A cyanobacterial dual fructose-1,6/sedoheptulose-1,7-bisphosphatase gene expression and its effect under high light in Chlamydomonas reinhardtii

Microalgae such as C. reinhardtii are extensively explored for various applications such as fuel, food, feed, wastewater treatment, bioremediation, etc. Limited inorganic CO₂ absorption and fixation in the Calvin-Benson-Bassham (CBB) cycle and photo-inhibition at higher lights are two significant challenges responsible for limited biomass productivity at large-scale cultivation. Overcoming these limitations is crucial to attaining biomass productivity at a commercial scale. In this study, Calvin cycle's rate-limiting enzymes, fructose-1,6 bisphosphatase (FBPase) and sedoheptulose 1,7-bisphosphatase (SBPase) were overexpressed in the chloroplast of C. reinhardtii using unique dual-activity FBP/SBPase with an antibiotic-free phototrophic selection system. Expression of this dual enzyme resulted in 1.5–2.6-fold higher biomass accumulation under both phototrophic and mixotrophic conditions. The dual enzyme expression could thus alleviate the limitations of the CBB cycle, resulting in enhanced biomass accumulation. Overcoming CBB cycle limitations also conferred high light tolerance, resulting in reduced photo-inhibition. This is the first report demonstrating the expression of dual FBP/SBPase in C. reinhardtii.