stramenopila Amoebozoa (Opisthokonta) (Viridiplantae) Fungi Plants Euglenids Ciliates Parabasalids Dinoflagellates Oomycetes Diatoms Golden algae Brown algae Radiolarians Entamoebas Metazoans Red algae Diplomonads Kinetoplastids Apicomplexans Foraminiferans Gymnamoebas Chlorophytes Chlorarachniophytes Cellular slime molds Choanoflagellates Charophyceans Plasmodial slime molds Figure 28.4 Ancestral eukaryote

Diplomonads and parabasalids Are adapted to anaerobic environments Lack plastids Have mitochondria that lack DNA, an electron transport chain, or citric-acid cycle enzymes

Dinoflagellates Dinoflagellates Are a diverse group of aquatic photoautotrophs and heterotrophs Are abundant components of both marine and freshwater phytoplankton

Each has a characteristic shape That in many species is reinforced by internal plates of cellulose Two flagella Make them spin as they move through the water Figure 28.10 3 µm Flagella

Rapid growth of some dinoflagellates Is responsible for causing “red tides,” which can be toxic to humans

Ciliates, a large varied group of protists Are named for their use of cilia to move and feed Have large macronuclei and small micronuclei

Conjugation is separate from reproduction The micronuclei Function during conjugation, a sexual process that produces genetic variation Conjugation is separate from reproduction Which generally occurs by binary fission

FEEDING, WASTE REMOVAL, AND WATER BALANCE Exploring structure and function in a ciliate 50 µm FEEDING, WASTE REMOVAL, AND WATER BALANCE Paramecium, like other freshwater protists, constantly takes in water by osmosis from the hypotonic environment. Bladderlike contractile vacuoles accumulate excess water from radial canals and periodically expel it through the plasma membrane. Contractile Vacuole Paramecium feeds mainly on bacteria. Rows of cilia along a funnel-shaped oral groove move food into the cell mouth, where the food is engulfed into food vacuoles by phagocytosis. Oral groove Cell mouth Food vacuoles combine with lysosomes. As the food is digested, the vacuoles follow a looping path through the cell. Thousands of cilia cover the surface of Paramecium. Micronucleus Macronucleus The undigested contents of food vacuoles are released when the vacuoles fuse with a specialized region of the plasma membrane that functions as an anal pore. Figure 28.12

CONJUGATION AND REPRODUCTION 8 7 2 MICRONUCLEAR FUSION Diploid micronucleus Haploid micronucleus MEIOSIS Compatible mates Key Conjugation Reproduction Macronucleus Two cells of compatible mating strains align side by side and partially fuse. 1 Meiosis of micronuclei produces four haploid micronuclei in each cell. 2 3 Three micronuclei in each cell disintegrate. The remaining micro- nucleus in each cell divides by mitosis. The cells swap one micronucleus. 4 The original macro- nucleus disintegrates. Four micronuclei become macronuclei, while the other four remain micronuclei. 8 The cells separate. 5 Micronuclei fuse, forming a diploid micronucleus. 6 Two rounds of cytokinesis partition one macronucleus and one micronucleus into each of four daughter cells. 9 Three rounds of mitosis without cytokinesis produce eight micronuclei. 7

Oomycetes (Water Molds and Their Relatives) Include water molds, white rusts, and downy mildews Were once considered fungi based on morphological studies

Most oomycetes Are decomposers or parasites Have filaments (hyphae) that facilitate nutrient uptake

germinate, growing into The life cycle of a water mold Encysted zoospores land on a substrate and germinate, growing into a tufted body of hyphae. 1 Several days later, the hyphae begin to form sexual structures. 2 Meiosis produces eggs within oogonia (singular, oogonium). 3 On separate branches of the same or different individuals, meiosis produces several haploid sperm nuclei contained within antheridial hyphae. 4 Figure 28.14 Cyst Zoospore (2n) ASEXUAL REPRODUCTION Zoosporangium Germ tube Zygote germination FERTILIZATION SEXUAL Zygotes (oospores) MEIOSIS Oogonium Egg nucleus (n) Antheridial hypha with sperm nuclei Key Haploid (n) Diploid (2n) Each zoospor- angium produces about 30 biflagellated zoospores asexually. 9 The ends of hyphae form tubular zoosporangia. 8 The zygotes germinate and form hyphae, and the cycle is completed. 7 Antheridial hyphae grow like hooks around the oogonium and deposit their nuclei through fertilization tubes that lead to the eggs. Following fertilization, the zygotes (oospores) may develop resistant walls but are also protected within the wall of the oogonium. 5 A dormant period follows, during which the oogonium wall usually disintegrates. 6

The ecological impact of oomycetes can be significant Phytophthora infestans causes late blight of potatoes

Diatoms are unicellular algae With a unique two-part, glass-like wall of hydrated silica Figure 28.15 3 µm

Diatoms are a major component of phytoplankton And are highly diverse Figure 28.16 50 µm

Accumulations of fossilized diatom walls Compose much of the sediments known as diatomaceous earth

Brown algae, or phaeophytes Are the largest and most complex algae Are all multicellular, and most are marine

Brown algae Include many of the species commonly called seaweeds Seaweeds Have the most complex multicellular anatomy of all algae Figure 28.18 Blade Stipe Holdfast

Kelps, or giant seaweeds Live in deep parts of the ocean Figure 28.19

Human Uses of Seaweeds Many seaweeds Are important commodities for humans Are harvested for food Figure 28.20a–c (a) The seaweed is grown on nets in shallow coastal waters. (b) A worker spreads the harvested sea- weed on bamboo screens to dry. (c) Paper-thin, glossy sheets of nori make a mineral-rich wrap for rice, seafood, and vegetables in sushi.

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

The life cycle of the brown alga Laminaria Figure 28.21 Sporophyte (2n) Zoospores Female Gametophytes (n) MEIOSIS FERTILIZATION Developing sporophyte Zygote Mature female gametophyte Egg Sperm Male Sporangia Key Haploid (n) Diploid (2n) The sporophytes of this seaweed are usually found in water just below the line of the lowest tides, attached to rocks by branching holdfasts. 1 In early spring, at the end of the main growing season, cells on the surface of the blade develop into sporangia. 2 Sporangia produce zoospores by meiosis. 3 The zoospores are all structurally alike, but about half of them develop into male gametophytes and half into female gametophytes. The gametophytes look nothing like the sporo- phytes, being short, branched filaments that grow on the surface of subtidal rocks. 4 The zygotes grow into new sporophytes, starting life attached to the remains of the female gametophyte. 7 Male gametophytes release sperm, and female gametophytes produce eggs, which remain attached to the female gameto- phyte. Eggs secrete a chemical signal that attracts sperm of the same species, thereby increasing the probability of fertilization in the ocean. 5 Sperm fertilize the eggs. 6

Concept 28.8: Red algae and green algae are the closest relatives of land plants Over a billion years ago, a heterotrophic protist acquired a cyanobacterial endosymbiont And the photosynthetic descendants of this ancient protist evolved into red algae and green algae

Red algae are reddish in color Due to an accessory pigment call phycoerythrin, which masks the green of chlorophyll

Red algae Are usually multicellular; the largest are seaweeds Are the most abundant large algae in coastal waters of the tropics Figure 28.28a–c (a) Bonnemaisonia hamifera. This red alga has a filamentous form. Dulse (Palmaria palmata). This edible species has a “leafy” form. (b) A coralline alga. The cell walls of coralline algae are hardened by calcium carbonate. Some coralline algae are members of the biological communities around coral reefs. (c)

Green Algae Green algae Are named for their grass-green chloroplasts Are divided into two main groups: chlorophytes and charophyceans Are closely related to land plants

Most chlorophytes Other chlorophytes Live in fresh water, although many are marine Other chlorophytes Live in damp soil, as symbionts in lichens, or in snow Figure 28.29

Chlorophytes include Unicellular, colonial, and multicellular forms Volvox, a colonial freshwater chlorophyte. The colony is a hollow ball whose wall is composed of hundreds or thousands of biflagellated cells (see inset LM) embedded in a gelatinous matrix. The cells are usually connected by strands of cytoplasm; if isolated, these cells cannot reproduce. The large colonies seen here will eventually release the small “daughter” colonies within them (LM). (a) Caulerpa, an inter- tidal chlorophyte. The branched fila- ments lack cross-walls and thus are multi- nucleate. In effect, the thallus is one huge “supercell.” (b) Ulva, or sea lettuce. This edible seaweed has a multicellular thallus differentiated into leaflike blades and a rootlike holdfast that anchors the alga against turbulent waves and tides. (c) 20 µm 50 µm Figure 28.30a–c

Most chlorophytes have complex life cycles With both sexual and asexual reproductive stages These daughter cells develop flagella and cell walls and then emerge as swimming zoospores from the wall of the parent cell that had enclosed them. The zoospores grow into mature haploid cells, completing the asexual life cycle. 7 In Chlamydomonas, mature cells are haploid and contain a single cup-shaped chloroplast (see TEM at left). 1 In response to a shortage of nutrients, drying of the pond, or some other stress, cells develop into gametes. 2 Gametes of opposite mating types (designated + and –) pair off and cling together. Fusion of the gametes (syngamy) forms a diploid zygote. 3 Figure 28.31 Flagella Cell wall Nucleus Regions of single chloroplast Zoospores ASEXUAL REPRODUCTION Mature cell (n) SYNGAMY SEXUAL Zygote (2n) MEIOSIS 1 µm Key Haploid (n) Diploid (2n)  + The zygote secretes a durable coat that protects the cell against harsh conditions. 4 When a mature cell repro- duces asexually, it resorbs its flagella and then undergoes two rounds of mitosis, forming four cells (more in some species). 6 After a dormant period, meiosis produces four haploid individuals (two of each mating type) that emerge from the coat and develop into mature cells. 5