Abstract
The marine diatom Phaeodactylum tricornutum can accumulate up to 30% of the omega-3 long chain polyunsaturated fatty acid (LC-PUFA) eicosapentaenoic acid (EPA) and, as such, is considered a good source for the industrial production of EPA. However, P. tricornutum does not naturally accumulate significant levels of the more valuable omega-3 LC-PUFA docosahexaenoic acid (DHA). Previously, we have engineered P. tricornutum to accumulate elevated levels of DHA and docosapentaenoic acid (DPA) by overexpressing heterologous genes encoding enzyme activities of the LC-PUFA biosynthetic pathway. Here, the transgenic strain Pt_Elo5 has been investigated for the scalable production of EPA and DHA. Studies have been performed at the laboratory scale on the cultures growing in up to 1 L flasks a 3.5 L bubble column, a 550 L closed photobioreactor and a 1250 L raceway pond with artificial illumination. Detailed studies were carried out on the effect of different media, carbon sources and illumination on omega-3 LC-PUFAs production by transgenic strain Pt_Elo5 and wild type P. tricornutum grown in 3.5 L bubble columns. The highest content of DHA (7.5% of total fatty acids, TFA) in transgenic strain was achieved in cultures grown in seawater salts, Instant Ocean (IO), supplemented with F/2 nutrients (F2N) under continuous light. After identifying the optimal conditions for omega-3 LC-PUFA accumulation in the small-scale experiments we compared EPA and DHA levels of the transgenic strain grown in a larger fence-style tubular photobioreactor and a raceway pond. We observed a significant production of DHA over EPA, generating an EPA/DPA/DHA profile of 8.7%/4.5%/12.3% of TFA in cells grown in a photobioreactor, equivalent to 6.4 μg/mg dry weight DHA in a mid-exponentially growing algal culture. Omega-3 LC-PUFAs production in a raceway pond at ambient temperature but supplemented with artificial illumination (110 μmol photons m-2s-1 ) on a 16:8h light:dark cycle, in natural seawater and F/2 nutrients was 24.8% EPA and 10.3% DHA. Transgenic strain grown in RP produced the highest levels of EPA (12.8%) incorporated in neutral lipids. However, the highest partitioning of DHA in neutral lipids was observed in cultures grown in PBR (7.1%). Our results clearly demonstrate the potential for the development of the transgenic Pt_Elo5 as a platform for the commercial production of EPA and DHA.
Highlights
Omega-3 long chain polyunsaturated fatty acids (LC-PUFAs) with 20 carbons or more in length containing three or more cis- double bond, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), play essential roles in human nutrition, including during neonatal development and in adult cardiovascular health
The highest content of omega-3 LC-PUFAs was observed when Instant Ocean (IO) and ESAW basal salts were supplemented with F/2 nutrients, in IO+F/2N and ES+F/2N media (21.5% and 21.1%, respectively, of total fatty acids as omega-3 LC-PUFAs)
The marine diatom P. tricornutum has been intensively investigated for the industrial production potential of EPA [3, 14, 17,18,19,20,21]
Summary
Omega-3 long chain polyunsaturated fatty acids (LC-PUFAs) with 20 carbons or more in length containing three or more cis- double bond, EPA and DHA, play essential roles in human nutrition, including during neonatal development and in adult cardiovascular health. LC-PUFAs can be classified into two main families, omega-6 (or n-6) and omega (or n-3) families, depending on the position of the first double bond proximal to the methyl end of the fatty acid. These two families are not inter-convertible and their metabolites have opposing physiological roles (For Refs see [1]). The main dietary source of EPA and DHA is marine fish. Marine algae are the primary producers of LC-PUFAs and represent a logical and promising alternative source to fish oils. The efficient production of high value products such as EPA and DHA from algae is expensive and significant efforts in strain development and cultivation technologies are still required to reduce the currently high production costs associated with algal biomass
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