INTRODUCTION

Energy plays a very vital role in the development f a region. Dwindling stock of fossil fuels coupled with the increased demand of conventional energy sources and impending changes in the climate due to enhanced levels of GHG’s (Green house gases) have resulted in the fuel crisis, necessitating for sustainable energy replacement. Major research emphasis have been on the development of petroleum, coal, and natural gas based refinery to exploit the cheaply available fossil feedstock. Declining global oil productive capacity, economic vulnerability and global climate change are the prime movers of a clean energy drive across the globe. Currently, the fossil resources are not regarded as sustainable and questionable from the economic; ecology and environmental point of view (Kamm et al., 2006). Cost-effective renewable energy sources and other energy alternatives is the need of the hour (Groom et al., 2008).

DIATOMS AS A PROMISING CANDIDATE FOR BIOFUEL PRODUCTION

Diatoms (Greek “cut in half”) under class Bacillariophyceae are eukaryotic, autotrophic microorganisms with a ubiquitous distribution. They are characterized by a unique siliceous (SiO2 x nH2O) cell wall (Round et al., 1990). Diatoms have large proportion of lipids in their body, about 70% of their dry weight. Diatom lipids have been suggested as a potential diesel with emphasis on the neutral lipids due to their lower degree of unsaturation and their accumulation in algal cells at the end of growth stage. Both polar lipids and non polar (neutral) lipids are found in diatoms.

STRESS ON DIATOMS

In diatoms stress or unfavourable condition leads to the production of more lipids (Hu et al., 2008). Environmental parameters like pH, temperature, light, nitrogen, carbon, silicon, phosphorous, iron, salt concentration, etc affect the lipid composition of the diatoms. Lipids act as a secondary metabolite for diatoms and these lipids maintain specific membrane functions and cell signaling pathways in microalgae and play a role in responding to changes in the environment (Hu et al., 2008). Stress condition ceases the growth of the species, thereby reducing the biomass content but enhances the lipid composition.

FACTORS RESPONSIBLE FOR ACCUMULATION OF TRIACYLGLYCEROL AND CHANGES IN FATTY ACID COMPOSITION

The lipid content, lipid class composition and the proportions of the various fatty acids in a microalgae vary according to the environmental or culturing variables such as light intensity, growth phase photoperiod, temperature, salinity, CO2 concentration, nitrogen and phosphorous concentration (Dunstan et al., 1993).

NUTRIENT STRESS

NITROGEN: Most critical nutrient affecting lipid metabolism in microalgae is nitrogen limitation (Kilham et al., 1997). Nitrogen limitation leads to a decrease in protein content in both freshwater algae  and diatoms. Cell size for nutrient limited cultures were significantly smaller than the non limited cells size and was more pronounced in N-limited cells (Lynn et al., 2000). Microalgae cells are capable of using organically combined nitrogen forms, especially amino acids, urea and purines, as their sole nitrogen source (Fabregas et al., 1997). Li et al., 2008 investigated the effect of different nitrogen sources sodium nitrate, urea and ammonium bicarbonate on the lipid content of green algae Neochloris oleoabundance. It turned out that sodium nitrate is the most favorable nitrogen source for both cell growth and lipid accumulation of N. oleoabundans.

SILICA: In diatoms, silicon is an equally important nutrient that affects cellular lipid metabolism. For example, silicon-deficient Cyclotella cryptica cells had higher levels of neutral lipids (primarily TAG) and higher proportions of saturated and mono-unsaturated fatty acids than silicon-replete cells (Roessler, 1988). Tilman (1977) studied the effect of wide range of Si: P supply ratios on the mixed population of the two diatoms A. formosa and Cyclotella meneghiniana grown in semicontinuous cultures. When the molar ratio of these elements was smaller than 6:1 by moles, A. formosa dominated, while C. meneghiniana dominated when the ratio exceeded 90:1 by moles.

OBJECTIVE: The objectives of this work were to find out whether different nitrogen sources contribute to the accumulation of lipids.