Bioprocessing of Microalgae for Bioenergy and Recombinant Protein Production

Abstract

This dissertation investigates harvesting of marine microalgae for bioenergy and production of two recombinant proteins for therapeutic applications in Chlamydomonas reinhardtii. The first study describes harvesting of marine microalgae by flocculation using aluminum chloride (AlCl_3), natural polymer chitosan, and synthetic cationic polymers.

Harvesting and concentration process of low concentration microalgae cultures ranging from 1 to 2 g dry weight per liter was affected by algogenic organic matter (AOM), ionic strength, cell concentration, polymer charge density, and media pH. Marine microalgae flocculation was greatly affected by the presence of AOM independently of the flocculant chemistry. Presence of AOM demanded extra flocculant dosage i.e., 3-fold of AlCl3, 7-fold of highly charged synthetic cationic polymer, and 10-fold of chitosan. Flocculant dosage required for > 90 % flocculation efficiency in the presence of AOM was 160 mg/L, 50 mg/L, and 20 mg/L when using AlCl_3, chitosan, and best (more efficient) synthetic polymer respectively. The high-ionic strength of saline water did not have a significant effect on flocculation efficiency when using AlCl_3. However, to achieve efficient algal biomass removal, application of highly-charged synthetic polymers was required to overcome the presence of electrolytes. The best synthetic cationic polymer tested herein, which achieved greater than 90 % flocculation efficiency at 20 mg/L dosage, was a polymer with 99 % cationic charge density. Cell concentration also affected flocculant dosage requirement; low density cultures (10^6 cells/mL) required 6-fold greater dosages than cultures grown until early stationary phase (10^7 cells/mL).

The second study addresses cultivation, extraction and purification challenges of two complex recombinant proteins, an immunotoxin molecule (MT51) and malaria vaccine antigen (Pfs25) produced in the chloroplast of C. reinhardtii. Main challenges identified were i) low transgene expression level, ii) proteolytic instability of MT51 immunotoxin, and iii) aggregation of Pfs25 antigen. Optimal expression and accumulation of Pfs25 antigen required growing C. reinhardtii cultures to late exponential phase (10^6 cells/mL) and inducing transgene expression for 24 h at a photon irradiance of 120 µmol/m^2s.