Effects of dissolved inorganic carbon concentrations on the stable growth of Ethiopian microalga Limnospira fusiformis under semi-continuous cultivation

Abstract Limnospira fusiformis LC-30 is a promising Ethiopian cyanobacterium for sustainable biomass production owing to its high growth potential and protein content. Dissolved inorganic carbon (DIC) is a key factor affecting both productivity and cultivation cost in Spirulina -related cyanobacterial production; however, the DIC concentration required to maintain stable biomass production during semi-continuous cultivation remains unclear. In this study, L. fusiformis LC-30 was cultivated at four DIC concentrations (0.02, 0.05, 0.10, and 0.20 M) to evaluate their effects on growth and biomass productivity. The highest dry weight and biomass productivity were obtained at 0.20 M DIC, reaching 1.14 ± 0.06 g L⁻¹ and 0.23 ± 0.01 g L⁻¹ d⁻¹, respectively, and were significantly higher than those under lower DIC conditions. However, L. fusiformis LC-30 maintained 65–74% of the maximum biomass productivity under 0.02–0.10 M DIC, demonstrating its ability to sustain substantial biomass production even with reduced inorganic carbon supply. Protein content remained high and stable at approximately 55–60% of dry weight, indicating that reduced DIC availability did not markedly compromise biomass quality. Among the tested conditions, 0.20 M DIC resulted in the highest biomass and protein productivity. Nevertheless, L. fusiformis LC-30 maintained substantial biomass productivity and a high protein content under reduced DIC conditions, highlighting the potential to reduce inorganic carbon input in semi-continuous cultivation.