(497c) Integrated Microalgal Biorefinery: Optimization of Process Conditions for Enhanced Biofuel and Antioxidant Production

Environmental concern for exhausting fossil fuel reserves has propelled investigation of microalgae as a viable and alternative source of biofuels. Microalgae are plant-like microbes with several advantages over conventional biofuel sources. However, current strategies for microalgae cultivation are limited by economic drawbacks, as biofuel alone cannot create sufficient revenues to ensure economic feasibility. The application of an integrated bio-refinery approach, which utilizes time and cost-effective strategies to enhance the production of both biofuel and high-value co-products, offers the possibility of enhancing overall economic feasibility while addressing issues regarding process sustainability and management of waste streams. Similar to an oil refinery, the main product of a bio-refinery is fuel, but the true economic value of the process lies in the production of other valuable chemicals and solvents. This research explores the design of cultivation strategies, which will enable production of selected products based on economic value and market size. A sustainable and feasible solution for a microalgal production system strengthens the bio-economy and renewable energy sector.

Enhancing biofuel and co-product production:

An integrated biorefinery framework is a promising approach for microalgal biofuels to reduce dependency on fossil fuel sources, which result in greenhouse gas emissions and contribute to global warming. A biorefinery is like a traditional oil refinery since it involves conversion of crude biomass to obtain biofuels, energy, and high-value co-products. Microalgae are a viable source of biofuels showing potential in producing co-products such as antioxidants, bioactive compounds and pigments [1]. As proof of concept, this project focuses on the simultaneous production of both fuels and antioxidants. Antioxidants such as Lutein and Zeaxanthin, specific for preventing and treating ophthalmic diseases, are high-value co-products found in significant quantities in the chloroplast membrane of the microalgal strain A. Protothecoides [2]. Moreover, literature have reported 40-50% lipid content for production of biofuel through the oleaginous A. Protothecoides [3], [4].

One of the main challenges of microalgal process systems is the low economic value of biodiesel as a product. As a result, the biorefinery must regulate product composition based on market demand to ensure high process economy and sustainability. The primary product (lipids) and the secondary co-products (antioxidants) require distinct nutrient conditions for accumulation [5]. In this research, microalgae consume organic substrates in the form of glucose or glycerol instead of using their photosynthetic functionality in order to prevent light dependency [3]. Glucose causes lipid accumulation but also chloroplast bleaching which limits antioxidant yields. On the other hand, glycerol modulates and enhances antioxidant production [6]. To maximize the economic value of the algal biomass, a two-stage strategy is proposed, where different stressors or modulators are used in each stage to achieve both optimal lipid and antioxidant accumulation.

Nitrogen is a primary source for cell division and growth in microalgae. However, nitrogen-rich cultures contribute to high biomass with limited lipid accumulation as most of the energy is utilized for cell division [7]. In this research, nitrogen-limited conditions are explored to enrich the intracellular organelles with more energy rich components such as lipids and carbohydrates as opposed to biomass growth. Experimental variation of the carbon to nitrogen ratio in batch cultivation shows high biomass growth for increasing nitrogen under nitrogen-limited conditions. When glucose and glycerol proportions were altered, the lipid content significantly decreased with lower glucose in the culture media. Glucose forms intermediate metabolites that circumvent the metabolic route through the chloroplast resulting in chloroplast bleaching. Since, chloroplasts are rendered inactive by glucose assimilation and nitrogen-limiting conditions hinder chloroplast production, the chloroplasts disintegrate for lipid accretion in the microalgal cell. Whereas, the presence of high glycerol content in culture media maintains metabolic activity through the chloroplast and in spite of nitrogen limiting conditions, glycerol metabolism modulates antioxidant and pigment production in microalgae.

Microalgal fed-batch cultivation system: Fed-batch cultures involve controlling nutrient feed at set time intervals by taking into account growth-limiting factors throughout the process duration [8]. A fed batch cultivation system allows microalgae to achieve high biomass concentration in a short timeframe because the osmotic and toxic effects of accumulating nutrients in the culture media no longer inhibit growth [9]. Alternatively, continuous supply of organic substrate, nitrogen and phosphate source would significantly contribute to the cost of the micro-algal process system. Hence, nutrient conditions such as organic substrate and nitrogen content are distributed to the culture media at specified stages, which promote accumulation of lipids and antioxidants at separate stages. Optimization studies were carried out to determine the optimal time to switch between nutrient conditions and harvest a portion of the microalgal culture. In effect, the intermittent supply of nutrients through a series of production stages, instead of a continuous supply, enhances the economic performance of the process.

Experimental results show that higher biomass and lipid yields were observed when the feed was rich in glucose. Therefore, a high glucose and nitrogen rich feed is preferable in the first stage to increase biomass growth and lipid accumulation. For the second stage, switching to a high glycerol content and nitrogen limited nutrient feed enhances and modulates the production of antioxidants in the biomass rich culture.

This research enables the development of an optimal cultivation strategy to maximize the economic value of algal biomass by regulating the production of biodiesel and high value co-products according to market demands. Since algal production systems are renewable, the proposed biorefinery process will result in a sustainable supply of both fuels and chemicals, replacing unsustainable and polluting fossil-based traditional refineries.

References:

[1] M. Bilal, T. Rasheed, I. Ahmed, and H. M. N. Iqbal, “High-value compounds from microalgae with industrial exploitability - A review.,” Front. Biosci. (Schol. Ed)., vol. 9, pp. 319–342, Jun. 2017.

[2] E.-S. M. Abdel-Aal, H. Akhtar, K. Zaheer, and R. Ali, “Dietary sources of lutein and zeaxanthin carotenoids and their role in eye health.,” Nutrients, vol. 5, no. 4, pp. 1169–1185, Apr. 2013.

[3] A. Patel, L. Matsakas, U. Rova, and P. Christakopoulos, “Heterotrophic cultivation of Auxenochlorella protothecoides using forest biomass as a feedstock for sustainable biodiesel production,” Biotechnol. Biofuels, vol. 11, p. 169, Jun. 2018.

[4] H. Rismani-Yazdi et al., “High-productivity lipid production using mixed trophic state cultivation of Auxenochlorella (Chlorella) protothecoides.,” Bioprocess Biosyst. Eng., vol. 38, no. 4, pp. 639–650, Apr. 2015.

[5] T. Dong et al., “Combined algal processing: A novel integrated biorefinery process to produce algal biofuels and bioproducts,” Algal Res., vol. 19, pp. 316–323, 2016.

[6] W.-B. Kong, H. Yang, Y.-T. Cao, H. Song, S.-F. Hua, and C.-G. Xia, “Effect of Glycerol and Glucose on the Enhancement of Biomass, Lipid and Soluble Carbohydrate Production by Chlorella vulgaris in Mixotrophic Culture,” Food Technol. Biotechnol., vol. 51, 2013.

[7] Y. X. Li, F. J. Zhao, and D. D. Yu, “Effect of nitrogen limitation on cell growth, lipid accumulation and gene expression in Chlorella sorokiniana ,” Brazilian Archives of Biology and Technology , vol. 58. scielo , pp. 462–467, 2015.

[8] O. Perez-Garcia and Y. Bashan, “Microalgal Heterotrophic and Mixotrophic Culturing for Bio-refining: From Metabolic Routes to Techno-economics BT - Algal Biorefineries: Volume 2: Products and Refinery Design,” A. Prokop, R. K. Bajpai, and M. E. Zappi, Eds. Cham: Springer International Publishing, 2015, pp. 61–131.

[9] M. I. Khan, J. H. Shin, and J. D. Kim, “The promising future of microalgae: current status, challenges, and optimization of a sustainable and renewable industry for biofuels, feed, and other products,” Microb. Cell Fact., vol. 17, p. 36, Mar. 2018.