Enhanced Production of Astaxanthin without Decrease of DHA Content in Aurantiochytrium limacinum by Overexpressing Multifunctional Carotenoid Synthase Gene

Integration of genetic engineering and multi-factor fermentation optimization for co-production of carotenoid and DHA in Schizochytrium sp

Schizochytrium sp., a microalga with high lipid content, holds the potential for co-producing docosahexaenoic acid (DHA) and carotenoids. In this study, the ability of Schizochytrium sp. to naturally produce carotenoids was systematically explored. Further, by enhancing the precursor supply of geranylgeranyl diphosphate, regulating carbon source through sugar limitation fermentation and employing a combination of response surface methodology and artificial neural networks to precisely optimize nitrogen sources, a new record of 43-fold increase in β-carotene titer was achieved in the 5L bioreactor (653.2 mg/L). Meanwhile, a high DHA content was maintained (13.4 g/L). Furthermore, the use of corn stover hydrolysate has effectively lowered the production costs of carotenoid and DHA while sustaining elevated production levels (with total carotenoid titer and DHA titer reached 502.0 mg/L and 13.2 g/L, respectively). This study offers an efficient and cost-effective method for the co-production of carotenoid and DHA in Schizochytrium sp..

Uncovering global lipid accumulation routes towards docosahexaenoic acid (DHA) production in Aurantiochytrium sp. SW1 using integrative proteomic analysis

Aurantiochytrium sp., a marine thraustochytrid possesses a remarkable ability to produce lipid rich in polyunsaturated fatty acids (PUFAs), such as docosahexaenoic acid (DHA). Although gene regulation underlying lipid biosynthesis has been previously reported, proteomic analysis is still limited. In this study, high DHA accumulating strain Aurantiochytrium sp. SW1 has been used as a study model to elucidate the alteration in proteome profile under different cultivation phases i.e. growth, nitrogen-limitation and lipid accumulation. Of the total of 5146 identified proteins, 852 proteins were differentially expressed proteins (DEPs). The largest number of DEPs (488 proteins) was found to be uniquely expressed between lipid accumulating phase and growth phase. Interestingly, there were up-regulated proteins involved in glycolysis, glycerolipid, carotenoid and glutathione metabolism which were preferable metabolic routes towards lipid accumulation and DHA production as well as cellular oxidative defence. Integrated proteomic and transcriptomic data were also conducted to comprehend the gene and protein regulation underlying the lipid and DHA biosynthesis. A significant up-regulation of acetyl-CoA synthetase was observed which suggests alternative route of acetate metabolism for acetyl-CoA producer. This study presents the holistic routes underlying lipid accumulation and DHA production in Aurantiochytrium sp. SW1 and other relevant thraustochytrid.

Improvement of Astaxanthin Production in Aurantiochytrium limacinum by Overexpression of the Beta-Carotene Hydroxylase Gene

Aurantiochytrium limacinum is a heterotrophic eukaryotic microorganism that can accumulate high levels of commercial products such as astaxanthin and docosahexaenoic acid. Due to its rapid growth and relatively simple extraction method, A. limacinum is considered a promising astaxanthin resource to replace the conventional microalgal production. However, the astaxanthin biosynthetic process in A. limacinum remains incompletely understood, especially in those catalysed by β-carotene hydroxylase (CrtZ) and ketolase. In this study, we overexpressed a crtZ candidate gene to increase astaxanthin production and expand our understanding of the conversion from beta-carotene to astaxanthin. The resultant transformant AlcrtZ#10 cultivated for 5 days showed a significant increase in astaxanthin production per culture (2.8-fold) and per cell (4.5-fold) compared with that of the wild-type strain. Strikingly, longer light exposure increased astaxanthin production and decreased the beta-carotene content in the wild-type strain, suggesting that light exposure duration is important for astaxanthin production in A. limacinum. Among several predicted intermediates, furthermore, the cantaxanthin produced from β-carotene by ketolase activity were enhanced in the transformant AlcrtZ#10. Although the further investigation is needed, this result suggested that the main route of astaxanthin was via cantaxanthin. Thus, our findings will be valuable not only for its application, but also for understanding the astaxanthin biosynthetic process in A. limacinum.

Efficient Co-production of Docosahexaenoic Acid Oil and Carotenoids in Aurantiochytrium sp. Using a Light Intensity Gradient Strategy

Aurantiochytrium is a promising source of docosahexaenoic acid (DHA) and carotenoids, but their synthesis is influenced by environmental stress factors. In this study, the effect of different light intensities on the fermentation of DHA oil and carotenoids using Aurantiochytrium sp. TZ209 was investigated. The results showed that dark culture and low light intensity conditions did not affect the normal growth of cells, but were not conducive to the accumulation of carotenoids. High light intensity promoted the synthesis of DHA and carotenoids, but caused cell damage, resulting in a decrease of oil yield. To solve this issue, a light intensity gradient strategy was developed, which markedly improved the DHA and carotenoid content without reducing the oil yield. This strategy produced 30.16 g/L of microalgal oil with 15.11 g/L DHA, 221 µg/g astaxanthin, and 386 µg/g β-carotene. This work demonstrates that strain TZ209 is a promising DHA producer and provides an efficient strategy for the co-production of DHA oil together with carotenoids.

Formulation of New Media from Dairy and Brewery Wastes for a Sustainable Production of DHA-Rich Oil by Aurantiochytrium mangrovei

Mozzarella stretching water (MSW) is a dairy effluent generated from mozzarella cheese production that does not have a real use and is destined to disposal, causing environmental problems and representing a high disposal cost for dairy producers. Spent brewery yeast (SBY) is another promising food waste produced after brewery manufacturing that could be recycled in new biotechnological processes. Aurantiochytrium mangrovei is an aquatic protist known as producer of bioactive lipids such as omega 3 long chain polyunsaturated fatty acids (ω3 LC-PUFA), in particular docosahexaenoic acid (DHA). In this work MSW and SBY have been used to formulate new sustainable growth media for A. mangrovei cultivation and production of DHA in an attempt to valorize these effluents. MSW required an enzymatic hydrolysis to enhance the biomass production. The new media obtained from hydrolysed MSW was also optimized using response surface methodologies, obtaining 10.14 g L^-1 of biomass in optimized medium, with a DHA content of 1.21 g L^-1.

Hyperaccumulation of fucoxanthin by enhancing methylerythritol phosphate pathway in Phaeodactylum tricornutum

The established human health benefits of carotenoids along with the contemporary consumption of natural carotenoids bring the necessity to sustainable production of carotenoids. Among, marine diatoms have emerged as the potential biological resources for carotenoid production; however, their relatively lower yield in native strains provides the impetus to genetically improve the diatoms to cope with the burgeoning demand. In this study, we genetically improved the diatom Phaeodactylum tricornutum by overexpressing key carotenogenic genes involved in methylerythritol phosphate (MEP) pathway. The genes with lower relative transcript level under optimum conditions such as CMK and CMS were selected and overexpressed in P. tricornutum individually. Both CMK and CMS overexpressing lines exhibited elevated growth and photosynthesis. The expression of key carotenogenic genes such as PSY, PDS, ZDS, CRT, and LCYB was significantly upregulated. Furthermore, total carotenoid content was significantly increased; particularly, fucoxanthin content was increased by 1.83- and 1.82-fold in engineered lines CMK and CMS, respectively. Together, the results identify the potential metabolic targets and also uncover the crucial role of MEP pathway in redirecting metabolic precursors towards carotenogenesis. KEY POINTS: • Low abundant genes CMS and CMK of MEP pathway were overexpressed in the diatom • Total carotenoid content was increased, particularly fucoxanthin • Critical metabolic nodes were uncovered to accelerate fucoxanthin biosynthesis.

Enhanced Lutein Production in Chlamydomonas reinhardtii by Overexpression of the Lycopene Epsilon Cyclase Gene

Chlamydomonas reinhardtii is a well-established microalgal model species with a shorter doubling time, which is a promising natural source for the efficient production of high-value carotenoids. In the microalgal carotenoid biosynthetic pathway, lycopene is converted either into β-carotene by lycopene β-cyclase or into α-carotene by lycopene ε-cyclase (LCYE) and lycopene β-cyclase. In this study, we overexpressed the LCYE gene in C. reinhardtii to estimate its effect on lycopene metabolism and lutein production. Chlamydomonas transformants (CrLCYE#L1, #L5, and #L6) produced significantly increased amounts of lutein per culture (up to 2.6-fold) without a decrease in cell yields. Likewise, the expression levels of LCYE gene in transformants showed a significant increase compared with that of the wild-type strain. These results suggest that LCYE overexpression enhances the conversion of lycopene to α-carotene, which in turn improves lutein productivity. Interestingly, their β-carotene productivity appeared to increase slightly rather than decrease. Considering that the inhibition of the lycopene cyclization steps often induces higher expression in genes upstream of metabolic branches, this result implies that the redirection from β-carotene to α-carotene by LCYE overexpression might also enhance upstream gene expression, thereby leading to auxiliary β-carotene production.