Multicellular algae are groups of mostly aquatic plants that derive their food from the manufacture of chlorophyll during photosynthesis. They are composed of several cells and reproduce somewhat differently from other types of plant. They do not have roots, leaves or stems just like ordinary plants; that is why they are classified under the Kingdom Protista. Algae can be classified as unicellular, multicellular, filamentous or colonial. Multicellular algae have usually two modes of reproduction: the sexual and asexual (meiosis and mitosis, respectively.)
In the sexual phase, the first step in the reproduction is the formation of gametes (ovum and sperms) in the female and male gametangia. After these are formed, the sperm then swims to the ovum to fertilize it. The fertilized ovum, called the zygote, turns into zoospores by the formation of walls. The zoospores then undergo reproduction through meiosis in which one cell forms the holdfast and another one, the filament. This mode of reproduction in algae is often triggered by environmental stress.
In the asexual stage, replication of the genetic code happens through the production of haploid zoospores through mitosis. The haploid contains only one set of chromosomes. Cleavage occurs, and then it goes on to form identical cells from one parent algae. The sexual reproduction involves the fusion of the sperm and the egg to combine the female and male genes, giving birth to new cells. Usually, there are three major asexual stages of embryogenesis; these are: cleavage, inversion and expansion.
An example of reproduction in multicellular algae is that of the multicellular green algae called Volvox. In cleavage, Volvox algae undergo several rapid divisions until the embryo is inside out, and its gonidia are outside with the flagella leaning towardthe body. In inversion, the algae return to its adult form through the conformation of the cells to their specific shapes. In expansion, the young algae are filled with extracellular material for their eventual hatching (immature gonidia) and release from their parent spheroid to become mature gonidia.
Some examples of multicellular algae are Phaeophyta or brown algae, some species of green algae (Volvox) and Rhodophyta or red algae. The Phaeophyta or brown algae grow in cooler places and include seaweeds and kelps, while the species of green algae, Volvox, live in ponds and ditches. The Rhodophyta or red algae thrive in warm marine habitats and include freshwater species. Through the sexual and asexual modes of reproduction, they multiply and provide a crucial role in the food chain and environmental balance of Mother Earth.