Life on earth is believed to have started with algae. Algae are photosynthetic organisms capable of converting solar energy into chemical energy and in the process, consume CO2 and release O2. With the advent of fossil fuels, the focus shifted from tapping solar energy via photosynthesis toward burning the fossil fuel to generate energy. The predominant use of fossil fuels over several years has made us realize the dangers that the GHGs released from burning these fuels, pose to the environment. The rapid depletion of these fuels and the millennia required for their renewal have now forced us to look for safer and renewable alternatives to the fossil fuels. Thus, we have now come a full circle toward again reverting to biomass, the unlimited solar energy and photosynthesis, to meet our ever increasing energy demands. Algae are considered to be the most photosyntheti­cally efficient plants on the earth. There is a large variety of algae ranging from small unicellular organisms to fairly complex and differentiated forms of multi­cellular organisms. They thrive on land as well as in water, using sunlight, CO2, and water for growth. Like other plants, algae use photosynthesis to convert solar energy into chemical energy and store it in the form of high energy substances such as oils, carbohydrates, and proteins. In other plants, the predominant store­house of this chemical energy is carbohydrates, whereas in algae, this energy is stored in the form of oils. A one-hectare algae farm on wasteland can produce over 10-100 times the oil produced by any other oil crop known till date [30]. The lipid content in different species of microalgae was found to vary from as low as 12-14% of dry weight in Scenedesmus obliquus, to as high as 80% of dry weight in Botyococcus braunii [31]. Algal energy is becoming increasingly popular because algae can be grown on wastelands and unarable lands, thus enabling all agri­cultural land completely available for growing food crops. Thus, third-generation biofuels, free from the food versus fuel controversy, can be obtained in abundance by efficient cultivation and harvesting of algae.

Algae can be classified on the basis of their fundamental cellular structure, life cycle, and pigmentation. According to the cellular structure, algae may be either unicellular or multicellular. The multicellular algae growing mainly in saltwater or freshwater are called macroalgae or ‘‘seaweeds’’. There are three different types of pigmentations seen in macroalgae: (1) green seaweed (Chlorophyceae), (2) red seaweed (Rhodophyceae), and (3) brown seaweed (Phaeophyceae). Microalgae which are microscopic photosynthetic organisms growing in both marine and freshwater environments are called microalgae (Cyanophyceae or blue-green algae).