Chapter 28: Protists

Concept 28.1: Most eukaryotes are single-celled organisms

• Protists are eukaryotes, in domain Eukarya.

• Morphological studies show some are more related to plants, fungi, or animals then to other protists.

• Due to such studies, kingdom Protista was abandoned and various lineages are recognized on their own.

• Protist is still used as a term, but it’s primarily a convenience for grouping together a diverse array of unicellular and simple multicellular organisms that do not fit neatly into the other kingdoms.

• Cells of protists have a nucleus and membrane-enclosed organelles.

• Lack a well-developed cytoskeleton, and rely on simpler structures for cellular support and shape, which distinguishes them from more complex eukaryotic organisms.

• Most are unicellular, .

Structural and Functional Diversity in Protists

• Most protists are unicellular, although there are some colonial and multicellular species.

• Unicellular protists are simplest of eukaryotes, but can very complex.

• They contain subcellular organelles, not multicellular organs which preform similar functions as eukaryotes.

  • Nucleus, endoplasmic reticulum, golgi apparatus, and lysosomes.

  • Contractile vacuoles are specialized organelles that help maintain osmotic balance by expelling excess water from the cell.

• Most diverse, functionally and nutrionally of all eukaryotes.

  • Photoautotrophs → contain chloroplasts

  • Heterotrophs → absorb organic molecules or ingest larger food particles

  • Mixotrophs → combine photosynthesis and heterotrophic nutrition.

• Some produce asexually, some produce sexually.

Endosymbiosis in Eukaryotic Evolution

• Endosymbiosis → process in which a unicellular organism engulfs another cell, which becomes an endosymbiont and then organelle in host cell.

• Mitochondria evolved by endosymbiosis of an aerobic prokaryote.

• Plastids evolved by endosymbiosis of photosynthetic cyanobacterium.

• Plastid-bearing lineage of protists evolved into red algae and green algae closely resemble the DNA of cyanobacteria.

  • Red and green algae underwent secondary endosymbiosis, in which they were ingested by a heterotrophic eukaryote.

Plastid Evolution:

• Evolution of mitochondria gave rise to eukaryotes.

• Plastids arose later when heterophic eukaryote engulfed a photosynthetic cyanobacterium.

• Two lineages of photosynthetic protists, red and green algae, evolved from the plastid-bearing ancestor.

• Like cyanobacteria, plastids of red algae and green algae have two membranes.

• Transport proteins in these membranes are homologous to those found in the inner and outer membranes of cyanobacteria.

• Red and green algae themselves were ingested by hetetrophic eukaryotes, a process called secondary endosymbiosis

  • chloracinophytes likely evolved when a heterotrophic eukaryote engulfed a green alga.

  • engulfed cell contains vestigial nucleus called a nucleomorph.

Concept 28.2: Excavates include protists with modified mitochondria and protists with unique flagella.

• Excavata are characterized by its cytoskeleton;

• Some members have a feeding groove.

• Controversial group including diplomonads, parabasalids, and euglenozoans.

Diplomonads and Parabasalids

• Both lack plastics, modified mitochondria and most live in anaerobic environments.

• Diplomonads

  • Have modified mitochondria called mitosomes.

  • Derive energy from anaerobic biochemical pathways.

  • Have two equal-sized nuclei and multiple flagella.

  • Often parasites.

• Parabasalids

  • Have reduced mitochondria called hydrogenosomes that generate some energy anaerobically.

  • Include pathogen which causes yeast infections in humans.

• Euglenozoans

  • Diverse clade that includes predatory heterotrophs, photosynthetic autotrophs, and parasites.

  • Main feature distinguishing them as a clade is a spiral or crystalline rod of unknown function inside their flagella.

  • Clade includes kinetoplastids and euglenids.

    • Kinetoplastids → Single mitochondrion with organized mass of DNA called a kinetoplast

      • Evade immune responses by switching surface proteins.

    • Euglenids → Have one or two flagella that emerge from a pocket at one end of the cell, enabling them to swim efficiently in aquatic environments.

      • Can be autotrophic or heterotrophic

SAR

• This supergroup includes three large clades: Stramenoplia, Alveolata, and Rhizaria.

  • Diatoms are most important photosynthetic stramenopiles.

  • Many rhizarians are amoebas with threadlike psuedopodia

Archaeplastida

• Supergroup includes red and green algae, and plants.

• Red and green algae include unicellular, colonial, and multicellular species.

  • Volvox is an multicellular green algae.

Unikonta

• Amoebas with lobe- or tube-shaped psuedopodia.

  • Amoeba proteus is a tubulinid amobea.

• Animals, fungi and non-amoeba protists closely related to animals or fungi.

Concept 28.5: Red and green algae are closest relatives of land plants.

• Over a billion years ago, a heterotrophic protist acquired a cyanobacterial endosymbiont,

• Photosynthetic descendants of this ancient protest evolved into red and green algae.

• Lands plants originated / descend from green algae.

• Archaeplastida → supergroup including red algae, green algae, and land plants.

Red Algae

• Reddish in color due to an accessory pigment called phycoerythin, whisk makes green of chlorophyll.

• Color varies from greenish-red in shallow water to dark red or almost black.

• Usually multicellular with largest being seaweeds.

• Most abundant large algae in coastal waters of the tropics.

Green Algae

• Paraphyletic group named for their green chloroplasts.

• Plants are descended from green algae.

• Two main groups are chlorophytes and charophycneans

  • Charophytes are most closely related to land plants.

  • Most chlorophytes live in fresh water, although some are marine.

  • Other chlorophytes live in damp soil, in symbionts in lichens or in snow.

• larger size and greater complexity evolved in chlorophytes by:

  • Formation of colonies from single cells.

  • Formation of true multicellular bodies by cell division and differentiation

    • Ulva → a genus of green algae known as sea lettuce, exemplifying the multicellular structures that can arise from chlorophytes.

  • Repeated division of nuclei with no cytoplasmic division.

• Most chlorophytes have complex life cycles with both sexual and asexual reproductive stages.

Concept 28.6: Unikonts include protists that are closely related to fungi and animals

• Supergroup Unikonta includes animals, fungi and some protists.

• Two clades are present: amoebozoans and opishtkonts

  • ambozoans are characterized by their amoeba-like cells, which move and feed using extensions of their cytoplasm known as pseudopodia.

    • Slime molds (mycetozoans)

    • Gymnamobas: unicellular organisms found in freshwater and marine environements; heterotrophic against bacteria and protists.

    • entamoebas: parasites of vertebraes and some invertebrates.

  • opishtkonts are divided into two major lineages: the nucleariids, which are closely related to fungi, and the choanoflagellates, which are the closest living relatives of animals.

• Root of eukaryotic tree is controversial; unclear when unikonts separated from eukaryotes.

Plasmodial Slime Molds

• Many species are brightly pigmented.

• Plasmodium → a mass of cytoplasm that is not divided into individual cells, allowing for multiple nuclei to exist within a single cellular structure.

  • extends psuedopodia through decomposing material.

  • absorbs nutrients through phagocytosis

Cellular Slime Molds

• Form multicellular aggregates in which cells are seperated by their membranes.

• Cells feed individually, but can aggregate to forma fruiting body.

Concept 28.7: Protists play key roles in ecological communities

• Found in diverse aquatic environments.

• Often play a role of symbiont or producer.

• Some protist symbionts benefit their hosts:

  • dinoflagellates nourish coral polyps that build reefs.

  • wood-digesting protists digest cellulose in gut of termites.

• Some protists are parasitic:

  • Plasmodium causes malaria.

  • Pfiesteria shumwayae is a dinoflagellate that causes fish death.

  • Phytophthora causes oak death.

• Some protists are photosynthetic:

  • Many are produces that obtain energy from the sun.

  • In aquatic environments, photosynthetic protists and prokaryotes are the main producers, only being limited by nutrients.

  • Biomass declined as sea surface temperatures rose, leading to reduced photosynthetic activity and a subsequent impact on the entire aquatic food web.