Introduction

Industrialization and the human population explosion have created a huge energy crisis worldwide due to their dependence on the non-conventional fossil fuels like petroleum, coal and natural gases to fulfil their daily energy requirements (Chandrasekhar et al., 2015; Kumar et al., 2017; Pandey et al., 2020). Extensive use of fossil fuels also generates a huge amount of harmful greenhouse gases which get accumulated in the environment and adversely affect the life on the planet. Harvesting solar energy via photosynthesis is one of nature’s noteworthy achievements that could also be a solution for the future world-wide energy need. Cyanobacteria which can fix CO2 into organic compounds using solar energy are of great significant cellular factories for producing biofuels (Knoll et al., 2008; Testa et al., 2019). In addition to sunlight as an energy source for carbon absorption, cyanobacteria require water and inorganic trace nutrients for growth. Among the photosynthetic organisms, cyanobacteria offer attractive systems for biotechnological applications due to their higher growth rate compared to plants and they are relatively acquiescent to genetic manipulation compared to microalgae which opens ample opportunities to modify complex biosynthetic pathways by synthetic biology approaches (Lu et al., 2010; Lin et al., 2021). Synthetic biology deals with this and aims to manipulate the existing genetic system and create the new one with greater capabilities. To fulfil these objectives, variety of genetic tools have been developed such as engineered promoters (constitutive and inducible), ribosome binding sites library (RBS), riboswitches, CRISPR/Cas system, vectors, etc (Li et al., 2016; Sengupta et al., 2020). These tools come under the common term“BioBricks” which stands for the part of the DNA. Some strains of cyanobacteria can accumulate large amounts of lipids and are excellent candidates for biodiesel production (Quintana et al., 2011; Eungrasamee et al., 2020). The concept of converting carbon dioxide into a desirable fuel gave rise to genetically engineered cyanobacteria for biofuel production (Atsumi et al., 2009: Lindberg et al., 2010; Knoot et al., 2018). The first model cyanobacterium was Synechocystis sp. PCC 6803 for which the complete genome was sequenced in 1996 (Kaneko et al., 1996). Some cyanobacterial model strains, Synechocystis sp . PCC 6803, Synechococcus elongatus sp. PCC 7942, Synechococcussp. PCC 7002, , Synechococcus elongatus UTEX 2973,Synechococcus elongatus PCC 11801 and Synechococcus elongatus PCC 11802 have been used in synthetic biology and metabolic engineering studies for biosynthesis of multiple fuel molecules including free fatty acids, isoprene, 2,3-butanediol, 1-butanol, squalene, n-alkanes, α- farnesene and hydrogen etc. Some attributes of cyanobacteria like high cell density growth, ability to grow on non-arable land, utilization of different water sources (fresh, marine and waste water) and capability to produce both biofuels and other economically important products qualifies them a suitable cell factory (Zahra et al., 2020).