Glucose and sucrose are two of the major sugar moieties easily obtained from cyanobacteria. Anaerobic fermentation of these simple sugars in the dark produces ethanol or ethyl alcohol. It is a colorless, flammable, and volatile liquid created by microbes by sugar fermentation. The ethanol made from renewable sources is a desirable source of energy, since it can be combined with diesel and utilized without requiring further modifications to present diesel engines (Kaygusuz, 2009). In the production of ethanol, cyanobacteria have an advantage over conventional energy crops, since they ferment naturally without any need for yeast cultures (fermentation initiators) that are needed by the latter, therefore making cyanobacteria better candidates for ethanol generation. Most algae and cyanobacteria do not need fermentation as their major source of energy. Fermentation occurs at a low level in these species, thus helping them to survive. Genetic modification may be a viable option for overcoming this difficulty and increasing ethanol output from engineered cyanobacteria that are a reliable source for ethanol production (Quintana et al., 2011). Synechococcus sp. PCC 7942 was the first genetically engineered cyanobacterial species utilized to produce ethanol. Coding sequences of Zymomonas mobilis pyruvate decarboxylase (PDC) and alcohol dehydrogenase II (ADH) were cloned in the pCB4 vector and utilized to transform Synechococcus sp. strain PCC 7942. The promoter from the rbcLS operon, which encodes cyanobacterial ribulose-1,5-bisphosphate carboxylase/oxygenase, increased the expression of the PDC and ADH genes (Deng and Coleman, 1999). Modifying various abiotic parameters like light intensity, carbon source, CO₂ concentration, pH, and medium composition can boost the production of certain molecules (Gao et al., 2012). In many situations, optimizing these abiotic and biotic elements together has led to increased yields (Andrews et al., 2021). Carrieri et al. (2010) evaluated the influence of salt stress on cyanobacterial fermentation rate. High salt concentration in a medium led to a 100-fold increase in ethanol production as compared to low salt concentration. The amount of ethanol released by cyanobacteria has a significant impact on two parameters, the amount of energy used and GHG emission. Cyanobacteria derived ethanol do not produce any remnant, ash, smoke and greenhouse gases in any form therefore serves as an eco-friendly fuel (Luo et al., 2010). Multiple ethanologenic cassette insertions inside the Synechocystis genome have been shown to be a successful technique for increasing ethanol titers (Gundolf et al., 2019; Wang et al., 2020). However, in cyanobacteria, plasmid-based ethanol-generating systems have been highly effective and have frequently resulted in rather large ethanol titers (Dühring et al., 2017). Using a plasmid-based system, the patented strain ABICyanol1 has attained an ethanol output of 0.55 g/L/d (Piven et al., 2015). According to an in silico study, the maximal amount of ethanol produced by Synechocystis sp. PCC 6803 was found to be 235 percent more than what was produced experimentally on the laboratory scale. The researchers were able to do this by developing a combined ethanol/biomass system (Lasry Testa et al., 2019). Cyanobacteria have the potential to produce lipids, which may be utilized as biofuel feedstock (Anahas and Muralitharan, 2018). Recently, Roussou et al. (2021) investigated the combined effects of Calvin-Benson-Bassham (CBB) enzymes like transketolase (TK), aldolase (FBA), and fructose-1,6/sedoheptulose-1,7-bisphosphatase (FBP/SBPase) on ethanol production in Synechocystis PCC 6803. The strain that overexpressed a combination of FBA and TK CBB enzymes produced the maximum amount of ethanol.

Butanol (butyl alcohol) is a 4-carbon alcohol (C₄H₉OH) that forms the main bulk chemical and an effective blend-in fuel sourced from non-renewable fossil resources (Liu et al., 2021). 1-Butanol is gaining interest as a potential fuel alternative and an important chemical feedstock. It can be produced by both the coenzyme A (CoA)-dependent route (Ezeji et al., 2007) and the ketoacid pathway (Shen and Liao, 2008) (Figure 2). There are also biological pathways for fermentative butanol synthesis (Wichmann et al., 2021). Natural cyanobacterial strains do not synthesize isobutanol or 1-butanol, suggesting that butanol metabolic genes and related pathways are missing. For heterotrophic 1-butanol synthesis, the clostridial route, a natural 1-butanol-generating pathway from Clostridium, was introduced into E. coli (Shen et al., 2011). In Synechococcus 7942, Atsumi et al. (2008) created a synthetic isobutanol pathway that generates isobutyraldehyde and isobutanol. Similarly, an alternative acetoacetyl-CoA biosynthesis pathway (ATP-driven) has been created in Synechococcus by overexpression of acetoacetyl-CoA synthase (NphT7) that condenses malonyl-CoA and acetyl CoA. In Synechococcus, the overexpression of three enzymes (phosphoglycerate mutase, enolase, and pyruvate kinase) from the three stages between 3-phosphoglycerate and the pyruvate of the Calvin–Benson–Bassham cycle boosted overall carbon output by 1.8-fold and 2,3-butanediol synthesis by 2.4-fold (Oliver and Atsumi, 2015). Miao et al. (2018) reported a maximum 1-butanol and isobutanol production in Synechocystis PCC 6803 during a long-lasting culture (0.9 g/L in 46 days). CRISPR interference (CRISPRi) was conducted in another investigation to repress the enzyme citrate synthase gltA, which arrests growth but increases carbon partitioning leading to the production of 1-butanol in Synechocystis (Shabestary et al., 2018).

Furthermore, the synthesis of 1-butanol was boosted by overexpressing a heterogeneous phosphoketolase (PK) route, one of the natural acetyl-CoA supporting routes in Synechocystis (Xiong et al., 2015; Liu et al., 2019). Wichmann et al. (2021) recently showed that photosynthetic butanol is created directly from CO₂, with a carbon partitioning of 60% to 1-butanol.

Biodiesel, unlike petroleum diesel, is biodegradable and non-toxic, and when burnt as a fuel, it considerably decreases toxicants and other emissions (Yusuf et al., 2011). Cyanobacteria are photosynthetic microorganisms that can convert solar energy and CO₂ into biofuels in a single biological system. Synthesizing fatty acid-based compounds using solar energy as the energy source, CO₂ as the carbon source, and cyanobacteria as the biological system would be a potential technique for developing sustainable biofuels (Quintana et al., 2011). In combined liquid–gaseous fuel-processing solutions, Spirulina also demonstrated the greatest overall utilization efficiency. In cyanobacteria, overproduction and transesterification of fatty acids to generate fatty acid methyl esters, fatty alcohols, or fatty alkanes are the two processes involved in the production of fatty acid-based biofuels (Mata et al., 2010). These lipid feedstocks are made up of 90–98 percent triglycerides and trace quantities of monoglycerides and diglycerides, 1–5 percent free fatty acids, and small amounts of phosphatides, phospholipids, carotenes, sulfur compounds, tocopherols, and water (Chisti, 2007). Because of their high fatty acid content, Hossain et al. (2020) discovered that cyanobacteria represent a promising source for the biodiesel sector. Cyanobacteria have high lipid content, with Oscillatoriales having the greatest overall lipid concentration. More studies are needed to improve the mass culture conditions for increasing the lipid content of cyanobacterial biomass, and research on value addition of residual biomass is also required. Nagappan et al. (2020) evaluated numerous nitrogen-fixing cyanobacteria for biodiesel production based on biomass production, lipid productivity, lipid profile, and harvesting capability. The evaluation resulted in the selection of Nostoc sp. MCC41, a nitrogen-fixing cyanobacterium and a promising species. It contains a high proportion of palmitic acid, indicating its applicability for biodiesel production. Nostoc sp. MCC41 can be regarded as a promising and eco-friendly resource for efficient biodiesel production because of its ease of harvesting, capacity to grow under mixotrophic conditions, and ability to fix atmospheric nitrogen. To investigate the activity of certain enzymes used in saturated fatty acid production in cyanobacteria, genetically altered Synechococcus elongatus PCC 7942 was generated by overexpression or deletion of genes coding for fatty acid synthase system enzymes; its lipid profile was evaluated, and it was found that the functioning of some of these enzymes differed. Modifications resulting from gene overexpression or deletion were then conducted to increase the production of alpha-linolenic acid in cyanobacterium. When combined with the overexpression of Synechococcus sp. desA and desB desaturase genes, the mutant generated by fabF overexpression and fadD deletion, PCC 7002, generated the most omega-3 fatty acids (Santos-Merino et al., 2018).

Because it is non-polluting and infinite, hydrogen (H₂) is a more appealing biofuel candidate for upcoming usage (Srirangan et al., 2011). According to Kruse et al. (2005), many cyanobacteria spontaneously create hydrogen as a secondary metabolite that balances redox energetics, and many other strains can create hydrogen by the reversible action of hydrogenase. The most significant impediment to cyanobacterial H₂ production is that hydrogenases are particularly sensitive to the O₂ produced during photosynthesis. Furthermore, the presence of reducing agents like ferredoxin and NADPH is a barrier, since they are involved in other processes such as respiration. To boost hydrogen generation, it will be necessary to reroute some of the electron flow to hydrogen-producing enzymes and to develop hydrogenases tolerant to oxygen (Weyman, 2010). When cyanobacteria are cultured under nitrogen-deficient conditions, H₂ evolves as a by-product of nitrogen fixation. It was also discovered that non-heterocystous cyanobacteria produce lesser hydrogen than heterocystous species. Several studies have looked at cyanobacterial species that can produce H₂ as a source of clean biofuel (Abed et al., 2009). Multiple attempts have been made to boost H₂ production rather than carbon flow to augment electron flux. Nitrogenase-based generation systems of H₂ have been created in Nostoc sp. PCC 7120 by inhibiting hydrogenase (Hup) absorption, resulting in increased hydrogen (Masukawa et al., 2002). Synechococcus 7002 mutants deficient in lactate dehydrogenase showed a 5-fold increase in total H₂ generation when compared with the wild type (McNeely et al., 2010). The production of an exogenous ferredoxin by Clostridium acetobutylicum during the fermentation process might boost electron flow to the hydrogenase (HydA). In light-dependent anoxic circumstances, H₂ production was increased twice its original amount (Ducat et al., 2011). Several initiatives have been launched to hinder competitive pathways for reductant usage and to promote H₂ production. As a consequence, the ldhA gene was inactivated in Synechococcus sp. 7002, NADH/NAD ratios were significantly increased, and H₂ production was significantly enhanced when native bidirectional [NiFe] hydrogenase was increased (five-fold in anoxic and dark circumstances) (McNeely et al., 2010). Engineered Anabaena variabilis ATCC 29413, Nostoc sp. PCC 7422, and Nostoc linckia HA-46 generated more H₂ than Synechococcus sp. PCC 7002, Synechocystis sp. PCC 6803, and Nostoc punctiforme ATCC 29133. In contrast, the lowest H₂ producers have been found to be Synechococcus elongatus PCC 7942, Synechococcus sp. PCC 7002, and Synechocystis sp. PCC 6803d. It implies that during H₂ synthesis, oxygen may function as an inhibitor in these cyanobacteria. As a consequence, an artificial oxygen-consuming device for O₂ consumption may be built in the near future, resulting in enhanced H₂ production in Synechocystis sp. PCC 6803 and Synechococcus sp. PCC 7002 (Srirangan et al., 2011). Khetkorn et al. (2012) used multiple inhibitors to promote electron flow toward nitrogenase and bidirectional Hox-hydrogenase in Anabaena siamensis TISTR 8012 to study the downregulation of those competing for metabolic pathways. Increased H₂ generation produced by inhibitors corresponded to increase in respective Hox-hydrogenase activity and nif D transcript levels and decrease in hupL transcript levels. The NiFe-hydrogenase HoxYH from the cyanobacterium Synechocstis sp. PCC 6803 was coupled with the photosystem I component PsaD. The resultant psaD-hoxYH mutant grew photoautotrophically, accumulated a large concentration of photosynthetically produced hydrogen in the light under anaerobic conditions, and did not consume hydrogen. According to the findings, psaD-hoxYH photosynthetic hydrogen production is most likely a mix of anoxygenic and oxygenic photosynthesis (Appel et al., 2020).

PHAs are linear polyesters having 3–6 hydroxy acids and more than 150 monomers accounting for around 2 million Daltons (Figure 4) (Chen and Wu, 2005; Gomes Gradíssimo et al., 2020). Alcohol, sugar, and alkane production can act as a substrate for the formation of polymers. The varied physical properties are due to the different chemical structures of PHAs produced from different bacterial genera becoming more suitable for wide applications (Raza et al., 2018). PHAs, however, are one of the largest categories of natural bio-based polyesters composed of more than 150 different monomers of PHAs (Pittman et al., 2011). Varied changes in structural configurations and variations of monomers can form homopolymers of PHAs and copolymers of PHAs, and an example of a homopolymer is PHB and a copolymer is poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHVB) (Chen, 2009). Based on polymerization, PHAs can be of different lengths such as short-chain length (SCL) including PHB, poly(3-hydroxyvalerate)-PHV, and their copolymer PHBV, while another type of PHAs is medium chain length (MCL) that includes poly(3-hydroxynonanoate)-PHN and poly(3-hydroxyoctanoate)-PHO.

Cyanobacterial PHAs exhibit thermostability and physical properties like those of conventional plastics derived from fossil origin (Gadgil et al., 2017). The physical and chemical properties are determined by their chemical structure, i.e., the number of monomers. The shorter PHA is brittle and shows high crystallinity, while polymers with more number of monomers are flexible and elastic. It covers biopolymers that are biodegradable as well as biocompatible (Chen and Wu, 2005; Tokiwa et al., 2009; Tan et al., 2014; Gomes Gradíssimo et al., 2020). The decomposition of PHAs under natural conditions or by bacterial action depends on various factors like polymer composition, humidity, and temperature of the surrounding. It also depends on the type of decomposing microorganism, as it affects the duration of degradation because different bacteria express diverse types of PHA-depolymerase that degrades the biopolymer. Moreover, the chemical and physical properties of bioplastics result in sinking of these bioplastics in an aquatic environment that facilitates their transformation into carbon and water by decomposers (Tokiwa et al., 2009; Raza et al., 2018).

Interestingly, PHAs like synthetic plastics can be customized and thus are attractive biomaterials, as they can be designed to suit a specific function. Blending PHAs with other biodegradable polymers can considerably improve the physical properties like crystallinity, glass transition temperature, and mechanical properties (Li et al., 2016). Owing to their unique versatility and ecologically friendly biodegradable properties, PHAs are of great use, as they have a wide range of applications and show to be a promising substitute to petrochemical polymers. Usually, petrochemical plastic wastes are either fragmented into microplastics or are being collected into the oceans as garbage patches. However, bioplastics have met the standard specifications for marine degradability. Based on growing needs, PHAs nowadays are most attractive to producers, and the bioplastic market is expected to strongly increase the production capacity of PHAs by many folds in the future (Aeschelmann et al., 2016; Rosenboom et al., 2022). They are widely used in industries to manufacture biodegradable water-resistant surfaces, for controlled pesticide delivery, nanocomposite materials, mulch films, medical devices, tissue scaffolding, plastic packaging, etc. (Philip et al., 2007; Gomes Gradíssimo et al., 2020).

PHAs producing organisms are categorized into two groups: one group that produces PHAs during their growth period and another group that synthesizes PHAs when they grow under limited nutrient conditions (Basnett et al., 2017). Cyanobacteria are the only prokaryotic species known to produce the homopolymer of PHB naturally in photoautotrophic and chemoheterotrophic environments (Rani and Sharma, 2021). The unsteady growth during the fermentation process promotes PHA production. The accumulation depends on the presence of various elements in the growth medium as well as the ionic strength of the medium (Chen, 2009). Recently, cyanobacteria were genetically transformed with PHB and PHA synthesis encoding genes (Noreen et al., 2016).

Three biosynthetic pathways are involved in the assimilation of carbon into different polymers. The foremost well-described pathway found in cyanobacteria for PHA production, especially PHB, is similar to earlier documented pathways in heterotrophic and other archaebacteria species (Lim et al., 2002; Lee et al., 2005; Singh and Mallick, 2017). Among cyanobacteria, Synechocystis PCC6803 is widely used as a model organism for gaining insight into the generation of PHBs. PHB biosynthesis involves majorly three enzyme-mediated steps: condensation, reduction, and polymerization. Step one includes reversible condensation (Claisen type) of the precursor, i.e., two molecules of acetyl-CoA derived from the tricarboxylic acid (TCA) cycle with an enzyme, ketothiolase, and produces acetoacetyl-CoA. The first step is followed by reduction of acetoacetyl-CoA (encoded by gene phaA) with NADPH-linked enzyme acetoacetyl-CoA reductase (encoded by phaB) and produces d(-)-3-hydroxybutyryl-CoA. Furthermore, in the last step, d (-)-3-hydroxybutyrate-CoA gets polymerized by the PHA-synthase enzyme (encoded by genes phaC and phaE) and finally produces poly(3-hydroxybutyrate) biopolymer (PHB) (Figure 2). The four genes, slr1993- phaA, slr1994- phaB, slr1830- phaC, and slr1829- phaE encoding the three enzymes are involved in PHB synthesis. In anoxygenic purple sulfur bacteria, the PHA synthase coding mediated by genes phaC and phaE is similar to that in a cyanobacterium (Lane and Benton, 2015). The role of Rubisco in CO₂ assimilation is additionally observed in the production of PHB involving the conversion of glycolate to 2-phosphoglycolate synthesis under photosynthetic conditions. Along with the conjugation of PHB synthesis, the substrate propionic acid is used in the synthesis of PHBV copolymer (Balaji et al., 2013). Reportedly, different conditions showed different production rates of PHBs. Under photo-autotrophic conditions in Synechocystis sp., the production of these biopolymers was up to 20% (DCW, dry cell weight); however, under heterotrophic conditions, acetate supplementation, and phosphorus deprivation, the production was 28.8% (DCW), and when genetically modified organisms were supplemented with acetate, they showed a higher yield of PHB at 35% (DCW). Interestingly, the highest yield of 55% was obtained when Synechococcus sp. MA19 was grown with Ca₃(PO₄)₂ (Panda et al., 2006; Khetkorn et al., 2016; Kamravamanesh et al., 2017; Gomes Gradíssimo et al., 2020).

However, another interesting biosynthetic pathway involves metabolism of lipids and transformation of different alkanes, alkenes, and alkanoates to generate MCL-PHA polymers. Furthermore, PHA synthase enzymes polymerize different hydroxyalkanoate monomers during the fatty acid oxidation pathway (Hein et al., 2002; Lim et al., 2002).

In the next and third biosynthetic low production cost involved pathway, MCL-PHAs are polymerized from monomers that are produced from glucose, fructose, and sucrose (Philip et al., 2007). Here, with a glycolic precursor and intermediates of the fatty acid biosynthesis pathway, lipids and carbohydrates are used in the generation of PHAs. PHA generation, accumulation, and growth of microbes are also affected by the availability or deprivation of many nutrients and their concentrations, mainly including nitrogen, phosphate, sulfur, and oxygen. Optimum changes in carbon-to-nitrogen ratio are used for culture optimization, whereas abundant carbon is favorable for the production of biomass, and limiting the concentration of phosphorus and/or nitrogen is found to beneficial for the generation of polymers of PHAs (Salehizadeh and Van Loosdrecht, 2004; Wen et al., 2010; Montiel-Jarillo et al., 2017). In cells, nitrogen is required for manufacturing proteins and nucleic acids, and the availability and deprivation of nitrogen levels influence NAD(P)H concentration, NAD(P)H/NAD(P) ratio, and, hence, the TCA cycle (Lee et al., 2005; Albuquerque et al., 2010; Reddy and Mohan, 2015). In a balanced well-supplemented nutrient culture, the TCA cycle is maintained as the concentration of NAD(P)H, and NAD(P)H/NAD(P) ratio remains constant. On the contrary, in nitrogen deficiency, amino acid synthesis, α-ketoglutarate conversion into glutamate, assimilation of ammonium ions, and NAD(P)H accumulation get affected (Panda et al., 2006; Liu et al., 2011) and, hence, influence PHB production.

In cyanobacteria, other than nitrogen, the availability and deprivation effect of phosphorus are found to be more significant. Despite the role in cell maintenance and lipid and carbohydrate assimilation, mainly the inorganic form of phosphorous is a part of protein and nucleic acid (Reddy and Mohan, 2015; Gomes Gradíssimo et al., 2020). Therefore, in well-balanced culture, nutritional growth conditions with a suitable concentration of phosphorous and high coenzyme-A (CoA-SH) concentration inhibit the synthesis of PHA, whereas during phosphorous deprivation, the Krebs cycle gets restricted (Montiel-Jarillo et al., 2017). The deprivation metabolism of nitrogen and phosphorous promotes the reducing factor, NADH accumulation; citrate synthase concentration decreases and isocitrate dehydrogenase and acetyl-CoA precursor concentration increases and, hence, influence PHA biosynthesis (Singh and Mallick, 2017; Gomes Gradíssimo et al., 2020).

Cyanobacteria serve as a promising and sustainable substitute to produce biopolymers like PHB and offer several advantages in comparison to heterotrophic bacteria (Khetkorn et al., 2016). Despite various advantages, still, they have not found acceptance in the market as cyanobacterial strains are still not optimized for industrial processes. Genetic engineering is one of the techniques that have been exploited to improve the production of PHBs. There have been numerous research studies on genetic engineering in plants for the production of bioplastics, which are referred to as first- and second-generation bioplastics, but now the third generation that is commonly known as “algal bioplastics” is gaining immense importance as it offers a substitute for sustainable production of bioplastics. Genetic studies on cyanobacteria have a major advantage in terms of genetic manipulation in comparison to plants as their genome is less complex and, thus, is easy to manipulate (Koksharova and Wolk, 2002; Koller, 2020). However, most studies on biosynthesis and metabolic engineering of cyanobacterial PHB have been conducted with restricted strains of cyanobacteria. Out of all, Synechocystis sp. PCC 6803 is a largely applied model organism. It was the first phototrophic organism to be completely sequenced with mutant variants readily available and thus is largely used in gene manipulation to improve PHA production. There are a number of reports where it has been efficiently transformed for the production of PHBs and for the synthesis of biohydrogen, isoprene, and other compounds (Kaneko and Tabata, 1997; Kamravamanesh et al., 2017; Yashavanth et al., 2021).

Photosynthetic prokaryotes can be genetically engineered by transformation with genes encoding the enzymes required for PHB biosynthesis like acetoacetyl-CoA reductase, β-ketothiolase, and PHB synthase. The wild type cyanobacterial strains like Aulosira fertilissima, Nostoc muscorum, and Synechocystis sp. PCC 6803, which exhibit native PHB biosynthesis pathway, can be genetically engineered for achieving high yield and PHB productivity (Bhati et al., 2010). Mobilization of complete operons or genes from some other microorganisms that produce PHBs into the cyanobacteria can also boost PHB biosynthesis. In addition to the genes associated with the PHB pathway, the overexpression or deletion of other genes has been found to amplify the level of acetyl-CoA and PHB (Yashavanth et al., 2021). Takahashi et al. (1998) reported that recombinant Synechococcus sp. PCC7942 obtained by transformation with genes from Alcaligenes eutrophus that encode poly-3-hydroxybutyrate (PHB)-synthesizing enzymes showed improved PHB content in response to CO₂ enrichment under photoautotrophic conditions and nitrogen starvation. Moreover, the addition of acetate enhanced PHB content by more than 25% under conditions of nitrogen starvation. Miyake et al. (2000) found that inserting the PHB synthase gene obtained from Ralstonia eutropha in Synechococcus sp. MA19 resulted in the production of soluble PHB synthase that led to the synthesis of PHB granules with no pigment. In another study by Akiyama et al. (2011), Synechococcus PCC7002 was transformed with PHA genes from Cupriavidus necator enhancing PHB production by up to 52% under heterotrophic conditions. In another study by Wang et al. (2013), synthetic metabolic pathways were constructed, and Synechocystis 6803 was standardized to synthesize (S)- and (R)-3-hydroxybutyrate (3HB) using CO₂ directly. It was observed that Synechocystis cells easily secreted both forms of 3HB molecules without overexpressing the transporters. Furthermore, the competing pathway was suppressed by deleting the genes coding for PHB polymerase (slr1829 and slr1830) from the genome of Synechocystis, which led to enhanced production of 3HB. The photosynthetic cultivation of recombinant Synechocystis TABd (for 21 days) produced 533.4 mg/L of 3HB. The accumulation of PHB is reported to increase under the conditions of nitrogen deficiency, and sigE (the Sigma factor) is also found to induce PHB synthesis and carbohydrate metabolic pathways. The sigE overexpression in Synechocystis 6803 improved PHB biosynthesis under nitrogen-deficient conditions. Interestingly, the monomer units and molecular weight of produced PHB are identical to those of PHB from the wild type (Osanai et al., 2013). Similarly, in another report, the recombinant Synechocystis sp. PCC 6803 obtained by transformation with the PHA synthase gene from Cupriavidus necator displayed enhanced activity of the PHA synthase enzyme; however, the total PHB content was not found to increase (Sudesh et al., 2002; Katayama et al., 2018). Synechocystis 6803 transformed with pha genes was found to be effective, and the recombinant cells had higher PHB content (12-fold high) in comparison to the wild type in response to nitrogen stress (Hondo et al., 2015). In Synechocystis 6803, phaAB overexpression and use of acetate (4 mM) were found to increase PHB content by 35% (DCW) (Khetkorn et al., 2016). In Synechocystis, overexpressing the acetoacetyl-CoA reductase gene improved the production of R-3-hydroxybutyrate to 1.84 g L⁻¹, and maximum volumetrical production was 263 mgL⁻¹ day⁻¹ in Synechocystis (Wang et al., 2018). In another study, deletion of the agp gene encoding for ADP-glucose pyrophosphorylase from Synechocystis sp. PCC 6803 altered the partitioning of cellular carbon, and that consequently led to increased PHB production (18.6%), titer (232 mg/L), and productivity (7.3 mg/L/day) (Wu et al., 2002). Enzymes like acetyl-CoA hydrolase, phosphotransacetylase, and phosphoketolase control the level of acetyl-CoA in microbial cells. Acetyl-CoA can be converted into a large number of compounds based on the cell's requirement by employing different enzymes, particularly those that are used for PHB biosynthesis. Phosphotransacetylase (pta gene) and acetyl-CoA hydrolase (ach gene) are enzymes that convert acetyl-CoA into acetate. Phosphoketolase (encoded by xfpk gene) is an enzyme that, on the other hand, increases the level of acetyl-CoA in cells. Carpine et al. (2017) used a different strategy to amplify the production of PHB. For this experiment, recombinant Synechocystis PCC 6803 was designed by engineering central carbon metabolism instead of overexpressing or introducing PHB synthesis genes such that the PHB synthesis pathway gets downregulated and acetyl-CoA level increases. For this, seven diverse mutants were constructed that harbored individually or in the arrangement of three dissimilar genetic modifications to the metabolic pathway of Synechocystis. These included deletions of acetyl-CoA hydrolase-Ach and phosphotransacetylase-Pta and expression of heterologous phosphoketolase-XfpK (Bifidobacterium breve). It was observed that a strain having all the three recombinations in consolidation, i.e., xfpk overexpression in a background of double deletion (pta, ach), displayed a maximum PHB production of 12% PHB yield, titer of 232 mg/L, and productivity of 7.3 mg/L/day. Koch et al. (2020) engineered a strain that lacked PirC (the regulatory protein, product of sll0944) that exhibited high phosphoglycerate mutase activity and increased level of PHB in response to nutrient stress. This was further modified to produce more PHBs by transferring phaA and phaB (PHA metabolism genes) from a PHB-synthesizing bacterium, Cupriavidus necator. This strain was termed as PPT1 (ΔpirC-REphaAB), and the production of PHB was found to be high (constant light as well under day-night conditions). Under nitrogen- and phosphorus-deficient conditions, it produced 63% of PHBs, and this level further increased by 81 percent when acetate was added (under identical culture conditions). The PHB production achieved by PPT1 was the maximum ever documented for any known cyanobacteria. Thus, the above studies indicate that genetic modification can be a promising tool in cyanobacteria to achieve sustainable and cost-effective PHB production on an industrial scale. Some of the wild-type and recombinant cyanobacterial strains used for biosynthesis of the polymer are listed in Table 2.