Algae grows via photosynthesis, which requires the presence of carbon dioxide, light (usually from the sun) and water. Photosynthesis is the process of using light energy to convert carbon dioxide and water into glucose, a sugar which can be broken down by an organism into chemical energy. This chemical energy then fuels the organism's function and growth.
A broad spectrum of organisms fall under the classification of algae including microscopic single-celled organisms such as diatoms, as well as giant, complex organisms like seaweed. Certain kinds of seaweed, such as the eastern Pacific kelp, can grow to several hundred feet in size. Although all algae nourish themselves via photosynthesis, they reproduce in a variety of ways, both sexual and asexual.
In asexual reproduction, an organism uses only its own genetic material to reproduce. Many of the single-celled algae reproduce via binary fission, meaning an alga simply splits in two, yielding two new individual algae. Even some of the more complex, multicellular algae reproduce asexually. For instance, some seaweeds reproduce by fragmentation or budding, where parts of the seaweed break off into fragments or buds that develop into new algae.
In sexual reproduction, two individual organisms recombine their DNA to form a new individual. In a process similar to land plants, many seaweeds reproduce sexually. Although, in a stunning display of versatility, some seaweeds are able to reproduce both sexually and asexually.
The reproductive versatility of algae works to the benefit of the planet's health. One of the by-products of photosynthesis is oxygen. Algae produce more of the oxygen in the atmosphere than all land plants combined. Oxygen is essential for the survival of many of the planet's species, including human beings.