Mechanisms of breakdown of Haematococcus pluvialis cell wall by ionic liquids, hydrochloric acid and multi‐enzyme treatment

Abstract

SummaryThe robust cell wall structure of Haematococcus pluvialis (H. pluvialis) consists of polysaccharides and tough non‐hydrolysable sporopollenins, which makes it difficult to extract superpotent antioxidant, astaxanthin from these cells. Therefore, breakdown of cell wall is a key step in the overall process of astaxanthin recovery. In this study, the mechanism of three well‐established chemical techniques for cell disruption of H. pluvialis cysts [ionic liquids (IL), hydrochloric acid (HCl) and multiple enzymes (multi‐enzyme, ME)] on deconstruction of the cyst cell wall of H. pluvialis was explored and characterised by Fourier transform infrared spectroscopy (FTIR), X‐ray diffraction (XRD), nuclear magnetic resonance (NMR) and gas chromatography–mass spectrometry (GC‐MS) analyses. The results demonstrated that the three cell wall breakdown techniques exhibited high extraction efficacies for the recovery of astaxanthin from H. pluvialis [IL (86.71 ± 2.06%), HCl (80.52 ± 2.28%) and ME (71.08 ± 2.49%)]. However, their performances on disrupting the trilayered cell walls of H. pluvialis were significantly different, which were confirmed by distinct morphologies of the treated cell walls visualised by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Meanwhile, the results of FTIR confirmed that, to some extent, cellulose, hemicellulose and lignin in the cell walls were hydrolysed by HCl, IL and ME treatments. However, ME exhibited a less hydrolytic effect on lignin than HCl and IL. Moreover, XRD and NMR analyses implied that the amorphous region of cell wall was susceptible to hydrolysis/breakdown by the three techniques.