Improvement of stress multi-tolerance and bioethanol production by Saccharomyces cerevisiae immobilised on biochar: Monitoring transcription from defence-related genes

The study determined the protective role of using biochar as immobilisation carrier against multiple stresses encountered by Saccharomyces cerevisiae assessing transcription from important metabolic routes involved in the molecular mechanisms triggered during inhibitory bioprocess conditions. Immobilised cells exhibited higher bioethanol titre (39 g L-1) and productivity (7.72 g L-1 h(-1)) at elevated temperatures compared with the suspended culture that yielded 34 g L(-1 )and 1.99 g L-1 h(-1) respectively. Fermentation at 39 ? resulted in 2.15-fold increase of HSP104 relative mRNA expression in suspended cells, while the gene was induced by 0.5-fold using the immobilised biocatalyst. A similar response occurred for HSF1 and TPS exhibiting 3.0- and 3.8-fold increase using suspended cells as opposed to the application of immobilised cells where transcription of the aforementioned genes was raised by 0.0- and 2.6-fold upon temperature increase respectively. Transcription from MSN2/ MSN4 under the aforementioned conditions indicated the protective role of cell attachment on the biomaterial against stimulation of the heat shock response route and oxidative stress. Although fermentations conducted under ethanol stress resulted in failure of the conventional process, immobilised cells produced 21 g L-1 bio-ethanol exhibiting 7 g L-1 h(-1) productivity, while monitoring transcription of HSP12 and HSP104 demonstrated the beneficial use of the proposed technology. Proline accumulation during osmotic stress further supported the elevated bioethanol productivity achieved by the immobilised system, which was 74% higher as opposed to the conventional process. The study confirmed that S. cerevisiae immobilisation on biochar conferred cells with heat tolerance, ethanol tolerance, osmotolerance and improved fermentation capacity. The technology proposed constitutes a sustainable technological alternative to strain modification improving multiple stress-tolerance in bioethanol fermentations.