Effect of culture density on biomass production and light utilization efficiency of <i>Synechocystis</i> sp. PCC 6803

Abstract

Abstract The viability of large‐scale microalgae cultivation depends on providing optimal growth conditions, for which a key operational parameter is culture density. Using Synechocystis sp. PCC 6803, we conducted a series of fixed‐density, steady‐state experiments and one batch‐growth experiment to investigate the role of culture density on biomass production and light utilization efficiency. In all cases, the fixed‐density, steady‐state experiments and batch‐growth experiment showed good agreement. The highest biomass production rates (260 mg L −1 d −1 ) and efficiency for converting light energy to biomass (0.80 μg (μmol photons) −1 ) occurred together at a culture density near 760 mg L −1 , which approximately corresponded to the lowest culture density where almost all incident light was absorbed. The ratio of OD 680 /OD 735 increased with culture density up to the point of maximum productivity, where it plateaued (at a value of 2.4) for higher culture densities. This change in OD 680 /OD 735 indicates a photoacclimation effect that depended on culture density. Very high culture densities led to a sharp decline in efficiency of biomass production per photons absorbed, likely due to a combination of increased decay relative to growth, metabolic changes due to cell‐cell interactions, and photodamage due to mixing between regions with high light intensity and zero light intensity.