Bioplastics are increasingly being recognized as a viable substitute for conventional plastics across a broad spectrum of applications. Microalgae, such as Spirulina platensis (Geitler) and Haematococcus pluvialis (Flotow), are emerging as sustainable sources for bioplastic production, aligning with the principles of a circular bioeconomy. This study employed response surface methodology (RSM) to optimize growth conditions for bioplastic-producing microalgae, Spirulina platensis and Haematococcus pluvialis, with the aim of maximizing polyhydroxybutyrate (PHB) accumulation. The results showed that aerated Zarrouk's medium optimized S. platensis growth at 20°C, pH 8, 5000 lux, and 0.025 M salinity, while medium growth was optimized at 30°C, pH 9, 5000 lux, and 0.3 M salinity. In contrast, aerated BG11 medium optimized H. pluvialis growth at 26.5°C, pH 7.9, 6500 lux, and 0 M salinity, with medium growth optimized at 27.5°C, pH 5.5, 3500 lux, and 0.2 M salinity. Notably, H. pluvialis under medium growth conditions yielded the highest PHB content (61.04±0.3mg/ g), whereas S. platensis under maximum growth conditions yielded the lowest PHB content (20.43±0.56mg/ g). Furthermore, PHB production was significantly enhanced for H. pluvialis by the addition of sodium acetate in phosphorus-deficient media, reaching a maximum of 197.58±0.6mg/ g with 2g/ L acetate.