Premed · Premed · General Biology 2
Lecture 7: Protists
General Biology II — Organismal, Evolution & Ecology
Learning Objectives
By the end of this lecture, students will be able to:
- Explain why "Protista" is not a valid monophyletic group and describe the diversity of eukaryotic supergroups
- Compare modes of nutrition, locomotion, and reproduction across major protist lineages
- Describe the ecological roles of protists in aquatic and terrestrial ecosystems
- Identify key protist groups and their distinguishing characteristics
- Discuss the medical and economic importance of selected protists
- Explain the endosymbiotic origins of plastids in different protist lineages (primary and secondary endosymbiosis)
Lecture Content
I. What Are Protists?
Protists are eukaryotes that are not plants, animals, or fungi -- a definition notable for what it excludes rather than what it includes. "Protista" is not a valid monophyletic group but rather a paraphyletic catchall that encompasses the vast diversity of eukaryotic life outside the three well-defined kingdoms. Protists include unicellular, colonial, and simple multicellular organisms, and they display enormous diversity in morphology, metabolism, ecology, and reproduction. Most are aquatic, inhabiting freshwater or marine environments, though some live in soil or as parasites within other organisms.
Protists span the full range of nutritional strategies. Photoautotrophic protists (the algae) carry out photosynthesis. Heterotrophic protists (the protozoans) feed by phagocytosis or absorption. Mixotrophic protists combine both strategies, switching between photosynthesis and heterotrophy depending on conditions. Movement is equally diverse: flagella propel organisms like Euglena and Trypanosoma, cilia cover the surface of Paramecium, pseudopodia allow amoeboid movement in Amoeba and foraminifera, and some protists are entirely non-motile. Reproduction may be asexual (through binary fission, budding, fragmentation, or the multiple fission process called schizogony) or sexual, with many protists capable of sexual reproduction under stress conditions. Some algae exhibit true alternation of generations.
II. Eukaryotic Supergroups
Modern molecular phylogenetics has reorganized the diversity of eukaryotes into several supergroups that reflect evolutionary relationships far more accurately than the traditional kingdom system.
A. Excavata
The Excavata are named for the feeding groove -- an "excavation" -- found on one side of the cell in many members. Many excavates lack typical mitochondria or possess modified versions such as hydrogenosomes or mitosomes, likely the result of secondary loss rather than primitive absence.
The Diplomonads possess two nuclei, no functional mitochondria (though they retain mitochondria-derived organelles), and multiple flagella. Giardia lamblia, a diplomonad, causes giardiasis -- a common intestinal infection acquired from contaminated water. The Parabasalids have an undulating membrane and hydrogenosomes instead of mitochondria; Trichomonas vaginalis, a parabasalid, causes the most common non-viral sexually transmitted infection in humans.
The Euglenozoans are a diverse group that includes the euglenids and the kinetoplastids. Euglenids are mixotrophic organisms that acquired their chloroplasts through secondary endosymbiosis from green algae. They possess a distinctive protein pellicle instead of a rigid cell wall, and many have an eyespot that allows them to orient toward light. Kinetoplastids are characterized by a single large mitochondrion containing a kinetoplast -- a concentrated mass of DNA. This group includes several devastating human pathogens: Trypanosoma brucei causes African sleeping sickness (transmitted by the tsetse fly), Trypanosoma cruzi causes Chagas disease, and Leishmania species cause leishmaniasis.
B. SAR Clade (Stramenopila + Alveolata + Rhizaria)
The SAR clade unites three major lineages that molecular data have shown to be closely related despite their dramatically different appearances and lifestyles.
The Stramenopila (or Heterokonts) are characterized by a distinctive hairy flagellum bearing hair-like projections. Diatoms are unicellular stramenopiles encased in intricate silica (glass) cell walls called frustules. They are major primary producers in marine ecosystems, responsible for approximately 20% of global photosynthesis. Brown algae (Phaeophyta) are large multicellular marine algae that dominate temperate coastal ecosystems; the giant kelp, reaching lengths of up to 60 meters, is among the largest of all protists. Oomycetes, or water molds, superficially resemble fungi but are not related to them -- they have cellulose cell walls rather than chitin and are classified among the stramenopiles. Phytophthora infestans, an oomycete, caused the Irish potato famine of 1845-1849.
The Alveolata are united by the presence of membrane-bound sacs called alveoli beneath their plasma membranes. Dinoflagellates possess two flagella and are frequently photosynthetic; some are bioluminescent. Zooxanthellae, the symbiotic dinoflagellates that live within coral tissues, provide up to 90% of a coral's energy supply. Toxic dinoflagellate species such as Karenia brevis cause harmful algal blooms known as red tides. Apicomplexans are obligate intracellular parasites equipped with an apical complex of organelles used to penetrate host cells. Plasmodium species cause malaria, which kills approximately 600,000 people per year globally, and Toxoplasma gondii causes toxoplasmosis, dangerous to immunocompromised patients and during pregnancy. Ciliates are covered in cilia and possess two types of nuclei: a macronucleus that handles everyday gene expression and a micronucleus reserved for reproduction. Paramecium, a classic model organism, uses contractile vacuoles to manage osmoregulation in its freshwater habitat and can exchange genetic material through conjugation.
The Rhizaria are amoeboid organisms that extend thin pseudopodia. Foraminifera (forams) are marine protists that secrete calcium carbonate (CaCO3) shells perforated with pores through which threadlike pseudopodia extend. Their shells are important index fossils in paleontology and play a significant role in the global carbon cycle. Radiolaria produce intricate silica skeletons with pseudopodia radiating outward.
<image>A composite figure showing representative protists from major groups. Panel A: A diatom (Stramenopila) — showing the intricate silica frustule with radial symmetry, with a magnified inset of the pore pattern. Panel B: Plasmodium life cycle (Alveolata/Apicomplexa) — showing the mosquito vector injecting sporozoites, liver stage producing merozoites, red blood cell infection and lysis cycle, and gametocyte formation taken up by mosquito. Panel C: A foraminiferan (Rhizaria) — showing the CaCO3 shell with pores and threadlike pseudopodia extending outward to capture food particles. Panel D: Paramecium (Alveolata/Ciliata) — labeled cross-section showing cilia, macronucleus, micronucleus, food vacuoles, oral groove, and contractile vacuole.</image>
C. Archaeplastida
The Archaeplastida includes all organisms whose plastids arose through primary endosymbiosis -- the direct engulfment of a cyanobacterium by a heterotrophic eukaryote. Red algae (Rhodophyta) are mostly marine and derive their characteristic color from the pigment phycoerythrin. They lack flagellated stages and produce cell walls containing agar and carrageenan, substances of considerable commercial value. Green algae (Chlorophyta and Charophyta) share chlorophyll a and b with land plants, store starch, and have cellulose cell walls. Charophytes are the closest living relatives of land plants and are therefore of immense evolutionary significance. Green algae range from unicellular forms (Chlamydomonas) to colonial organisms (Volvox) to multicellular species (Ulva, or sea lettuce). Land plants (Embryophyta) evolved from charophyte ancestors and are covered in subsequent lectures.
D. Amoebozoa
The Amoebozoa move using lobe-shaped pseudopodia (lobopods) and include both free-living amoebas and the remarkable slime molds. Entamoeba histolytica, a parasitic amoeba, causes amoebic dysentery. Plasmodial slime molds (Myxomycetes) form a large multinucleate mass called a plasmodium that streams over surfaces engulfing bacteria before producing spore-bearing fruiting bodies. Cellular slime molds, exemplified by Dictyostelium, lead solitary lives as individual amoeboid cells when food is abundant but aggregate into a multicellular slug when food becomes scarce, eventually forming a fruiting body. Dictyostelium has become an important model organism for understanding cell signaling and the evolution of multicellular cooperation.
E. Opisthokonta
The Opisthokonta unites fungi and animals as sister groups, along with several protist lineages. The Choanoflagellates are collar-flagellated protists that bear a striking resemblance to the choanocytes (collar cells) of sponges, making them the closest living protist relatives of animals. Some choanoflagellates form colonies, and these colonial forms may model the transition from unicellular to multicellular animal life.
III. Endosymbiosis and Plastid Diversity
The distribution of photosynthesis across the eukaryotic tree reflects a complex history of endosymbiotic events. Primary endosymbiosis occurred when an ancestral heterotrophic eukaryote engulfed a cyanobacterium, giving rise to the Archaeplastida (red algae, green algae, and land plants). The plastids produced by this event are surrounded by two membranes -- the inner and outer membranes of the original cyanobacterium.
Secondary endosymbiosis occurred when a different heterotrophic eukaryote engulfed a photosynthetic eukaryote that already carried a primary plastid. This explains the plastids found in stramenopiles (derived from a red algal endosymbiont), euglenids (derived from a green algal endosymbiont), and dinoflagellates. The additional membranes acquired during secondary endosymbiosis -- from the engulfed eukaryote's plasma membrane and the host's food vacuole -- explain why these plastids are surrounded by three or four membranes rather than two. Tertiary endosymbiosis has occurred in some dinoflagellates that replaced their original plastids by engulfing other algae. This layered history of endosymbiotic events explains the mosaic distribution of photosynthesis across the eukaryotic tree.
<image>A diagram tracing the evolutionary history of plastids through endosymbiotic events. Panel A (Primary endosymbiosis): A heterotrophic eukaryote engulfs a cyanobacterium; the cyanobacterium becomes a chloroplast with two membranes. An arrow shows this leading to the Archaeplastida lineage (red algae, green algae, land plants). Panel B (Secondary endosymbiosis): A heterotrophic eukaryote engulfs a red alga (or green alga); the algal cell is reduced but retains its chloroplast, now surrounded by 3–4 membranes. Arrows show this leading to stramenopiles (from red algal endosymbiont) and euglenids (from green algal endosymbiont). Each stage is illustrated with cell diagrams showing the number of surrounding membranes and the origin of each membrane layer labeled.</image>
IV. Ecological and Medical Importance
The ecological significance of protists can hardly be overstated. Phytoplankton -- primarily diatoms, dinoflagellates, and coccolithophores -- are responsible for approximately 50% of all global photosynthesis and oxygen production. Marine protists such as foraminifera and coccolithophores fix carbon into calcium carbonate shells; when these organisms die, their shells sink to the ocean floor, representing long-term carbon sequestration that eventually forms chalk and limestone deposits. Protists form the base of aquatic food webs, with zooplankton grazing on phytoplankton and transferring energy to higher trophic levels. Symbiotic protists are critical to several major ecosystems: coral reef ecosystems depend on zooxanthellae, lichens contain algal symbionts (or cyanobacteria), and termite gut protists digest the cellulose that termites cannot break down on their own.
On the medical front, protists cause some of the world's most devastating diseases. Malaria (Plasmodium), sleeping sickness (Trypanosoma), giardiasis, amoebic dysentery, and toxoplasmosis collectively afflict hundreds of millions of people. Harmful algal blooms (HABs), fueled by nutrient pollution and eutrophication, produce toxins that threaten marine life and human health.
