Flocculation activity of carp protamine in microalgal cells

Abstract

Protamine, a polymer cationic peptide, originates from fish milt, an aquatic bioresource produced in massive quantities as a byproduct of fish processing. Protamine has been used as a heparin antagonist, hemostatic agent in the medical field and as an antimicrobial agent in the food industry. However, few studies have examined its flocculation activity. To investigate the applications of protamine extracted from carp (Cyprinus carpio Linnaeus) as a flocculating agent in microalgal cells, protamine extracts were prepared by digestion with sulfuric acid and purified by ion-exchange chromatography. The results showed that purified protamine was rich in lysine (39.24%) but low in arginine (1.92%). Carp protamine showed good flocculation activity towards Tetraselmis subcordiformis, with the flocculation rate reaching 85.00% at a protamine concentration of 20 mg/L. However, lower flocculation rates were observed for Dunaliella salina (23.56%) and Chlorella vulgaris (9.65%) at the same protamine concentration. After flocculation, precipitated T. subcordiformis cells were viable and could be recultured in fresh medium, showing similar growth curves as the original culture. Additionally, residual cells in the upper liquid were recultured and flocculated by protamine again, with a flocculation rate reaching 84.20% at a protamine concentration of 20 mg/L. The flocculating effect may be related in part to the electrostatic affinity of protamine to the negative surface charges of microalgal cells. The flocculation activity of carp protamine showed potential for use in harvesting of microalgal cells in the aquaculture industry. Furthermore, our results may have applications in the development of new approaches for promoting the high-value utilization of fish milt.