New insights into the effects of growth phase and enzymatic treatment on the cell-wall properties of Chlorella vulgaris microalgae
Résumé
Green microalgae are a natural source of oil for commercial biodiesel production. However, their cell-wall
barrier remains a major obstacle for effective intracellular lipid extraction. Solution to this issue includes the
use of surface-digesting enzymes and the control of the so-far poorly understood changes in structural and
mechanical properties of the microalgal envelops during growth and upon action of enzymes. Here, we used a
combination of atomic force microscopy (AFM) and confocal microscopy to decipher the variation of cell-wall
ultrastructure, composition and nanomechanics of Chlorella vulgaris upon culture ageing and lysozyme treat-
ment. AFM imaging revealed the presence of a fibrillated mesh at the surface of cells harvested in the stationary
phase, hardly distinguishable on cells from younger culture (mid-log phase). The fibrils form a chitin-like
network containing the N-acetyl-D-glucosamine unit, and this structured network is severely damaged upon
lysozyme treatment, a property we identified at the cellular and molecular scales by fluorescent-lectin staining
and AFM-based force spectroscopy using lectin-modified tips, respectively. The enzyme was also found to act on
algal physiology, to trigger oxidative stress and changes in cell lipid content. Detailed single-cell mapping of the
nanomechanical properties of C. vulgaris further indicated that microalgal cells soften upon ageing and it
confirmed that lysozyme affects both their surface and intracellular compartments. Altogether, our results
emphasize that AFM-based multi-parametric analysis in combination with confocal microscopy allows accurate
evaluation of physico-chemical surface properties of microalgae and offers an exciting perspective on cell culture
condition optimization for facilitated oil extraction.
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