Microalgae are single-cell microscopic organisms which are naturally found in fresh water and marine environment. Their position is at the bot­tom of food chains. Microalgae are considered to be one of the oldest living organisms in our planet. There are more than 300,000 species of micro algae, diversity of which is much greater than plants [3]. They are thallophytes — plants lacking roots, stems, and leaves that have chlorophyll as their primary photosynthetic pigment and lack a sterile covering of cells around the reproductive cells [4]. While the mechanism of photosynthesis in these microorganisms is similar to that of higher plants, microalgae are generally more efficient converters of solar energy thanks to their simple cellular structure. In addition, because the cells grow in aqueous suspen­sion, they have more efficient access to water, CO2, and other nutrients [2, 5]. Generally, microalgae are classified in accordance with their co­lours. The current systems of classification of microalgae are based on a) kinds of pigments, b) chemical nature of storage products, and c) cell wall constituents [2]. Some additional criteria are also taken into consid­eration including cytological and morphological characters: occurrence of flagellate cells, structure of the flagella, scheme and path of nuclear and cell division, presence of an envelope of endoplasmic reticulum around the chloroplast, and possible connection between the endoplasmic reticu­lum and the nuclear membrane [6]. Some major groups of microalgae are shown Table 1.

The oil contents of various microalgae in relation to their dry weight are shown in Table 2. It is clear that several species of microalgae can have oil contents up to 80% of their dry body weight. As mentioned ear­lier, some microalgae can double their biomasses within 24 hours and the shortest doubling time during their growth is around 3.5 hours which makes microalgae an ideal renewable source for biofuel production [7]. The oil content and types of microalgae available at fresh water and ma­rine water are shown separately in Tables 3 & 4.