Optimisation of microalgal growth conditions for production of eicosapentaenoic acid (EPA)

Abstract

Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are long chain polyunsaturated fatty acids (LC-PUFA) belonging to the omega-3 fatty acids, which have demonstrated to be nutritionally important for the development, cardiovascular and brain function of higher eukaryotes. Exponential growth of the omega-3 fatty acids market has dramatically reduced the stocks of traditional sources for those fatty acids that are typically derived from small fatty fish. Microalgae are the major provider of LC-PUFAs in the marine ecosystem; hence research has turned towards commercial land-based cultivation of algae with high nutritional content. Heterotrophic culture systems of microalgae are already being used commercially, in particular for the production of DHA as a food supplement. However, production has high setup and operational costs, and faces limitations for scale-up as contamination of these heterotrophic cultivation systems cannot be easily controlled. Autotrophic production systems may allow further expansion of the use of microalgae as LC-PUFA biofactories, and therefore EPA obtained from marine microalgae can form the basis for sustainable industrial production, when optimal culturing conditions specifically stimulate the synthesis of these bio-products. The proposition was made at the start of this thesis, that optimal environmental cultivation conditions for EPA production in marine microalgae can be promoted under certain conditions and optimized to achieve higher yields. Among others, two potential EPA-producing microalgae were further investigated based on initial production data using unoptimized standard laboratory conditions. These include the microalga Nannochloropsis sp. BR2, originally isolated from the Brisbane River, Australia, as well as the marine microalga Tetraselmis sp. M8, that was first isolated from a rock pool at Australia’s Sunshine Coast. In the present study, it was found that the highest EPA content can be achieved for Nannochloropsis sp. when conditions were optimal for growth and high biomass productivity. For example, a well-controlled pH or the use of additional CO2 during Nannochloropsis sp. cultivation led to the highest overall EPA productivity, with EPA proportions of up to 40% of total isolated fatty acids. However, the total EPA contents per dry weight in this alga can be higher when stimulated by external stress factors, but these typically slow down growth. For Tetraselmis sp. M8, optimal EPA productivity was achieved with progressive nutrient stress. LC-PUFA biosynthesis in this alga correlated well with gene expression and differed according to nutrient and growth phase in this microalgal strain. However, no single stress factor was identified that specifically induced only EPA biosynthesis. In conclusion, this study demonstrates that improved EPA production can be achieved for autotrophic microalgae. LC-PUFAs, including EPA were highest in relation to total fatty acids during optimal growth phases, suggesting that they are required in larger amounts under these conditions for cellular functions, such as membrane assembly. Stress conditions, such as nutrient deprivation and UV-C light exposure can lead to higher total amounts of EPA, most likely to repair cellular damage, but overall EPA productivities were highest during exponential growth for the high yielding Nannochloropsis sp. BR2 strain. Large-scale cultivation of autotrophic microalgae should be developed to determine whether additional optimization measures can be identified to enable more sustainable supplies of omega-3 fatty acids via land-based grown autotrophic microalgae.