Protists Chapter 28

YOU MUST KNOW Protista are no longer considered a kingdom. How chloroplasts and mitochondria evolved through endosymbiosis. The major clades of protists: text page 598.

The Classification of Protists Most eukaryotes are single-celled organisms! The term protist is now used to refer to any eukaryote that is neither a plant, animal, or fungi. Biologists no longer consider Protista a kingdom because it is paraphyletic. Protists vary in structure and function more so than any other group of eukaryotes. Most are unicellular Most use aerobic metabolism and have mitochondria.

Endosymbiosis Mitochondria and chloroplasts evolved through endosymbiosis. They were originally unicellular organisms engulfed by other cells that ultimately became organelles in the host cell. How did compartmental organization of the eukaryotic cell evolve from the simpler prokaryotic condition? In one process, the endomembrane system – the nuclear envelope, ER, Golgi, and related structures – may have evolved from specialized infoldings of the prokaryotic plasma membrane. Another process, called endosymbiosis, probably led to mitochondria, plastids, and some other features of eukaryotic cells. Mitochondria & plastids evolved from endosymbiotic bacteria: Plastids are a general term for the class of eukaryotic organelles that includes chloroplasts Chloroplasts and mitochondria are descendants of cyanobacteria and aerobic, heterotrophic prokaryotes, respectively, that took up residence within evolving eukaryotic cells.

Groups of Protists There are three major categories of protist: Plant-Like (photosynthetic algae) Animal-Like (heterotrophic protozoans) Fungus-Like (absorptive) Most protists are aquatic and are important constituents of plankton. All protists are capable of asexual reproduction, but some may reproduce sexually as well. Many other protists live as symbionts in other organisms. Because they are such a diverse group, the classification of protists continues to undergo revision.

Protozoa: Animal-Like Protists Includes all protists that ingest food. Rhizopoda: the amoebas that travel from place to place using pseudopods (false feet) that are extensions of their cells. As they eat, they undergo phagocytosis. Foraminifera: Marine protists whose name is derived from the word foramen because of the presence of calcium carbonate shells full of holes. Actinopoda: Major constituent of plankton. Apicomplexa: Parasitic protists (formerly known as sporozoans). An example is plasmodium, the causative agent of malaria. Zooflagellates: known for their flagella and can be parasitic. Tryanosoma causes African sleeping sickness. Ciliophora: use cilia to travel. Live in water and have two types of nuclei: a macronucleus (controls daily activities) and a micronuclei (functions in conjugation. Paramecium is an example.

Exploring structure and function in a ciliate 50 µm 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

Life Cycle of Plasmodium An infected Anopheles mosquito bites a person, injecting Plasmodium sporozoites in its saliva. 1 The sporozoites enter the person’s liver cells. After several days, the sporozoites undergo multiple divisions and become merozoites, which use their apical complex to penetrate red blood cells (see TEM below). 2 Figure 28.11 Inside mosquito Inside human Sporozoites (n) Oocyst MEIOSIS Liver Liver cell Merozoite Red blood cells Gametocytes FERTILIZATION Gametes Zygote (2n) Key Haploid (n) Diploid (2n) cell Apex 0.5 µm An oocyst develops from the zygote in the wall of the mosquito’s gut. The oocyst releases thousands of sporozoites, which migrate to the mosquito’s salivary gland. 7 The merozoites divide asexually inside the red blood cells. At intervals of 48 or 72 hours (depending on the species), large numbers of merozoites break out of the blood cells, causing periodic chills and fever. Some of the merozoites infect new red blood cells. 3 Gametes form from gametocytes. Fertilization occurs in the mosquito’s digestive tract, and a zygote forms. The zygote is the only diploid stage in the life cycle. 6 Another Anopheles mosquito bites the infected person and picks up Plasmodium gametocytes along with blood. 5 Some merozoites form gametocytes. 4

Ecology of Animal Like Protists Not so Good: Can be parasitic/cause disease Malaria, African Sleeping Sickness, Cryptosporidium Good: Symbiosis Termites have beneficial animal like protists called Trichonympha in their stomachs Break down cellulose in wood so termites can use it as food 9

Molds: Fungus-Like Protists Fungus-like protists include slime molds and water molds. Myxomycota: heterotrophic; includes plasmodial slime molds – they do not eat alone, but instead they eat and grow as a single, clumped unicellular mass known as a plasmodium. Acrasiomycota: the cellular slime molds, when food is plentiful, they eat alone as solitary beings…but when food becomes scarce, they clump together in a manner similar to slime molds and work together as a unit. Oomycota: the water molds – can be parasitc or saprobes. Use filaments known as hyphae to release digestive enzymes on food. Often found to be multicellular, or coenocytic.

Life Cycle of a Plasmodial Slime Mold The plasmodium erects stalked fruiting bodies (sporangia) when conditions become harsh. 3 The feeding stage is a multinucleate plasmodium that lives on organic refuse. 1 The plasmodium takes a weblike form. 2 Figure 28.26 Feeding plasmodium Mature (preparing to fruit) Young sporangium Spores (n) Germinating spore Amoeboid cells Zygote (2n) 1 mm Key Haploid (n) Diploid (2n) MEIOSIS SYNGAMY Stalk Flagellated cells The cells unite in pairs (flagellated with flagellated and amoeboid with amoeboid), forming diploid zygotes. 7 These cells are either amoeboid or flagellated; the two forms readily convert from one to the other. 6 The resistant spores disperse through the air to new locations and germinate, becoming active haploid cells when conditions are favorable. 5 Within the bulbous tips of the sporangia, meiosis produces haploid spores. 4

The life cycle of a cellular slime mold (Dictyostelium) 12

Ecology of Fungus-Like Protists The ecological impact of oomycetes can be significant Phytophthora infestans causes late blight of potatoes – Irish Potato Famine Overgrowth of water mold caused by wet and cool conditions Slime molds and water molds are the MOST important recyclers of organic material Why is the earth not littered with dead organisms? Tissues broken down by Fungi Like Protists and other decomposers

Algae: Plant-Like Protists Dinoflagellates: have 2 flagella, important producers in aquatic food chains. Golden Algae: chrysophyta, constituent of plankton. Diatoms: bacillariophyta, major constituent of phytoplankton – most abundant organisms on Earth - have cell walls made of silica to protect them (diatomaceous Earth/fossilized diatom walls). Major primary producers in freshwater ecosystems! Green Algae: chlorophyta – have chlorophyll a and b. Brown Algae: phaeophyta – multicellular and live in marine environments (kelp and seaweed), contain fucoxanthin pigments. Red Algae: rhodophyta – contain phycobilin pigments, mostly multicellular, live in ocean. Many live in deep water and absorb nonvisible light via accessory pigments.

Euglenophytes 15

Algal Blooms

Red Tides Great blooms of the dinoflagellates have occurred in recent years on the east coast. These blooms are known as “red tides”. These species release a potentially harmful toxin that infect certain shellfish. Eating these infected shellfish can be harmful or fatal. 17

Ecology of Plant-Like Protists Most unicellular species beneficial Act as producers in the marine food chain Form Phytoplankton for consumer organisms to eat Approx. ½ of the photosynthesis on earth!!! Produce large amount of oxygen Symbiosis: Coral Reefs, Clams Provide food via photosynthesis, receive a home 18

Beneficial Aspects of Algae Food for humans Food for invertebrates and fishes in mariculture Animal feed Soil fertilizers and conditioners in agriculture Treatment of waste water Diatomaceous earth (= diatoms) Chalk deposits Phycocolloids (agar, carrageenan from red algae; alginates from brown algae) Drugs Model system for research Phycobiliproteins for fluorescence microscopy 19

Detrimental Aspects of Algae Blooms of freshwater algae Red tides and marine blooms Toxins accumulated in food chains Damage to cave paintings, frescoes, and other works of art Fouling of ships and other submerged surfaces Fouling of the shells of commercially important bivalves 20