What is the difference between a protist and an alga?

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The other day in class, someone asked what the real difference is between a protist and an alga. I remembered reading that while algae are technically protists, not all protists are algae. This answer breaks it down clearly with real examples, evolutionary context, and how classification has changed over time. It’s super helpful if you’re studying microbiology or teaching it.

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Jan Biermann 2025-06-12T18:01:44+00:00 1 Answer 4 views
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  1. fleming
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    2025-06-12T18:03:10+00:00
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    Relationship

    Algae as a Subset of Protists: Algae are traditionally considered a subset of protists. All algae are protists, but not all protists are algae.

    Modern Understanding: With advances in molecular phylogenetics, both terms have been recognized as artificial groupings that don’t perfectly reflect evolutionary relationships. However, they remain useful in certain contexts, particularly in ecology, biotechnology, and education.

    Definitions

    Protist: A term of convenience for eukaryotic organisms that are not animals, plants, or fungi. Protists represent a paraphyletic assemblage of diverse lineages distributed across multiple supergroups in modern eukaryotic classification.

    Alga (plural: algae): Photosynthetic protists that contain chlorophyll and perform oxygenic photosynthesis. The term encompasses a diverse array of organisms ranging from unicellular microalgae to large, complex seaweeds.

    Key Characteristics

    Protists (Broader Group):

    • Include photosynthetic forms (algae), heterotrophic forms (protozoa), fungus-like forms (slime molds, water molds), and mixotrophs
    • Exhibit diverse cellular structures, nutritional modes, and reproductive strategies
    • Distributed across multiple supergroups in modern eukaryotic classification

    Algae (Subset of Protists):

    • Primarily photosynthetic, containing chlorophyll and producing oxygen during photosynthesis
    • Possess chloroplasts (derived from cyanobacteria through endosymbiosis)
    • Range from unicellular forms to complex multicellular organisms
    • Lack the specialized tissues and organs found in land plants
    • Typically aquatic or found in moist environments

    Major Groups of Algae

    Algae are not a monophyletic group but rather represent several distinct evolutionary lineages that independently acquired chloroplasts:

    1. Red Algae (Rhodophyta):
      • Contain chlorophyll a and phycobilins (red pigments)
      • Lack flagella at all life stages
      • Include many seaweeds and some unicellular forms
      • Example: Porphyra (nori, used in sushi)
    2. Green Algae (Chlorophyta and Charophyta):
      • Contain chlorophyll a and b, similar to land plants
      • Store energy as starch
      • Charophytes are the closest relatives to land plants
      • Examples: Chlamydomonas, Spirogyra, Ulva (sea lettuce)
    3. Brown Algae (Phaeophyceae, within Stramenopiles/Heterokonta):
      • Contain chlorophyll a and c, plus fucoxanthin (brown pigment)
      • Exclusively multicellular, including the largest and most complex algae
      • Examples: Kelps (Macrocystis, Laminaria), Fucus (rockweed)
    4. Diatoms (Bacillariophyceae, within Stramenopiles/Heterokonta):
      • Unicellular algae with distinctive silica cell walls
      • Contain chlorophyll a and c, plus fucoxanthin
      • Major primary producers in oceans and freshwater
      • Example: Pinnularia
    5. Dinoflagellates (Dinophyceae, within Alveolata):
      • Typically have two flagella and distinctive cell coverings
      • Contain chlorophyll a and c, plus peridinin
      • Many are mixotrophic (both photosynthetic and heterotrophic)
      • Some cause red tides and produce toxins
      • Example: Ceratium
    6. Euglenoids (Euglenophyceae, within Excavata):
      • Flagellated protists with chlorophyll a and b
      • Many can lose chloroplasts and live heterotrophically
      • Example: Euglena

    Examples of Protists that are NOT Algae

    • Protozoa: Amoeba, Paramecium, Plasmodium (malaria parasite)
    • Slime Molds: Physarum polycephalum, Dictyostelium discoideum
    • Water Molds/Oomycetes: Phytophthora infestans (potato late blight pathogen)

    Ecological and Economic Significance

    Algae:

    • Primary producers in aquatic ecosystems, producing approximately 50% of the oxygen in Earth’s atmosphere
    • Form the base of aquatic food webs
    • Used in food (nori, spirulina), biofuels, fertilizers, and various industrial applications
    • Some produce toxins causing harmful algal blooms

    Other Protists:

    • Include important decomposers and consumers in ecosystems
    • Many are human, animal, or plant pathogens
    • Some have industrial or research applications

    Classification Challenges

    Several factors complicate the traditional distinction between algae and other protists:

    1. Secondary Loss of Photosynthesis: Some heterotrophic protists evolved from photosynthetic ancestors through the loss of chloroplasts.
    2. Mixotrophy: Many algae can switch between photosynthesis and heterotrophy, blurring the line between “algae” and “protozoa.”
    3. Endosymbiosis: Some protists harbor photosynthetic symbionts but are not themselves considered algae.
    4. Evolutionary Relationships: Molecular studies have shown that some algal groups are more closely related to certain non-algal protists than to other algae.

    Modern Classification Approach

    In modern eukaryotic classification, both protists and algae are distributed across multiple supergroups based on evolutionary relationships rather than traditional morphological or functional categories:

    • Archaeplastida: Includes red algae, green algae, and land plants
    • Stramenopiles/Heterokonta: Includes brown algae, diatoms, and water molds
    • Alveolata: Includes dinoflagellates, apicomplexans (like Plasmodium), and ciliates
    • Excavata: Includes euglenoids and many non-photosynthetic flagellates
    • Rhizaria: Includes some photosynthetic forms like chlorarachniophytes
    • Amoebozoa: Primarily non-photosynthetic, includes amoebas and slime molds

    Algae represent the photosynthetic subset of the broader protist category. While both terms are somewhat outdated from a strict phylogenetic perspective, they remain useful in certain contexts, particularly in ecology, biotechnology, and education. Modern classification systems focus on evolutionary relationships rather than traditional morphological or functional categories, distributing former “protists” and “algae” across multiple supergroups of eukaryotes.

    Source: Graham, L.E., et al. Algae; Adl, S.M., et al. (2019). Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes. Journal of Eukaryotic Microbiology.

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