The Algae is a collective name traditionally given to several phyla of primitive, and mostly aquatic plants, making up a highly diverse group of over 30 000 species. They display a wide variety of structure, habitat and life-cycle, ranging from single-celled forms to massive seaweeds tens of metres in length. Most algae share a number of common features which caused them to be grouped together. Among these are:
The Protista is a grouping of convenience, containing organisms not easily accommodated elsewhere. It includes all unicellular and colonial eucaryotic organisms, but is often expanded to include multicellular algae.
• possession of the pigment chlorophyll
• deriving energy from the sun by means of oxygenic photosynthesis
• fixing carbon from CO2 or dissolved bicarbonate (see Chapter 6).
Modern taxonomy attempts to reflect more accurately the relationship between organisms with an assumed common ancestor. Thus, in the following pages, the unicellular 'algae' are discussed in relation to other unicellular eucaryotes. Multicellular forms, including the Phaeophyta (brown algae) and Rhodophyta (red algae), are not discussed at great length and are included for the sake of completeness.
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