Promoting Synechocystis sp. PCC 6803 Harvesting by Cationic Surfactants: Alkyl-Chain Length and Dose Control for the Release of Extracellular Polymeric Substances and Biomass Aggregation

Development of an efficient biomass-harvesting technology for microalgae would achieve cost and energy savings in large-scale microalgae biomass cultivation. Cationic surfactants could improve biomass harvesting, but determining the optimal type and dose of surfactant requires mechanistic understanding. In this study, we evaluated how the alkyl-chain length and dose of three cationic surfactants - hexadecyltrimethylammonium bromide (CTAB), myristyltrimethylammonium bromide (MTAB), and dodecyltrimethylammonium bromide (DTAB) - affected biomass harvesting of Synechocystis. Flow cytometry (FC) with the nucleic-acid (NA) stain SYTOX Green (SG) was used to differentiate the release of extracellular polymeric substances (EPS) from cell lysis. All selected cationic surfactants dramatically improved the biomass-harvesting efficiency, and harvesting kinetics were represented well with a first-order kinetic model. The efficiency of biomass harvesting correlated positively with the alkyl-chain length; i.e., CTAB > MTAB > DTAB. A longer alkyl chain increased EPS release, which made it easier to achieve a less-negative zeta potential but without cell lysis. For CTAB, the largest cationic surfactant tested, a dose of 4.5 mM and treatment for 60 min achieved the maximum harvesting efficiency of ∼91%. This work lays the foundation for optimizing surfactant species and dose for biomass harvesting.