Microalgae Flocculation: Assessment of Extraction Yields and Biological Activity

Abstract

Downstream costs represent one of the main obstacles to enabling microalgae to become widespread. The development of an economical, easily scaled-up strategy could reduce the overall process costs. Here, different flocculants were tested on different microalgae strains and a cyanobacterium. The results indicate that flocculation could be an alternative to centrifugation, as CaCl₂ induced a complete flocculation of green and red marine strains (96 ± 4% and 87.0 ± 0.5%, respectively), whereas Chitosan was the only agent able to induce flocculation on the cyanobacterium (46 ± 1%). As for the thermoacidophilic red microalga, 100% flocculation was achieved only by increasing the pH. Carotenoids were extracted from the flocculated biomass, and the strategy improved with the use of the wet biomass. The results indicate that flocculation does not affect carotenoid yield, which is at least the same than that obtained upon centrifugation and extraction from the wet biomass. Then, for the first time, the biological activity of the extracts obtained from the flocculated biomasses was evaluated. The results indicate that only the green microalga extract shows increased antioxidant activity. In conclusion, this work highlights that a general downstream procedure cannot be developed for microalgae strains but should be rationally tailored.

Keywords: antioxidant activity; flocculation; green extraction; harvesting; industrial processes; microalgae.

Figure 1

Flocculation efficiency (%) of chitosan (CS). Effect of different CS concentrations on the flocculation efficiency of microalgae strains analyzed immediately (0 min, black bars), 10 min (grey bars) and 20 min (white bars) after stirring. (A), P. cruentum; (B), G. phlegrea; (C), P. simplex; (D), S. bigranulatus upon different flocculation times. Data are obtained by means of three independent experiments (mean ± S.D.). * p < 0.05. The lines above the bars indicate the two samples compared for statistical analysis.

Figure 2

Flocculation efficiency (%) of calcium chloride (CC). Effect of different CC concentrations on the flocculation efficiency of microalgae strains analyzed immediately (0 min, black bars), after 10 min (grey bars) and after 20 min (white bars) from stirring. (A), P. cruentum; (B), G. phlegrea; (C), P. simplex; (D), S. bigranulatus upon different flocculation times. Data are obtained by means of three independent experiments (mean ± S.D.). * p < 0.05, ** p < 0.005. The lines above the bars indicate the two samples compared for statistical analysis.

Figure 3

Flocculation efficiency (%) after pH increase. Effect of pH on the flocculation efficiency of microalgae strains analyzed immediately (0 min, black bars), after 10 min (grey bars) and after 20 min (white bars) from stirring. (A), P. cruentum; (B), G. phlegrea; (C), P. simplex; (D), S. bigranulatus upon different flocculation times. Data are obtained by means of three independent experiments (mean ± S.D.). * p < 0.05, ** p < 0.005. The lines above the bars indicate the two samples compared for statistical analysis.

Figure 4

Pigments extraction. Extraction yields (%) are reported as mg of extract with respect to mg of D.W. biomass. Black bars indicate centrifuged dry biomass, dark grey bars indicate centrifuged wet biomass, light grey bars indicate flocculated wet biomass. The white bar refers to flocculated biomass at pH 13. (A), P. cruentum; (B), G. phlegrea; (C), P. simplex; (D), S. bigranulatus. Results are expressed as the mean ± S.D. of three independent experiments. * indicates p < 0.05, ** indicates p < 0.01. The lines above the bars indicate the two samples compared for statistical analysis.

Figure 5

Antioxidant activity of ethanol extracts. Values are expressed as IC₅₀ (μg/mL), i.e., the concentration required to scavenge 50% of free radical ABTS. Black bars indicate centrifuged wet biomass, dark grey bars indicate flocculated wet biomass, and the light grey bar indicates flocculated wet biomass at pH 13. (A), P. cruentum; (B), G. phlegrea; (C), P. simplex; (D), S. bigranulatus. Data are shown as means ± S.D. Three independent measurements were carried out. * indicate p < 0.01; ** indicate p < 0.001 with respect to centrifuged wet biomass. The lines above the bars indicate the two samples compared for statistical analysis.