Improvement of eicosapentaenoic acid (EPA) production by the alga Trachydiscus sp. in concentric airlift photobioreactors.

The New Zealand microalga Trachydiscus sp. Awa9-2 is a recently identified freshwater microalga containing a high percentage of eicosapentaenoic acid (EPA), an essential omega-3 fatty acid for humans. The ability of species of microalgae to produce a high percentage of EPA makes them candidates for commercial production of this bioproduct. This study investigated the growth of Trachydiscus sp. Awa9-2 in order to evaluate its cultivation requirements and to determine the optimal growth conditions for production of EPA in bench-scale concentric airlift photobioreactors (PBRs). Preliminary studies in shake flasks and PBRs were performed to find a suitable culture medium, cultivation vessels and pH. The effect of culture medium, time of harvest, temperature and light intensity (photon flux density) were investigated using four PBRs. EPA productivity was investigated in four monochromatic lights (blue, green, red and photo red) and two combined wavelength ranges (white and blue+photo red) in PBRs. In addition, the study explored the impact of twelve wavelength ranges on algal growth and the effect of these in two- phase wavelength stress experiments on fatty acid composition and EPA accumulation of the algal biomass. To perform these, a special multi-chamber LED box was designed and constructed. The box was able to illuminate cultures at each of twelve wavelength ranges, all at the same light intensity. An experiment examined the effect of dark incubation at a range of temperatures on fatty acid composition of harvested biomass. Furthermore, an evaluation was undertaken of heterotrophic and mixotrophic ability of Trachydiscus sp. Awa9-2 and the impact of these cultivation methods on fatty acid composition and yield. Results showed that optimal growth and EPA production occurred in cultures in a PBR containing Z medium at pH 7, and with harvest of cells in the exponential phase. Temperatures of 15-25 °C were suitable for growth but growth was very poor at ≥ 28 °C. 25 °C resulted in the highest EPA and biomass productivity. Dark incubation of cultures caused the dry biomass and percentage of EPA, as a constituent of total fatty acids, to gradually decrease as incubation temperatures increased from 4 to 40 °C. Of the three light intensities tested, 130 µmol m⁻² s⁻¹ was optimal for growth and EPA productivity. Trachydiscus sp. Awa9-2 was able to grow heterotrophically on a range of organic carbon sources but the yield was considerably lower than phototrophic growth. The mixotrophic growth yield of the cultures supplemented with galactose and lactose was about three times higher than the controls in Z medium. However, mixotrophic cultivation did not enhance EPA productivity. In wavelength studies, the alga grew at all tested wavelength ranges but there was very little growth under true violet. Despite low whole-cell absorption of green light, a relatively large amount of biomass was obtained at this wavelength range. In PBRs, biomass grown in blue light resulted in the highest percentage of EPA and EPA productivity, while red and photo red showed the poorest results. Highest EPA productivity of Trachydiscus sp. Awa9-2 was observed in phototrophic conditions when the alga was in optimal growth conditions. The alga did not increase EPA productivity when placed under light or temperature stress. Overall, the highest EPA productivity (15.6 mg L⁻¹ day⁻¹) was achieved by cultivation of the alga in PBRs containing Z medium, illuminated with blue light (448 nm) at light intensity of 130 µmol m⁻² s⁻¹, at 25 °C and with the harvest of algal biomass during the exponential growth phase. Under these conditions, 22.7% of biomass dry weight was fatty acid of which 38.3% was EPA