Table 4.

Culture and Genetic variables affecting PHB yield in Recombinant Cyanobacteria and Microalgae

Recombinant microalgae	Culture conditions	Genetic modifications	PHB content	Key bottlenecks or yield determination	References
C. reinhardtii	Photoautotrophic	Introduction of phbB and phbC genes from R. eutropha	6 µg/g	limited expression of only two PHB pathway genes; potential lack of precursor (acetyl CoA)availability under photo auto trophy	Chaogang et al. (2010)
Synechocystis sp. PCC6803	Photoautotrophic	Double PTA (phosphotransacetylase) and ACH (acetyl-CoA hydrolase) deletion and heterologous overexpression of phosphoketolase (xfpk) from Bifidobacterium brevephosphoketolase	12.4%	Enhanced carbon flux through heterologous phosphoketolase pathway; suggests flux redistribution improved acetyl-CoA availability	Carpine et al. (2017)
Synechocystis sp. PCC 6803 (GOX50)	depletion of N2 sources naturally	SigE overexpression	1.4 mg/100 mg	Nitrogen limitation affects global metabolism; SigE regulates sugar catabolism, but may not sufficiently direct flux to PHB	Osanai et al. (2013)
Synechocystis sp. PCC 6714 (mutant MT_a24)	starvation of phosphorus and nitrogen	Point mutation in integral membrane protein A of the phosphate-specific transport system caused by random UV mutagenesis (PstA)	37%	Phosphate uptake disruption may activate PHB accumulation as a stress response	Kamravamanesh et al. (2018)
Synechocystis sp. PCC6803	Photoautotrophic; 21-day batch; phosphate limitation	Slr1829 and Slr1830 deletion	533.4 mg/L	Disruption of carbon storage regulatory genes enhances carbon flux towards PHB under phosphate limitation	Wang et al. (2013)
Phaeodactylum tricornutum	Autotrophic; NO3-induced synthesis of PHA	R. eutropha H16 PHA production gene insertion	10.6%	Nitrogen induction supports PHA biosynthesis, but native carbon flux in diatoms may limit polymer yield	Hempel et al. (2011)
Synechococcus elongatus UTEX 2973	Photoautotrophic	insertion of the phaABC operon from C. necator	16.7%	Fast growing chassis; PhaABC expression enables high yield; efficient precusor generation; supports improved PHB	Roh et al. (2021)
Synechococus elongatus PCC 7942	Photoautotrophic	insertion of the phaA and phaB genes from C. necator, tesB from E. coli and P. putida, and nphT7 from Streptomyces species as well as thioesterase genes	1.2 g/L	Multi gene engineering addresses multiple bottlenecks; tailored expression of thioesterases enhances precursor availability	Ku and Lan (2018)