Influence of light intensity and single or mixed heavy metals (Cr, Cd, Pb) on Thermosynechococcus sp. CL-1 (TCL-1) CO₂ fixation, pollutant removal, and byproducts

Due to its survivability in the presence of pollutants, cultivation of photosynthetic microorganisms has a broad implementation in wastewater treatment. Besides, accumulation of valuable byproducts during the cultivation is also favorable. In this study, Thermosynechococcus sp. CL-1 (TCL-1), a cyanobacterium, was assessed under the effect of light intensity and combined with the presence of single (Cr) or mixed (Cr, Cd, and Pb) heavy metals. Performance was examined for its fixation rate of carbon, removal of heavy metals, and accumulation of byproducts (lipid, carbohydrate, and phycocyanin). Higher light intensity promoted the growth of TCL-1 along with the CO₂ fixation rate. Compared to the control with absence of heavy metals, the presence of 0.5 mg/L Cr as single heavy metal induced a better performance of TCL-1, which might be due to the hormesis phenomenon. Nevertheless, higher concentrations of added Cr (VI) or mixed with other types of heavy metal led to a decline in TCL-1 performance. TCL-1 implemented bioadsorption as the major heavy metal removal mechanism, which might be supported by the variation in the generated exopolysaccharide/EPS content. Mixed heavy metals medium improved the removal capacity of TCL-1 with 0.140, 0.332, and 0.882 mg/g for Cr, Cd, and Pb, respectively, in the illumination of 2000 μE/m²/s. During the cultivation with exposure of heavy metals, the production of lipid and carbohydrate by TCL-1 was insignificantly influenced. However, the production of allophycocyanin was significantly affected by the level of Cr content.