What are algae?



Algae are photosynthetic protists and bacteria that can be thought of as simple plants.  They are organized by genetic similarities by kingdom and division.  The casual observer also can view them as large or small:  MACRO- large or MICRO- small algae.  


Phaeophyta: The Brown Algae

The brown algae, or Phaeophyta, are brown seaweeds that are especially abundant along rocky coasts in cold water. Brown algae are Macrophytes that include the giant kelps, which are located along the Pacific coast and form forests and are keystone species to a wide range of marine life. Some species of brown algae have a part of their thallus similar to roots called a holdfasts that anchor the algae to the substrate.   Photosynthesis mainly occurs in leaf-like structures called 'blades'.  These blades absorb nutrients and perform photosynthesis.  The blades are buoyed up toward the water's surface by air bladders called pneumatocysts.  Brown algae contain a brown-colored pigment called fucoxanthin that gives the plants their characteristic dark color. Other well-known brown algae are the common rockweed Fucus and Nereocystis (bull whip kelp).

  • Pigments include chlorophyll-a and -c, and fucoxanthin
  • They store their energy as Laminarin (type of oil)
  • Cell walls composed of cellulose with alginic acids


 Phaeophyte, Macrocystis Rockweed Nereocystis
Phaeophyte- Macrocystis pyrifera: giant kelp Phaeophyte- Fucus:
Phaeophyte- Nereocystis:  Bull whip kelp




Rhodophyta: The Red Algae

The red algae, or Rhodophyta, are marine algae that are most often found in shallow waters and deep tropical seas. A few also occur in freshwater. Their morphology ranges from single celled microalgae to macroalgae.  The larger species have filaments that are massed together and resemble the leaves and stems of plants. They have no flagella and typically grow attached to a hard substrate or on other algae as an epiphyte. Some species contain a red pigment phycoerythrine; others range in color from green to red, purple, and greenish-black. The cell walls of coralline red algae become heavily encrusted with calcium carbonate. 

  • Pigments include Chlorophylls -a and -d and the red pigment phycoerythrine
  • They store energy as starch
  • Cell walls are composed of cellulose or pectin, many strains can produce calcium carbonate structures similar to corals 


Rhodophyta, Bossiella Nori  Chondracanthus

Rhodophyte- Bossiella: 
coralline red algae

Rhodophyte- Porphyra: Commonly called nori and used to wrap sushi Rhodophyte- Chondracanthus: The Turkish Towel




Cyanobacteria, The Blue-Green Algae

Blue-green algae make up the division Cyanophyta in the kingdom Eubacteria.  They are prokaryotic and have no membrane-bound organelles.  Cyanobacteria gets its common name from the blue-green pigment, phycocyanin, which along with chlorophyll a gives cyanobacteria a blue-green appearance. Phycocyanin is a protein that functions as the photosynthetic pigment in photosystem II, whereas in plants chlorophyll b is the pigment in photosystem II.

Cyanobacteria have a wide variety of habitats that range from frozen lakes, to acidic bogs, to deserts and volcanoes. They are most commonly found in alkaline aquatic environments (but also in aquatic environments ranging in salinity and acidity), they can also be found in soil, on rocks, and even in the atmosphere.

Nostoc spirulina

Cyanobacteria- Nostoc:  
A nitrogen fixing bacteria

 Cyanobacteria- Arthrospira:  
Spirulina, an edible algae


Chlorophytes, the Green Algae

The green algae, or Chlorophyta, occur mainly in freshwater although are common in brackish and marine ecosystems. Green algae are usually single-celled and microscopic, although some form colonies that are considered macroscopic. The colonies have very interesting morphology forming spherical (round) colonies composed of many cells or occurring as straight or branched filaments (long, thin series of cells). Green algae are thought to be in the evolutionary line that gave rise to the first land plants and are often called grass-green algae because of the similarities in pigments to terrestrial plants.  


chlorella Ulva green algae

Chlorophyte- Chlorella: an
edible green algae

Chlorophyte- Ulva: sea lettuce



Dinoflagellata, the Dinoflagellates

Dinoflagellates, have two flagella used for locomotion, one around their thallus like a belt, and the other at their bottom.  Most of these microscopic species live in saltwater, with few occurring in freshwater. Some species of dinoflagellates, including Pyrocystis fusiformis, emit bright flashes of light called bioluminescence when disturbed.  This is believed to 'blind' a predator, or if the predator eats the cell, the bioluminescence will light up the predator exposing it predation by a larger organism.



Dinoflagellate- Pyrocystis
Bioluminescent dinoflagellate

Dinoflagellate-  Gymnodinium


Bacillariophytes, the Diatoms

Bacillariophytes, or diatoms, are single-celled algae that have silica shells (frustules) with very intricate patterns and symmetrical shapes. They are one of the most common forms of phytoplankton and are an important food source for aquatic life, living in both fresh- and saltwater environments. Diatomaceous earth is mined deposits of fossilized diatoms; due to diatom's hollow and silica frustules, diatomaceous earth has a wide selection of commercial uses, such as in “natural” pest control, cosmetic abrasives, and water filtration.

To read more about the specific diatom strain Thalassiosira click here.


SEM diatoms
 Bacillariophyte- Thalassiosira
eccentrica: living in a glass house
(photo: Swedish Meterological
and Hydrological Inst)

Bacillariophyte- bunch o' diatoms





Haptophytes are another clade of photosynthetic unicellular algae, containing fucoxanthin and diadinoxanthin to give them a golden-brown color. They can be responsible for large, biologically harmful algal blooms- although they don't produce a toxin, large quantities of these alage can clog the gills of fish. Under normal circumstances, however, they are a great source of food for marine life. Unlike bacillariophytes, the shells of haptophytes are either unmineralized carbohydrates or calcium carbonate. The most common representative are coccolithophores which look like spheres of calcified pineapple rings. When coccolithophores die, they sink to the bottom of the ocean where their plates form carbonate oozes, contribute to sediment, and create limestone and chalk.


Haptophyte- Coccolithophore
Emiliania huxleyi (photo: Young
el al. 2003)
White Cliffs of Dover made from
fossilized coccolithophores
(photo: Immanuel Giel)


Young, J. R., Geisen, M., Cros, L., Kleijne, A., Probert, I. and Ostergaard, J. B. 2003. A guide to extant coccolithophore taxonomy, Journal of Nannoplankton Research, 1, 1–132.