One of the most primitive plants, one-celled or multicellular plant-like organisms that lack true stems, roots, and leaves, but usually contain chlorophyll. Algae can range from the microscopic (microalgae), to large (seaweed; macroalgae).They are distributed worldwide in the sea, in fresh water, and in moist environments on land. It is believed that the majority of oil and natural gas originates from algae – that died millions of years ago. With deeper burial, sufficient time and temperature, the soft parts of these organisms were slowly converted to hydrocarbons over millions of years.