Best practices in heterotrophic high-cell-density microalgal processes: achievements, potential and possible limitations

Microalgae of numerous heterotrophic genera (obligate or facultative) exhibit considerable metabolic versatility and flexibility but are currently underexploited in the biotechnological manufacturing of known plant-derived compounds, novel high-value biomolecules or enriched biomass. Highly efficient production of microalgal biomass without the need for light is now feasible in inexpensive, well-defined mineral medium, typically supplemented with glucose. Cell densities of more than 100 g l−1 cell dry weight have been achieved with Chlorella, Crypthecodinium and Galdieria species while controlling the addition of organic sources of carbon and energy in fedbatch mode. The ability of microalgae to adapt their metabolism to varying culture conditions provides opportunities to modify, control and thereby maximise the formation of targeted compounds with non-recombinant microalgae. This review outlines the critical aspects of cultivation technology and current best practices in the heterotrophic high-cell-density cultivation of microalgae. The primary topics include (1) the characteristics of microalgae that make them suitable for heterotrophic cultivation, (2) the appropriate chemical composition of mineral growth media, (3) the different strategies for fedbatch cultivations and (4) the principles behind the customisation of biomass composition. The review confirms that, although fundamental knowledge is now available, the development of efficient, economically feasible large-scale bioprocesses remains an obstacle to the commercialisation of this promising technology

Heterotrophic growth and oil production from Micractinium sp. ME05 using molasses

In this study the thermo-resistant green alga Micractinium sp. ME05 was cultivated in media containing molasses as a carbon source. Shake flask experiments and 2-L bioreactor experiments were conducted at different inoculum ratios, aeration rates, and agitation speeds. The experimental condition which resulted in the highest biomass concentration (3.73 +/- 0.45 g L-1) with 10% inoculum in 500-mL flasks was scaled up to 2-L flasks at two aeration rates (0.25 and 0.5 L min(-1)). An increase in biomass concentration from 2.35 +/- 0.53 to 3.06 +/- 0.21 g L-1 was observed with an increase of aeration rate from 0.25 to 0.50 L min(-1), which demonstrated significant effect of aeration rate on biomass concentration (p = 0.000 < 0.05). In 2-L bioreactor experiments, highest biomass productivity (0.53 +/- 0.076 g L-1 day(-1)) and lipid productivity (7.7 +/- 1.6 g L-1 day(-1)) were obtained with 5% (v/v) inoculum and 50 rpm agitation speed. The principal fatty acids were palmitic acid (C16:0) and linoleic acid (C18:2) comprising 30.2 +/- 1.01 and 45.2 +/- 1.32% of the total fatty acid content, respectively. Thus, the present study highlights the possibility of using molasses for biomass and lipid production with Micractinium sp. ME05 under different cultivation conditions. Using low cost feedstock such as molasses would be valuable in terms of evaluating waste materials for further biodiesel production