Enhancing lipid production rates by increasing the activity of enzymes via genetic engineering
Lipid accumulation in algae typically occurs during periods of environmental stress, including growth under nutrient-deficient conditions. The lipid and fatty acid contents of microalgae vary in accordance with culture conditions. In some cases, lipid content can be enhanced by the imposition of nitrogen starvation or other stress factors. Biochemical studies have also suggested that acetyl-CoA carboxylase (ACCase), a biotin-containing enzyme that catalyzes an early step in fatty acid biosynthesis, may be involved in the control of this lipid accumulation process. Therefore, it may be possible to enhance lipid production rates by increasing the activity of this enzyme via genetic engineering.
Induction of Lipid Synthesis by Nutrient Deprivation in Microalgae - In an experiment, microalgal strains were screened in the laboratory for their biomass productivity and lipid content. Four strains (two marine and two freshwater), selected because of their robustness, high productivity and relatively high lipid content, were cultivated under nitrogen deprivation in 0.6-L bubbled tubes. Only the two marine microalgae accumulated lipid under such conditions; they are eustigmatophyte & Nannochloropsis sp. F&M-M24, which attained 60% lipid content after nitrogen starvation. These were subsequently grown in a photobioreactor to study the influence of irradiance and nutrient (nitrogen or phosphorus) deprivation on fatty acid accumulation. Fatty acid content increased with high irradiances (up to 32.5% of dry biomass) and following both nitrogen and phosphorus deprivation (up to about 50%). Further tests proved that under nutrient sufficient and deficient conditions, for specific strains, lipid productivity increased from 117 mg/L/day in nutrient sufficient media (with an average biomass productivity of 0.36 g/L/day and 32% lipid content) to 204 mg/L/day (with an average biomass productivity of 0.30 g/L/day and more than 60% final lipid content) in nitrogen deprived media. In a two-phase cultivation process (a nutrient sufficient phase to produce the inoculum followed by a nitrogen deprived phase to boost lipid synthesis) the oil production potential could be projected to be more than 90 kg per hectare per day. This is the first report of an increase of both lipid content and areal lipid productivity attained through nutrient deprivation in an outdoor algal culture. The experiments showed that this marine eustigmatophyte has the potential for an annual production of 20 tons of lipid per hectare in the Mediterranean climate and of more than 30 tons of lipid per hectare in sunny tropical areas. (Reference: Title: Microalgae for oil: Strain selection, induction of lipid synthesis and outdoor mass cultivation in a low-cost photobioreactor; University: Dipartimento di Biotecnologie Agrarie, Università degli Studi di Firenze, Piazzale delle Cascine 24, 50144 Firenze, Italy, Source: Biotechnol Bioeng, 2008 Jun 18)
Si Depletion - research into diatom lipid accumulation by silicon depletion
Cyclotella cryptica accumulated more lipid more rapidly after Si depletion. Further studies (by NREL, during the ASP Program) identified two factors that seemed to be at play in this species:
• Si-depleted cells direct newly assimilated carbon more toward lipid production and less toward carbohydrate production.
• Si-depleted cells slowly convert non-lipid cell components to lipids.
• During the ASP research at NREL, the highest lipid content occurred with Navicula, which increased from 22% in exponential phase cells to 49% in Si-deficient cells and to 58% in N-deficient cells.
• Coomls, et al. reported that the lipid content of the diatom Navioua pelliculosa increased by about 60% during a 14-hour silicon starvation period. Similarly, Werner also reported an increase in cellular lipids during a 24 hours silicon starvation period. The switch from carbohydrate accumulation to lipid accumulation in these Diatoms occurs very rapidly, though mechanisms involved are not yet fully understood.
