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Kingdom Protista: The Macroalgae or Seaweeds

Most of the photosynthetic organisms found in the ocean belong to a group of organisms classified by biologists as Protistans. Protistans include unicellular organisms such as formaniferans and diatoms—organisms that may be found in the intertidal zone but that are too small to be seen with the naked eye. The most familar types of Protistans are the seaweeds, which are multi-cellular organisms that are more complex than their unicellular counterparts in the Kingdom Protista.

"Seaweed" is an unfortunate term, because these organisms are not weeds. Biologists prefer the term macroalgae or macrophytes. Macroalgae are eukaryotic, photosynthesizing multi-cellular organisms that lack the specialized structures and reproductive mechanisms characteristic of true plants. Macroalgae lack the true leaves, stems, and roots of plants but have analagous parts: the thallus, blades, pneumatocysts, stipe, and holdfast. The thallus is the complete "body" of the macroalgae and includes the blades, stipe, and pneumatocysts. The blades are the leaf-like flattened portions of the thallus, and the stipe is the stemp-like sstructure that provides support. The pneumatocysts are gas-filled bladders present on some macroalgae to help keep the blades near the sunlit surface. A structure that looks like a root, the holdfast, attaches the thallus to the bottom.

There are three types of macroalgae: green algae (Phylum Chlorophyta), the brown algae (Phylum Phaeophyta), and the red algae (Phylum Rhodophyta). The macroalgae in these phyla are distinguished by the different types of photosynthetic pigments found in their cells.

Click on the links below to learn more about different types of macroalage that are found in the intertidal of the Gulf of California. To go back to the Gallery of Marine Life, click here.

To take a practice quiz on the Macroalgae, click here.


page last updated on 7/15/03

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