Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 28.3 SAR Is a Highly Diverse Group of Protists Defined by DNA Similarities
The SAR supergroup—short for Stramenopiles, Alveolates, and Rhizarians—is a diverse and important clade of protists united by genetic evidence. These organisms include many key photosynthesizers, parasites, and ecological players in both marine and terrestrial environments.
Stramenopiles
- Diatoms:
- Unicellular algae with glass-like silica cell walls.
- Extremely abundant in oceans and lakes.
- Major photosynthetic producers; impact global CO₂ levels.
- During blooms, uneaten diatoms sink, pumping CO₂ to the ocean floor—a potential climate change mitigation method.
- Brown algae:
- Largest and most complex algae; all multicellular.
- Common along cold marine coasts.
- Seaweeds like kelp form underwater forests.
- Show analogous (not homologous) structures to plants (e.g., holdfasts, blades).
- Exhibit alternation of generations with heteromorphic forms.
- Oomycetes:
- Include water molds and downy mildews.
- Once grouped with fungi but have cellulose walls (not chitin).
- Many are decomposers or plant parasites (e.g., Phytophthora infestans—Irish potato famine).
Alveolates
Defined by alveoli—membrane-bound sacs beneath the plasma membrane.
- Dinoflagellates:
- Marine and freshwater phytoplankton; spin with two flagella in armored grooves.
- Some species cause red tides by forming toxic blooms.
- Can be heterotrophs, photoautotrophs, or mixotrophs.
- Apicomplexans:
- Animal parasites with an apical complex for host cell invasion.
- Include Plasmodium, the malaria-causing parasite.
- Complex life cycles involving multiple hosts.
- Plasmodium evades the immune system by changing surface proteins.
- The apicoplast (a relic plastid) is a target for vaccine development.
- Ciliates:
- Use cilia for movement and feeding (e.g., Paramecium).
- Have two types of nuclei: macronucleus (daily functions) and micronucleus (genetic exchange).
- Reproduce asexually by binary fission and exchange genes through conjugation.
Rhizarians
Mostly amoeboid protists with threadlike pseudopodia.
- Radiolarians:
- Marine protists with silica skeletons.
- Pseudopodia radiate outward to trap food.
- Skeletons accumulate as thick ocean sediments.
- Forams (Foraminiferans):
- Have porous calcium carbonate shells (tests).
- Use pseudopodia for movement and feeding.
- Some host symbiotic algae.
- Have an excellent fossil record useful in climate studies.
- Cercozoans:
- Amoeboid and flagellated protists found in soil and aquatic environments.
- Include heterotrophs, predators, and mixotrophs.
- Paulinella chromatophora represents a rare second case of primary endosymbiosis.
In a Nutshell
The SAR supergroup includes ecologically vital and diverse protists such as diatoms, kelp, malaria parasites, and ciliates. Defined by DNA similarities, not obvious morphology, SAR protists demonstrate key evolutionary themes: endosymbiosis, complex life cycles, and adaptations for photosynthesis, parasitism, and structural innovation.