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Many people, if not most, believe seaweed to be a plant. Is it?

Seaweed is actually a plant-like protist, which are also known as algae. The green color is due to what pigment? Algae, like plants, obtain their energy through photosynthesis.

Plant-Like Protists: Algae

Plant-like protists are called algae (singular, alga). They are a large and diverse group. Some algae, the diatoms , are single-celled. Others, such as seaweed , are multicellular (see Figure below ).

Both unicellular diatoms and multicellular seaweed are algae

Diatoms are single-celled algae. Other forms of algae are multicellular.

Why are algae considered plant-like? The main reason is that they contain chloroplasts and produce food through photosynthesis . However, they lack many other structures of true plants. For example, algae do not have roots, stems, or leaves. Some algae also differ from plants in being motile. They may move with pseudopods or flagella. Although not plants themselves, algae were probably the ancestors of plants.

Ecology of Algae

Algae play significant roles as producers in aquatic ecosystems. Microscopic forms live suspended in the water column. They are the main component of phytoplankton . As such, they contribute to the food base of most marine ecosystems.

Multicellular seaweeds called kelp may grow as large as trees. They are the food base of ecosystems called kelp forests (see Figure below ). Kelp forests are found throughout the ocean in temperate and arctic climates. They are highly productive ecosystems.

Kelp forests help sustain large communities

Kelp Forest. This kelp forest supports a large community of many other types of organisms.

Classification of Algae

Types of algae include red and green algae, euglenids , and dinoflagellates (see Table below for examples). Scientists think that red and green algae evolved from endosymbiotic relationships with cyanobacteria. Their chloroplasts have two membranes because the cell membranes of the cyanobacteria became additional plasma membranes of the chloroplasts. Scientists think that euglenids and dinoflagellates evolved later, from endosymbiotic relationships with green and red algae. This is why their chloroplasts have three membranes. Differences in the types of chlorophyll in the four types of algae also support the hypothesized evolutionary relationships.

Type of Algae Origin of Chloroplast Type of Chloroplast

Red algae

cyanobacteria two membranes, chlorophyll like the majority of cyanobacteria

Green algae

cyanobacteria two membranes, chlorophyll like a minority of cyanobacteria


green algae three membranes, chlorophyll like green algae


red algae three membranes, chlorophyll like red algae

Reproduction of Algae

Algae have varied life cycles. Two examples are shown in Figure below . Both cycles include phases of asexual reproduction (haploid, n ) and sexual reproduction (diploid, 2n ). Why go to so much trouble to reproduce? Asexual reproduction is fast, but it doesn’t create new genetic variation. Sexual reproduction is more complicated and risky, but it creates new gene combinations. Each strategy may work better under different conditions. Rapid population growth (asexual reproduction) is adaptive when conditions are favorable. Genetic variation (sexual reproduction) helps ensure that some organisms will survive if the environment changes.

Life cycles of algae

Life Cycles of Algae: Zygotic Meiosis (A), Gametic Meiosis (B) and Sporic Meiosis (C). In life cycle A (left), diploid (2n) zygotes result from fertilization and then undergo meiosis to produce haploid (n) gametes. The gametes undergo mitosis and produce many additional copies of themselves. How are life cycles B and C different from life cycle A?

KQED: Algae Power

QUEST explores the potential of algae-–once considered nothing more than pond scum–-to become the fuel of the future. Entrepreneurs from throughout California are working to create the next generation of biofuels from algae. But will you ever be able to run your car off it? See http://www.kqed.org/quest/television/algae-power for additional information.


  • Plant-like protists are called algae. They include single-celled diatoms and multicellular seaweed.
  • Like plants, algae contain chlorophyll and make food by photosynthesis.
  • Types of algae include red and green algae, euglenids, and dinoflagellates.


Use this resource to answer the questions that follow.

  1. What is the main characteristic of plantlike protists?
  2. Where do most plantlike protists live?
  3. What is a red tide?
  4. Describe the chromists. Give two examples.
  5. List three characteristics of brown algae.
  6. Describe red algae.
  7. What are three forms of green algae?


1. Compare and contrast algae and plants.

2. State pros and cons of asexual and sexual reproduction in algae.

3. Explain why dinoflagellates and euglenids have chloroplasts with three membranes instead of two.

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