In this study, the geothermal water (GW) was replaced with 25% (GW25), 50% (GW50), 75% (GW75) and 100% (GW100) of the volume of the Schlosser culture medium (SM). The biomass concentration was determined to the higher in the GW50 (1.324 g/L) than in other groups (GW25, GW75, GW100 and SM). The highest protein content and phycocyanin purity ratio were found in the GW75 medium containing 116.6 times higher phosphate than the SM. The chlorophyll, phycocyanin, protein, dry biomass, mineral and amino acid in groups prepared by mixing Schlosser medium and geothermal water mix (GW25, GW50 and GW75) were better than geothermal water (GW100) and the SM. It is concluded that using partial geothermal water in Spirulina production is efficient considering the parameters, i.e., biomass yield, protein, mineral and chlorophyll contents, and also return of investment. Spirulina (Arthrospira platensis Gomont, 1892) is grown in highly alkaline waters and is an important ingredient for many industries. However, analytical grade-based media in used commercial-scale production of Spirulina is quite expensive. Spirulina cultivation was studied by replacing the Schlosser medium with geothermal water in terms of its biomass, biochemical content, amino acid and mineral production. Spirulina was cultured in a semi-open raceway system with a 420 L culture volume for three weeks. Then, the geothermal water (GW) was replaced with 25% (GW25), 50% (GW50), 75% (GW75) and 100% (GW100) of the volume of the Schlosser culture medium (SM). The biomass concentration was determined to the higher in the GW50 (1.324 g/L) than in other groups (GW25, GW75, GW100 and SM), while the minimum yield was in GW100 (0.624 g/L) group. The highest protein content and phycocyanin purity ratio were found in the GW75 medium containing 116.6 times higher phosphate than the SM. The chlorophyll content of SM, GW25 and GW50 was higher than that of the GW100 (P < 0.05). The highest lipid content was determined in the GW100 (P < 0.05). Isoleucine, leucine and valine levels of the GW50 and GW75 were higher than those of the other groups (P < 0.05). The iron levels of the GW50 and GW75 groups, which have the highest nitrate content in the culture mediums, were significantly lower. The chlorophyll, phycocyanin, protein, dry biomass, mineral and amino acid in groups (GW25, GW50 and GW75) prepared by mixing Schlosser medium and geothermal water mix were better than geothermal water (GW100) and SM. As a result, it is concluded that it is possible to use partial geothermal water in Spirulina production under this study conditions.
