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).