The role of micro-algae in triglycerides production: assessment of challenges, sustainable developments, future prospects and circular bioeconomy opportunities in Africa

Abstract

Microalgae cultivation provides a strategic opportunity for Africa to concurrently boost food security, stimulates renewable energy development, enhances environmental pollution control and management through wastewater and food treatments, and promotes sustainable bio-economy development. This systematic literature review critically addresses the various operational challenges, sustainable solutions and recent advances in commercial micro algae production in Africa most especially by using photobioreactor systems (PBRs). These include: cost, temperature maintenance, intensive energy demand for cooling, biofouling, high risk of contamination, accumulation of dissolved oxygen and challenges with scale up. Furthermore, the merits and demerits of different photobioreactor designs including flat panel, vertical column (M-PBR and N-PBR) and tubular reactors were thoroughly discussed. Also, the use of mixotrophy microalgae cultivation system as an alternative hybrid nutritional strategy for sustainable biomass production were analyzed. The paper also addresses the key policies, institutional and socio-economic barriers affecting micro-algae biomass deployment in Africa. Extensive literature review revealed that the accumulation of fatty acids in micro-algae is significantly controlled by the synergistic interactions and biochemical integration of glycolysis, pentose phosphate pathway, TCA, enzymatic activities, redox balance, and metabolic fluxes within the micro-algae cells. Higher lipids accumulation usually occurs under heterotrophic and mixotrophic conditions because the processes are not limited by sunlight and ATPs are mainly used by photosynthetic micro-algae in protein, carbohydrates and pigments synthesis. Whereas, excess sugars are usually converted and stored as lipids by heterotrophic and mixotrophic micro-algae. The lipids productivities of micro-algae are usually in the range of 15-25%, 40-60% and 45-65% on dry weight basis in photoautotrophic, heterotrophic and mixotrophic cultures respectively depending on micro-algae strain, photobioreactor design and the extents of nutrients stress. Schizochytrium and Nannochloropsis micro-algae species contain high PUFAs that are rich in DHA and EPA respectively which are used as essential dietary supplements. The Schizochytrium, Botryococcus, Chlorella and Nannochloropsis micro-algae species are generally cultivated for their high lipids’ productivities in the range of 50-80%. Chlorella. vulgaris CCALA 256 is the most productive chlorella vulgaris micro-algae strain with lipids contents of 57.25±0.17% on dry weight basis. Whereas, N. oceanica IMET 1 strain is widely cultivated among the Nannochlropsis species due to its high lipids’ productivity (about 69% dry weight). Also, Botryococcus braunii micro-algae strain (SCCAP 1761) has a high lipid value of 79.92 ±1.9%. Interestingly, the bubble column photobioreactors offer higher biomass and lipids production with the microbubble assisted photobioreactor (M-PBR) and nano-technology enhanced photo-bioreactor (N-PBR) exhibiting enhanced gas-liquid mass transfer characteristics, more uniform CO 2 and light distribution tendencies. Moreover, digital technologies such as artificial intelligence, machine learning algorithms, internet of things (IoT) sensors, advanced data analytics software, multi-sensing monitoring tools and smart farming operations through digitalization and automation are potential future strategies for efficient and sustainable micro-algae cultivation in Africa with enhanced biomass and lipids productivity as well as appreciable resource recovery. Finally, carbon capture and sequestration, valorization of industrial and agricultural wastes (sludge water effluents) and waste food residues (WFR) (waste food effluents) are important sustainable strategies for micro-algae circular bioeconomy developments in Africa.