Structural and biochemical characterization of Nannochloropsis oceanica IMET1 cell wall and its implications for enzyme-assisted extraction

Microalgae are considered a potential sustainable feedstock for producing ingredients used in food, feed and cosmetics. However, the rigid cell wall structure presents a significant challenge for the efficient extraction of intracellular products. Enzymatic hydrolysis of cell wall polymers could offer a mild and environmentally sustainable cell disruption approach. Nevertheless, incomplete knowledge of the structural and architectural properties of the cell wall limits the commercial application of this method. In this study, a combination of biochemical and microscopy techniques was used to decipher the composition, topography, ultrastructure and the architecture of the cell wall of Nannochloropsis oceanica IMET1. Acid hydrolysis of the extracted cell wall released neutral sugars, constituting 80 % of its total weight (w/w). Of these sugars, 64 % was glucose, which primarily constitutes the cellulose layer. This was further confirmed through calcofluor white fluorescence imaging using a scanning confocal microscope. Additionally, the presence of small amounts of arabinose, fucose, galactose, and mannose suggests the presence of polysaccharide networks connecting the outer layer, the cellulose-based layer and the plasma membrane. Scanning electron microscopy revealed a rugged cell surface covered with N-acetyl-glucosamine units as confirmed by fluorescent-lectin staining. Transmission electron microscopy revealed a double-layered cell wall structure. Hydrolysis of the extracted cell wall with lichenases and cellulases generated mostly glucose monomers alongside traces of other neutral sugars. Overall, this study provides insights into the complex structure of the cell wall of N. oceanica IMET1, which can enhance enzyme assisted extraction strategies in microalgae.