Protists: Eukaryotic Diversity

• Protists are mostly unicellular eukaryotes, that are neither plants, animals, nor fungi.
• Protists are eukaryotes with organelles and are more complex than prokaryotic cells.
• Advances in eukaryotic systematics have changed the classification of protists, making Kingdom Protista no longer valid.

Eukaryotic Supergroups

• There are four supergroups of Eukaryota: Unikonta, Excavata, SAR, and Archaeplastida.

Structural and Functional Diversity

• Most protists are unicellular, though some are colonial or multicellular.
• Protists are nutritionally diverse, including photoautotrophs, heterotrophs, and mixotrophs.
• Photoautotrophs contain chloroplasts.
• Heterotrophs absorb organic molecules or ingest larger food particles.
• Mixotrophs combine photosynthesis and heterotrophic nutrition; Euglena is an example.
• Some protists reproduce asexually via fission
• Some reproduce sexually via processes of conjugation, fertilization, and meiosis

Protistan Diversity

• Protistan diversity can be traced to endosymbiosis.
• Endosymbiosis is when one organism lives inside another.
• Mitochondria and plastids are derived from prokaryotes engulfed by early eukaryotic cells.
• Mitochondria descended from an alpha proteobacterium engulfed by an archaeal lineage.
• Over time, the endosymbiont bacterium could no longer lead an independent life because the genes originally present on its chromosomes moved to the host's nuclear genome via horizontal gene transfer.
• Plastids evolved later from a photosynthetic cyanobacterium engulfed by a host heterotrophic eukaryote.
• Plastid-bearing protists evolved into photosynthetic protists, for example, red and green algae.

Primary and Secondary Endosymbiosis

• Plastids evolved initially from cyanobacteria engulfed by an ancestral heterotrophic eukaryote (primary endosymbiosis).
• Descendants diversified into red algae and green algae
• Some red and green algae were then engulfed by other eukaryotes (secondary endosymbiosis).
  • A heterotrophic eukaryote engulfs a cyanobacterium, leading to primary endosymbiosis resulting in three membranes
  • The cyanobacterium inside the eukaryote has three membranes, one of which will be lost in red and green algal descendants.
• Secondary endosymbiosis led to:
  • Stramenopiles and alveolates with plastid and nucleus
  • Euglenids with plastid, nucleus, and flagella
  • Chlorarachniophytes with plastid, nucleus, and pseudopodia

Supergroup Excavata

• Are protists with modified mitochondria, unique flagella and cytoskeleton.
• Diplomonads and parabasalids live in anaerobic environments and are parasitic
  • Diplomonads have two equal-sized nuclei and multiple flagella, for example, Giardia intestinalis.
  • Giardia intestinalis are found in mammalian intestines, and can cause parasitic infection if consumed in drinking water.
  • Parabasalids have reduced mitochondria called hydrogenosomes that generate energy anaerobically
  • Trichomonas vaginalis is a parabasalid and the pathogen that causes yeast infections in human females.
  • Trichomonas vaginalis was not a pathogen originally, but acquired a pathogenic gene via transformation from vaginal bacteria.
• Euglenozoa are a diverse clade including predatory heterotrophs, autotrophs, parasites, & mixotrophs.
  • Euglenozoa main feature is a spiral or crystalline rod inside their flagella
  • Euglenids have one or two flagella that emerge from a pocket at one end of the cell
  • Euglena are found in pond water, with some species being autotrophic and heterotrophic
  • Kinetoplastids have a single mitochondrion with an organized mass of DNA called a kinetoplast.
  • Free-living species are consumers of prokaryotes in freshwater, marine, and moist terrestrial ecosystems.
  • Some species are parasitic
  • Trypanosoma causes African sleeping sickness in humans.
  • Another Trypanosoma species causes Chagas' disease.
  • Some Trypanosomes evade immune detection by switching surface proteins from generation to generation, preventing the host from developing immunity.

Supergroup SAR

• "SAR" is a diverse monophyletic supergroup named for the first letters of 3 major clades:
  • Stramenopiles
  • Alveolates
  • Rhizarians

Stramenopiles

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

Diatoms

• Are unicellular algae with a unique two-part, glass-like wall of silicon dioxide
• Their wall is divided into two overlapping parts
• Parts enable live diatoms to withstand immense pressure, preventing crushing jaws of predators.
• Are a major component of phytoplankton and are highly diverse.
• Dead diatom walls compose sediments known as diatomaceous earth
• After a diatom population has bloomed, individuals fall to the ocean floor where they're not broken down by decomposers.
• This removes carbon dioxide from the atmosphere and "pumps” it to the ocean floor.
• Through carbon fixation, diatoms remove carbon dioxide from the atmosphere
• The carbon dioxide is converted to organic carbon and oxygen is released as a product

Brown Algae

• Largest and most complex protists
• Are multicellular; most are marine
• Include many species commonly called "seaweeds"
• Kelps live in deep parts of the ocean.
• Algal seaweeds have plantlike bodies with rootlike holdfasts that anchor them to the ocean floor and stemlike stipes which support their blades.
• Similarities between algae and plants are examples of analogous structures.
• Algin in their cell walls is used to thicken processed food.

Oomycetes

• Are Stramenopiles that include water molds, white rusts, and downy mildews.
• Were misidentified as fungi due to their multinucleate filaments that resemble fungal hyphae.
• Oomycetes cell walls are composed of cellulose, rather than chitin, unlike fungi.
• Based on molecular analysis, oomycetes and fungi are not closely related.
• Oomycetes are related to plastid-bearing groups, but do not have plastids or perform photosynthesis.
• Nutrients are acquired through parasitism or decomposition.
  • Phytophthora infestans is a parasite that causes potato late blight, and kills potato crops.

Alveolates

• Members of the clade Alveolata have membrane-enclosed sacs (alveoli) just under the plasma membrane
• This clade includes:. Dinoflagellates, Apicomplexans, and Ciliates

Dinoflagellates

• Have two flagella; each cell has cellulose plates
• Are an abundant unicellular algae, components of both marine & freshwater plankton
• Dinoflagellates in symbiosis nourish coral polyps building reefs.
• Are a diverse group of aquatic phototrophs, mixotrophs & heterotrophs
• Toxic "red tides" are caused by dinoflagellate blooms.
  • Dramatic reproduction of Karenia brevis in the Gulf can produce brevetoxins
  • Brevetoxins can kill fish, crabs, turtles, etc.

Apicomplexans

• One end, the apex (tip), contains a complex of organelles specialized for penetrating host cells and tissues.
• They spread through their host as infectious cells called sporozoites.
• Plasmodium causes malaria.
  • Plasmodium embeds itself in human red blood cells.
  • Requires two or more different host species for completion of life cycle
• Anopheles mosquitos and humans can both host it

Ciliates

• Contain motile cilia for locomotion and feeding
  • Cilia means "eyelash” in Latin
  • Cilia are organelles and slender protuberances.
• Contain two nuclei called macro and micro nuclei.
• Contractile vacuoles are present and live in fresh water
• They pump water out if placed in hypotonic solutions
• Example of ciliates is the paramecium

Reproductive Processes

• Conjugation = Sexual Reproduction
• Transverse Fission = Asexual Reproduction

Rhizarians

• Many species in the rhizarian clade are amoebas
  • Amoebas are in the SAR Clade, and the eukaryote supergroup Unikonta
• Marine protists called radiolarians have delicate, symmetrical internal skeletons made of silica.
  • Radiolarians' thread-like pseudopodia engulf microorganisms by phagocytosis.
  • Amoebas move and feed by extensions of the cell cytoplasm called pseudopodia.

Supergroup Archaeplastida

• Includes red & green algae and land plants.
• Land plants are descended from ancient protists that were green algae.
• Endosymbiosis of cyanobacterium to unicellular green algae to land plants was the evolution of chloroplasts.

Red Algae

• 6,000 species are "rhodophytes," which contain phycoerythrin, a red photosynthetic pigment.
• Species that are adapted to shallow water have less phycoerythrin and appear green.
• The red pigment masks the green chlorophyll if a significant amount is present.
• Most are multicellular and used in agar gel, carrageenan, and Sushi wraps.

Green Algae

• Descended from plants
• These are named for their green chloroplasts
• These are divided into Chlorophytes and Charophytes
  • Chlorophytes have many different specimens
  • Charophytes are closely related to land plants and ancestors

Green Algae Complexity

• Large size and greater complexity evolved in green algae by 3 different mechanisms:
  • Formation of colonies from individual cells, for example, Volvox
  • Formation of true multicellular bodies by cell division and differentiation, for example, Ulva
  • Repeated division of nuclei with no cytoplasmic division (Caulerpa)

Supergroup Unikonta

• Includes protists closely related to fungi & animals
• The root of the eukaryotic tree remains controversial
• It is unclear whether unikonts separated from other eukaryotes relatively early or late
• Unikontas are fungi, animals & protists (choanoflagellates & amoebozoans)
• Chaonoflagellates are related to animals than other protists

Amoebozoans

• Are amoebas that have lobe- or tube-shaped pseudopodia
• Include slime molds, entamoebas & gymnamoebas.
  • Dictyostelium discoideum is a slime mold used as a model organism for the study of multicellular evolution.

Entamoebas

• Are parasites of vertebrates and some invertebrates
• Some are free-living species. -Entamoeba histolytica causes amebic dysentery (infectious diarrhea) and is the 3rd leading cause of human death due to eukaryotic parasites.

Ecological Communities

• Protists are found in diverse aquatic and moist terrestrial environments
• Photosynthetic dinoflagellates form symbiotic relationships in coral reefs, which provide nourishment to hosts (coral polyps) building coral reefs.
• Dinoflagellates and diatoms constitute plankton
• Protists obtain energy from the sun that are important producers
  • Photosynthetic protists and prokaryotes are the main producers in aquatic environments
  • Biomass has declined because of sea surface temperatures increasing Growth of phytoplankton communities relies on nutrients delivered to the ocean bottom through upwelling
  • warm surface water acts as barrier that prevents it
  • marine ecosystems and fishery yields may face large effects from global warming due to the interference with the process of upwelling
• Current efforts explore the possibility of using algae to produce biodiesel fuel