Protists

1. The Origin of Eukaryotes: Endosymbiosis

Eukaryotic cells are chimeric—they combine features of their own lineage with those of engulfed bacteria. The endosymbiotic theory holds that mitochondria evolved from an aerobic bacterium taken in (or invading) an ancestral archaeal cell, and chloroplasts evolved from a cyanobacterium engulfed by an early eukaryote. Evidence includes double membranes, their own circular DNA, and bacteria-like ribosomes. A primary endosymbiosis gave rise to the chloroplasts of red and green algae (and their descendants, the plants); later, secondary endosymbioses—one eukaryote swallowing another photosynthetic eukaryote—spread photosynthesis into many other protist lineages.

2. A Paraphyletic "Kingdom" Organized into Supergroups

Because plants, animals, and fungi evolved from within protist lineages, the traditional kingdom Protista is paraphyletic. Modern biology instead groups eukaryotes into supergroups that reflect real relationships, such as Excavata (e.g., Giardia, Euglena, Trypanosoma), the SAR clade (diatoms, brown algae, dinoflagellates, ciliates, apicomplexans), Archaeplastida (red algae, green algae, and land plants), Amoebozoa (amoebas and slime molds), and Opisthokonta (animals, fungi, and their protist relatives). These groupings continue to be refined as genomes accumulate.

3. Nutrition, Ecology, and Disease

Protists use every major nutritional strategy. Photoautotrophs such as diatoms and dinoflagellates fix carbon and release oxygen, contributing about half of global photosynthesis. Heterotrophs feed by phagocytosis or absorption, and include many parasites. Mixotrophs, such as Euglena, photosynthesize in light but feed on organic matter in the dark. Ecologically, photosynthetic protists anchor aquatic food webs and drive the biological carbon pump, while parasitic protists cause major human diseases—malaria (Plasmodium), sleeping sickness and Chagas disease (Trypanosoma), giardiasis (Giardia), and toxoplasmosis (Toxoplasma).

How it works

How a photosynthetic eukaryote arose by endosymbiosis:

1. An early eukaryotic cell engulfs a cyanobacterium (a photosynthetic prokaryote).
2. Instead of digesting it, the host keeps the cyanobacterium alive inside a membrane-bound compartment.
3. The cyanobacterium divides inside the host and is passed to daughter cells at cell division.
4. Over time, the cyanobacterium loses genes it no longer needs, transferring many to the host nucleus.
5. The reduced, specialized cyanobacterium becomes a chloroplast—an organelle with its own DNA but dependent on the host.
6. The host gains photosynthesis, opening new niches and later, through secondary endosymbiosis, spreading chloroplasts to other lineages.

Common confusions

Memory aids

"Protists are the P-side of the tree: Paraphyletic, PhotoProducers, and Parasites." Or remember the supergroups by their first letters—"SEA of Amoebas and Opisthokonts" for SAR, Excavata, Archaeplastida, Amoebozoa, Opisthokonta.

Quick review

Topic Recap

• "Protists" are a paraphyletic assemblage of mostly single-celled eukaryotes, not a kingdom.
• Endosymbiosis explains how eukaryotes acquired mitochondria and chloroplasts.
• Supergroups (Excavata, SAR, Archaeplastida, Amoebozoa, Opisthokonta) organize eukaryotic diversity.
• Protists are photoautotrophs, heterotrophs, and mixotrophs.
• They produce much of Earth's oxygen, drive aquatic food webs, and include major human parasites.

Knowledge Check

1. Why is the traditional kingdom Protista considered paraphyletic rather than monophyletic?
2. What evidence supports the endosymbiotic origin of mitochondria and chloroplasts?
3. How do diatoms contribute to both global oxygen production and long-term carbon storage?
4. What is the difference between primary and secondary endosymbiosis?

Answers and Rationales

1. A monophyletic group includes an ancestor and all its descendants, but "Protista" excludes plants, animals, and fungi, which also descend from that ancestor—so it leaves descendants out.
2. Both organelles have double membranes, their own circular DNA, and bacteria-like ribosomes, and divide like bacteria—all signs they were once free-living prokaryotes.
3. Diatoms photosynthesize, fixing CO₂ and releasing oxygen; when they die, their heavy silica shells sink, carrying carbon to the deep sea and locking it away for long periods.
4. Primary endosymbiosis engulfs a prokaryote (a cyanobacterium becomes a chloroplast); secondary endosymbiosis engulfs a whole photosynthetic eukaryote, giving the host a chloroplast that still retains remnants of its own nucleus.