Unravelling the impact of nitrogen deprivation on the morphology and nanomechanical properties of Parachlorella kessleri

Abstract Nitrogen deprivation is commonly applied to enhance lipid accumulation in microalgae for third-generation biofuel production. However, its influence on cell wall morphology and nanomechanical properties, which critically affect downstream processing, remains poorly explored. To address this gap, this study investigated the impact of nitrogen deprivation on these properties of the freshwater microalga Parachlorella kessleri using a multi-microscopy approach. Starvation did not lead to any significant morphological changes in either cell size (diameter and height) or cell wall thickness, but it significantly impacted nanomechanical properties: the Young’s modulus decreased by approximately 60% (from 1320.7 ± 415.1 to 535.8 ± 346.5 kPa), while surface roughness remained comparable between conditions (1.3 ± 0.5 and 1.9 ± 1.4 nm). Regardless of these similar roughness values, starved cells showed a strong negative correlation between roughness and stiffness, suggesting underlying structural changes not captured by morphology alone. Although biomass freezing for storage and subsequent thawing for analysis may have influenced absolute mechanical values, the comparative trends observed indicate that nitrogen deprivation preconditions the structural and nanomechanical features of this microalga. Therefore, this work highlights the potential of nanomechanical descriptors as complementary indicators for evaluating biomass processability in biofuel and bioproduct applications, opening the door to the development of more efficient and subsequently cost-effective cell disruption strategies.