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Unicellular Algae
Unicellular algae are composed of a single cell. They are simply structured and more primitive in nature than the multicellular forms. Many are found free-floating in the ocean currents and some have flagellae that enable them to move slowly through the water. Some species of euglenoids, a protozoan-like algae, feed on organic matter in the water and do not depend on photosynthesis.
Multicellular Algae
Multicellular algae are made up of multiple cells and have a more complex tissue structure, which can be organized into specialized organelles, each serving a specific function. These algae are usually attached to a substrate, such as an ocean or stream floor. Some have leaf-like structures to aid with photosynthesis. Multicellular algae, like seaweed, also tend to be larger than unicellular algae.
Reproduction
With the term "algae," covering so many different divisions and species, reproduction strategies of algae run the gamut from simple asexual cell division to more complicated sexual reproduction processes. Many algae can also switch from one form to another. Asexual reproduction can include binary fission, budding, fragmentation or the formation of spores. Sexual reproduction involves the union of two gametes to form a zygote.
Importance
Algae are not only an important source of the world's oxygen; microscopic algae are essential primary producers in their ecosystems. These photosynthetic organisms are able to transform energy from the sun into readily available carbon sources for other organisms forming the basis of the food web. Algae are also used for food additives, such as agar, and they are gaining popularity as an alternative fuel source.