Recent developments in synthetic biology and metabolic engineering in microalgae towards biofuel production

Abstract

In the wake of the uprising global energy crisis, microalgae have emerged as an alternate feedstock for biofuel production. In addition, microalgae bear immense potential as bio-cell factories in terms of producing key chemicals, recombinant proteins, enzymes, lipid, hydrogen and alcohol. Abstraction of such high-value products (algal biorefinery approach) facilitates to make microalgae-based renewable energy an economically viable option. Synthetic biology is an emerging field that harmoniously blends science and engineering to help design and construct novel biological systems, with an aim to achieve rationally formulated objectives. However, resources and tools used for such nuclear manipulation, construction of synthetic gene network and genome-scale reconstruction of microalgae are limited. Herein, we present recent developments in the upcoming field of microalgae employed as a model system for synthetic biology applications and highlight the importance of genome-scale reconstruction models and kinetic models, to maximize the metabolic output by understanding the intricacies of algal growth. This review also examines the role played by microalgae as biorefineries, microalgal culture conditions and various operating parameters that need to be optimized to yield biofuel that can be economically competitive with fossil fuels.

Keywords: Biofuel; Biorefinery; Genome-scale reconstruction; Microalgae; Optimization models; Synthetic biology.

Fig. 1

Pictorial representation of the overall process towards biofuel production in microalgae using synthetic biology approach (i.e., isolation, selection of an ideal strain, redirecting the metabolism to maximize synthesis of the targeted biofuel)

Fig. 2

Scheme representing the synergy between enzymes that lead to the formation of lipid (CA carbonic anhydrase; RuBisCO Ru1,5BP carboxylase/oxygenase; PDC pyruvate dehydrogenase complex; ACC acetyl-CoA carboxylase; KAS 3-ketoacyl-ACP synthase; ACL ATP-citrate lyase; MDH malate dehydrogenase; MME NADP-malic enzyme; PDC pyruvate dehydrogenase complex; GPAT glycerol-3-phosphate acyltransferase; LPAAT lyso-phosphatidic acid acyltransferase; LPAT lyso-phosphatidylcholine acyltransferase; DGAT diacylglycerol acyltransferase; PDAT phospholipid diacylglycerol acyltransferase

Fig. 3

Key events that mark the development of synthetic biology in microalgae-based oil accumulation

Fig. 4

Hypothetical circuits proposed with the help of genetic modules (plasmids a, b, c) that can be applied to microalgae with light intensity stimulus. Case 1. Input (plasmid a + b) = light-inducible promoter::gRNA for transcription factor (PSR1/NRR1)^a + light-inducible promoter::dCas9/VP64(CRISPRa) = activation of lipid pathway (Output). Case 2. Input (plasmid a + c) = light-inducible promoter::gRNA for transcription factor (Zn(II)₂Cys₆)^b + light-inducible promoter::dCas9/SRDX(CRISPRi) = inactivation of lipid pathway suppressors (Output). ^aPSR1 and NRR1 are transcription factors that get induced during stress which leads to lipid accumulation [171, 172]. ^bZn(II)₂Cys₆ is a transcription factor that negatively regulates lipid accumulation under nitrogen stress [183]

Fig. 5

Multi-objective optimal cultivation of microalgae