From contamination to valorisation: a case study assessing culture contamination effect on the biofuel potential and nanomechanical properties of Parachlorella kessleri

Culture contamination is a critical problem in microalgae cultivation, often leading to the loss of affected batches and significant financial losses. Therefore, repurposing this contaminated biomass as a fuel source is one of the most viable valorisation outlooks, as the presence of contaminants in the energy sector is not as relevant as in cosmetic or food applications. This work evaluated the viability of this approach using a culture of Parachlorella kessleri contaminated with initially unknown micro-organisms. Microscopy analyses identified Tetradesmus sp. and bacilli as contaminants. The biochemical composition of this contaminated sample was compared to regular and nitrogen-starved cultures, showing no statistically significant differences from the regular batch. This resemblance also extended to the biodiesel profile, highlighting the suitability of valorising contaminated microalgae in biofuel applications. However, the evaluation of nanomechanical properties revealed that P. kessleri cells from the contaminated batch had Young's modulus remarkably higher than values reported in the literature, indicating that the cell disruption step could be more energy intensive. Hence, contaminated biomass remains biochemically suitable for biofuel valorisation, but potentially requires more energy-intensive processing.