Cyanobacteria as a Photosynthetic Chassis for Metabolic Pathway Engineering with Heterologous Gene Expression

Cyanobacteria are increasingly recognised as photosynthetic chassis for sustainable metabolic engineering because oxygenic photosynthesis generates ATP and NADPH via the photosynthetic electron transport chain, which drive CO2 fixation through the Calvin–Benson–Bassham cycle into carbon intermediates that can be redirected toward engineered heterologous pathways. Their genetic tractability, CO2-fixing capacity, ecological adaptability, and relatively simple cellular organisation make them attractive platforms for developing low-carbon biotechnological processes. This review explores recent progress in engineering cyanobacteria for heterologous pathway construction, critically evaluating genetic tools including transformation methods, genome integration strategies, promoter systems, and CRISPR-based editing, with specific emphasis on challenges of direct relevance to phototrophic chassis: host–pathway metabolic compatibility, precursor supply, cofactor balancing between photosynthetic output and heterologous pathway demand, and achieving genetic stability in polyploid cyanobacterial genomes. The review also addresses key limitations with mechanistic context: metabolic burden from multi-gene pathway expression reduces growth rate and selects against producing cells; polyploidy delays complete chromosomal segregation of engineered constructs; slow photoautotrophic growth constrains volumetric productivity; native regulatory networks resist carbon flux redirection; and cultivation constraints—including light attenuation in dense cultures and mismatches between photosynthetic ATP/NADPH supply and heterologous pathway demand—further limit achievable yields.