Comparative studies on biomass productivity and lipid content of a novel blue-green algae during autotrophic and heterotrophic growth

Algae have long been acclaimed as the attractive renewable source for generating third-generation biofuels, particularly biodiesel. Under the present investigation, the trends of production of biomass and lipid during the autotrophic and heterotrophic growth of newly isolated blue-green algae, Leptolyngbya subtilis JUCHE1, were compared and correlated with the variation in C-sources. In the autotrophic and heterotrophic growth studies, CO₂ and glycerol were respectively used as the inorganic and organic C-sources maintaining equivalence in the initial amount of carbon. Light was used as the source of energy in both cases. The concentration of CO₂ in the feed gas stream was varied from 5 to 20% (% v/v). Equivalent quantity of carbon was supplied through glycerol during heterotrophic growth. Small-scale closed algal bioreactors were used for growing the algae at 37 °C and 2.5 kLux light illumination in batch mode for 0–4 days. Primarily, higher biomass production from glycerol compared with CO₂ was observed. In case of photoautotrophic growth, the maximum values of biomass and lipid productivity, obtained at 15% CO₂, were 0.1857 g/L/d and of 0.020 g/L/d respectively. The maximum biomass productivity of 0.2733 g/L/d was obtained for photoheterotrophic growth at a glycerol concentration equivalent to 15% CO₂ (v/v). Under photoheterotrophic growth of Leptolyngbya subtilis JUCHE1, lipid productivity of 0.0702 g/L/d was obtained at glycerol concentration equivalent to 5% (v/v) CO₂, which is 4.66-fold higher than that obtained under corresponding photoautotrophic condition. The “switch-over” from the autotrophy to the photoheterotrophy instigated the oleaginous anabolism and consequent lipid enrichment in L. subtilis JUCHE1, which can be extracted and converted to biodiesel.