Protist Notes

Protists

Overview

• Protists are the first eukaryotic cells, arising through endosymbiosis.
• They are largely unicellular but vary in structure, nutrition, and sexual life cycle.
• Nutritional modes:
  • Photosynthetic (like land plants)
  • Heterotrophic by ingestion (like animals)
  • Heterotrophic by absorption (like fungi)
• Protists are classified into supergroups:
  • Chromalveolates
  • Archaeplastids
  • Excavates
  • Amoebozoans
  • Rhizarians
  • Opisthokonts

Cellular Characteristics

• Eukaryotic cells with membranous organelles.
• Endosymbiotic theory explains the origin of mitochondria and chloroplasts.
  • Mitochondria: Derived from aerobic bacteria.
  • Chloroplasts: Derived from cyanobacteria (engulfed in two separate events).
• Unicellular but complex.
• Contractile vacuole: Organelle for water regulation in amoeboids and ciliates.

Nutrient Acquisition

• Algae: Photosynthetic, gather energy from sunlight.
• Protozoans: Heterotrophic, some use endocytosis to form food vacuoles.
• Slime molds: Ingest decaying plant material.
• Parasitic protozoans: Absorb nutrients from their host.
• Euglena: Mixotrophic (autotrophic and heterotrophic).

Reproduction and Locomotion

• Asexual reproduction by mitosis is common.
• Sexual reproduction (meiosis and spore formation) occurs in hostile environments.
• Locomotion: Flagella, pseudopods, or cilia. Non-motile protists tend to be parasitic.

Archaeplastids

General Characteristics

• Include land plants, green algae, and red algae.
• Plastids derived from endosymbiotic cyanobacteria.

Green Algae

• Contain chlorophylls a and b.
• Most are unicellular, but filamentous and colonial forms exist.
• Color varies due to additional pigments.
• Subdivided into chlorophytes and charophytes based on molecular data.

Chlorophytes

• Chlamydomonas:
  • Minute, motile, freshwater chlorophyte.
  • Definite cell wall.
  • Single, large, cup-shaped chloroplast with a pyrenoid (starch synthesis).
  • Two flagella for breaststroke-like motion.
  • Asexual Reproduction: Mitosis produces up to 16 daughter cells within the parent cell wall. Daughter cells secrete a cell wall and acquire flagella. Daughter cells are released via enzymatic digestion of the parent cell wall.
  • Sexual Reproduction: Under unfavorable conditions, gametes of different mating types fuse to form a zygote. A heavy wall forms around the zygote and creates a resistant zygospore that undergoes dormancy. Upon germination, the zygospore produces four zoospores (flagellated spores) by meiosis, which grow into adults.

Charophytes

• Filamentous (branched or unbranched).
• Spirogyra:
  • Unbranched charophyte.
  • Grows epiphytically on aquatic plants or attaches to underwater objects; some are suspended in water.
  • Found in green masses on ponds and streams.
  • Ribbon-like, spiralled chloroplasts.
  • Sexual Reproduction: Spirogyra undergoes conjugation, a temporary union, facilitating the exchange of genetic material. Cell contents from one filament move into the cells of another, forming diploid zygotes. Resistant zygospores survive winter and undergo meiosis in spring to produce new haploid filaments.

Red Algae

• Multicellular seaweeds.
• Contain phycoerythrin (red), phycocyanin (blue), and chlorophyll.
• Live in warm seawater, in both shallow and deep waters (exceeding 70m).
• Filamentous or complexly branched.
• Economic Importance:
  • Agar: From Gelidium and Gracilaria. Used as a solidifying agent for bacterial culture medium, in electrophoresis gels, and as an anti-drying agent in baked goods.
  • Carrageenin: Emulsifying agent for chocolate and cosmetics.

Chromalveolates

General

• Includes stramenopiles and alveolates.

Stramenopiles

• Have flagella or are descended from ancestors with flagella; one flagellum is longer and has hair-like projections.

Brown Algae

• Contain chlorophylls a and c and fucoxanthin (carotenoid pigment).
• Reserve food is laminarin (carbohydrate).
• Range from simple filaments to large multicellular forms (up to 100 m).
• Seaweeds in north temperate zone rocky coasts; cell walls contain mucilaginous, water-retaining material.

Diatoms

• Silica shell (valve) resembling a petri dish.
• Orange-yellow color due to fucoxanthin.
• Approximately 100,000 species.
• Significant part of plankton, source of oxygen and food for heterotrophs in freshwater and marine ecosystems.
• Asexual Reproduction: Each diatom receives one old valve; new valves fit inside the old ones, resulting in smaller diatoms until they reach about 30% of the original size.
• Sexual Reproduction: The zygote grows and divides mitotically to produce diatoms of normal size.
• Diatomaceous earth: Remains of diatoms used as filtering agents, soundproofing materials, and gentle polishing abrasives.
  $Asexual \ Reproduction \rightarrow Meiosis \rightarrow Gametes \rightarrow Zygote \rightarrow Auxospore \rightarrow Sexual \ Reproduction$

Golden Brown Algae

• Distinctive color from yellow-brown carotenoid pigments.
• Unicellular or colonial protists with two flagella bearing tubular hairs.
• Cells may be naked, covered with organic or silica scales, or enclosed in a lorica.
• Many are mixotrophs (e.g., Ochromonas), capable of photosynthesis and phagocytosis.
• Contribute to freshwater and marine phytoplankton.

Water Molds

• Live in water, parasitizing fish or insects, and decompose remains.
• Saprotrophic, living off dead organic matter.
• Some live on land and parasitize insects and plants.
• Nearly 700 species described.
• Phytophthora infestans: Responsible for the 1840s potato famine in Ireland.
• Saprolegnia: Cotton-like white mass on dead organisms.
• Filamentous body, but cell walls are largely composed of cellulose, not chitin as in fungi.
• Life cycle differs from fungi.
• During asexual reproduction water molds produce motile spores (2n zoospores), which are flagellated. The organism is diploid (not haploid as in the fungi), and meiosis produces gametes. Their alternate name “oomycetes” refers to enlarged tips (called oogonia) where eggs are produced.

Alveolates

• Have alveoli (small sacs) beneath the plasma membranes, thought to support the cell surface.

Dinoflagellates

• Single cell bounded by cellulose plates impregnated with silicates.
• Typically have two flagella: one longitudinal (rudder-like) and one transverse (causes spinning).
• Chloroplasts vary in color (yellow-green to brown) due to chlorophylls a and c, and carotenoids.
• Approximately 4,000 species; some (e.g., Noctiluca) are bioluminescent.
• Important source of food for small animals in the ocean (plankton).
• Symbiotic dinoflagellates (zooxanthellae) lack cellulose plates and flagella; provide hosts with organic nutrients, and corals provide wastes that fertilize the algae.
• Photosynthetic dinoflagellates are often mixotrophs, ingesting food particles by phagocytosis.
• Can undergo population explosions, causing red tide (e.g., Gymnodinium brevis), leading to massive fish kills due to neurotoxins. Humans consuming shellfish during outbreaks may suffer paralytic shellfish poisoning.

Ciliates

• Unicellular protists that move by cilia.
• Structurally complex and specialized.
• Paramecium: Classic example.
• Hundreds of cilia beat rhythmically.
• Semi-rigid outer covering (pellicle).
• Trichocysts: Oval capsules beneath the pellicle that discharge barbed threads for defense and prey capture; some release poisons.
• Feeding: Food particles are swept down a gullet, forming food vacuoles; nutrients are absorbed, and non-digestible residue is eliminated at the anal pore.
• Asexual Reproduction: Divide by transverse binary fission.
• Two types of nuclei: macronucleus (controls metabolism) and micronuclei (involved in reproduction).
• Sexual Reproduction: Conjugation occurs where the macronucleus disintegrates and the micronuclei undergo meiosis. Two ciliates exchange haploid micronuclei. Then the micronuclei give rise to a new macronucleus, which contains copies of only certain housekeeping genes.
• Diverse group (10-3,000 μm; approximately 8,000 species); mostly free-living, but some are parasitic, sessile, and colonial.
• Didiniums expand to consume larger paramecia.
• Suctoria paralyze victims and use tentacles to suck them dry.
• Stentor resembles a blue vase.
• Ichthyophthirius causes “ick” in fish.

Excavates

General

• Zooflagellates with atypical or absent mitochondria and distinctive flagella and/or deep (excavated) oral grooves.

Euglenids

• Small (10–500 μm), freshwater unicellular organisms.
• Classification is problematic.
• About one-third have chloroplasts; the rest ingest or absorb food.
• Chloroplasts are like those of green algae, likely derived from endosymbiosis.
• Chloroplasts are surrounded by three membranes, not two. Pyrenoid: Region of chloroplast where carbohydrate formation occurs.
• Produce paramylon (unusual carbohydrate).
• Euglena deces: Inhabitant of freshwater ditches and ponds; mixotrophic, treated as plant-like in botany and animal-like in zoology texts.
• Have two flagella, one much longer with hairs (tinsel flagellum). An eyespot near the base shades a photoreceptor.
• Flexible pellicle made of protein bands; can assume different shapes.
• Contractile vacuole for ridding the body of excess water.
• Reproduce by longitudinal cell division; sexual reproduction is not known to occur.

Amoebozoans

General

• Protozoans that move by pseudopods (cytoplasm streams forward).
• Usually live in aquatic environments.
• Often part of the plankton.

Amoeboids

• Protists that move and ingest food with pseudopods.
• Amoeba proteus: Commonly studied freshwater member.
• Feed by surrounding and phagocytizing prey (algae, bacteria, other protists); digestion within a food vacuole.
• Freshwater amoeboids have contractile vacuoles to remove excess water.
• Entamoeba histolytica: Parasitic amoeboid in the human large intestine causing amoebic dysentery. Complications arise from intestinal lining invasion. Liver and brain involvement can be fatal.

Slime Molds

• Contribute to ecological balance by phagocytizing dead plant material and controlling bacteria populations.
• Once classified as fungi but lack cell walls and have flagellated cells during their life cycle.
• Vegetative state is mobile and amoeboid.
• Produce spores by meiosis, germinating to form gametes.

Plasmodial Slime Molds

• Exist as a plasmodium (diploid, multinucleated, cytoplasmic mass) enveloped by a slime sheath, creeping along and phagocytizing decaying plant material.
• Many species are brightly colored.
• Under unfavorable conditions (e.g., drought), the plasmodium develops sporangia (reproductive structures that produce spores; aggregate called a fruiting body).
• Spores survive until moisture is sufficient for germination, releasing a haploid flagellated or amoeboid cell.
• Two cells fuse to form a zygote that grows, producing a multinucleated plasmodium.

Cellular Slime Molds

• Exist as individual amoeboid cells in the soil, feeding on bacteria and yeasts.
• Small size prevents them from being seen individually.
• As food runs out, cells release a chemical, aggregating into a pseudoplasmodium (temporary stage).
• The pseudoplasmodium gives rise to a fruiting body in which sporangia produce spores.
• Under favorable conditions, spores germinate, releasing haploid amoeboid cells, and the asexual cycle begins again.

Opisthokonts

General

• Includes animals, fungi, and closely related protists.
• Includes both unicellular and multicellular protozoans.

Choanoflagellates

• Animal-like protozoans and near relatives of sponges.
• Filter-feeders with cells resembling choanocytes (feeding cells of sponges).
• Cells have a single posterior flagellum surrounded by a collar of microvilli.
• Flagellum beating creates a water current, where food particles are taken in by phagocytosis.
• Colonial choanoflagellates attach to surfaces with a stalk or float freely.

Rhizarians

General

• Consist of foraminiferans and radiolarians with fine, threadlike pseudopods.
• Assigned to a different supergroup from amoebozoans due to molecular data.

Foraminiferans and Radiolarians

• Have a skeleton called a test.
• Foraminiferans: Calcium carbonate test, often multi-chambered; pseudopods extend through openings.
• Radiolarians: Glassy silicon test, internal and usually with a radial arrangement of spines; pseudopods are external to the test.
• Tests of dead foraminiferans and radiolarians form a deep sediment layer (700–4,000 m) on the ocean floor.
• Radiolarians lie deeper due to the glassy test’s insolubility at greater pressures.
• Presence used as an indicator of oil deposits.
• Deposits of foraminiferans for millions of years, followed by geological upheaval, formed the White Cliffs of Dover.
• Shells are abundant in the ocean.