**1. Introduction**

In this twentieth century due to the shortage in the conventional energy sources as well as exponentially rising trend of environmentally harmful products, microalgae have been chosen as an alternative source for a wide variety of metabolic products, viz. dietary supplements, pharmacological compounds, lipids, enzymes, biomass, polymers, toxins, pigments, wastewater treatment, and "renewable energy".

The microalgal cultivation chiefly follows autotrophic growth. Almost all microalgae are photosynthetic in nature having chlorophyll a, chlorophyll b and bacterio-chlorophyll in some blue-green algae (cyanobacteria) [1], and thus are

significant solar energy convertors, so, they are cultivated in illuminated environments either in open or closed cultivation systems [2].

For the last two decade microalgae have been recognized as a prominent alternative source for oil production. Several oleo genic species of microalgae can be manipulated to overproduce specific lipids and fatty acids through alteration of their physical and chemical properties of the culture medium. Microalgae can accumulate substantial amounts of lipids – up to 50% of dry cell weight in certain species [3]. Many microalgal species can thrive well in water with high salt concentration, viz. brackish water or seawater, thereby avoiding the demand for fresh water which has been designated as a limited resource in many parts of the world [4].

In microalgae, lipids play an important role in the synthesis of plasma membranes lipid protein lipid structure, in maintaining buoyancy and as an energy reserve during adverse growth conditions [5]. Accumulation of lipids in the microalgae can be attributed to the consumption of sugars at a rate higher than that of the rate of cell doubling, which promote conversion of excess sugar into lipids which favor the algae in its stationary phase of growth to fight the nutrient depletion [6].

Different saturated Fatty acids (SFA), monounsaturated Fatty acids (MUFA), and polyunsaturated Fatty Acids (PUFA) have been reported in microalgae. Hexadecanoic acid (C16:0) and oleic acid (C18:1) are common fatty acids in the microalgae. Omega-3 (ω-3) FA) can be traced in some [7]. Significantly, the microalgae can produce essential Fatty Acids, viz. alphalinolenic acid, which can be converted to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in mammals by metabolic pathways [8]. The concentration of EPA is insignificant in the terrestrial plants. Seaweeds, such as Palmaria palmata [9] can produce EPA, but in lesser concentration in comparison to the microalgae [10].

Microalgae have been designated as a producer of different types of renewable biofuels viz. methane produced by anaerobic digestion of the biomass [11], biodiesel derived from oil [12–14]; and biohydrogen can be produced photo-biologically [15, 16]. The concept of using microalgae as a source of fuel is not new [17], but for the past few decades it has being taken seriously because of the escalating price of petroleum and, most importantly, the emerging concern about global warming that is associated with the burning of fossil fuels [18].
