Investigation of polysaccharide extraction efficiencies from diatom biomass using green extraction techniques Green extraction of diatom polysaccharides considering a biorefinery concept and preservation of biogenic silica.

Abstract

Diatoms, the abundant unicellular microalgae encapsulated by a silica cell wall called frustules, have received interest as a feedstock for biomass valorization. Despite its promising content, challenges remain regarding the down stream processing of diatoms which suffers from long processing time and reliance on toxic solvents. This study has explored two green extraction techniques, Ultrasound Assisted Extraction (UAE) and Microwave Assisted Extraction (MAE), to investigate extraction yields of polysaccharides. While polysaccharides were the extraction target, a biorefinery approach has been considered, meaning the extraction yields of soluble proteins, lipids and pigments, and the impact on the frustules have been tested. High pressure homogenization (HPH) treated biomass was subjected to extractions in water, which were performed in a 22-factorial design of experiments for both extraction methods. The extract supernatant was used for quantification of extraction yield, monosaccharide composition and soluble protein content. The separated pellet was used for lipid and pigment analysis, and observed under a scanning electron microscope (SEM) to assess the structural integrity of the cell walls. The green extraction methods were compared with a conventional control of hot water extraction (HWE). Overall, MAE presented a gentle physical treatment, with an increased extracellular monosaccharide content compared to HWE. The overall extraction yields of MAE and HWE were similar, displaying values in the range of 20-25%, where the MAE extracts contained the highest amount of monosaccharides (12%). However, there was no dramatic increase compared to the 9-10% content measured in UAE and HWE, or between the different conditions. SEM imaging confirmed a marginal impact on the frustule integrity by MAE. Moreover, pigment yield was enhanced compared to initial biomass, whereas soluble protein yield decreased, likely due to denaturation in elevated extraction temperatures. UAE conditions resulted in the highest extraction yield at approximately 45%, twofold that of MAE and HWE. Furthermore, UAE increased soluble protein yield by 85% compared with untreated biomass and showed a content twofold that of MAE, but induced the most damage to the frustules by fragmenting them. The disruption of cells increased the fraction of extracellular polysaccharides, but reduced the levels of glucose, possibly due to degradation of the polysaccharides into oligo- and monosaccharides, which should be studied further. The performed extraction screening presents promising results, with possibilities of method alterations and optimization for the future use of diatoms as a biorefinery feedstock.