Optimising pretreatment methodologies to improve the use of nuisance macroalgal biomass from bloom Ulva spp. prior to enzyme hydrolysis for downstream processing and application

The utilization of macroalgae for bioresource applications, particularly those derived from Ulva spp. is gaining global interest. This study presents an optimized process for the separation of Ulva biomass into its reducing sugar constituents, covering aspects of sourcing, washing, pre-treatment and enzyme saccharification. Tubular-form Ulva spp. from nuisance blooms were collected wild, with content analyses used to determine carbohydrate abundance relative to biomass weight. Optimal washing conditions were determined through tap-water wash cycles, with six washes resulting in 97.6% decrease in salinity compared with the initial wild Ulva composition. Subsequent trials used a commercially grown U. lacinulata; trialling excess water removal (dewatering) methods by comparing screw-pressing and electric spinner techniques. Here a higher efficacy of electric spinner batch dewatering was demonstrated, whilst retaining significantly higher carbohydrates than those following dewatering by screw-pressing. Comparative drying using different temperatures of oven drying and freeze-drying revealed that freeze-drying maintained a significantly higher carbohydrate abundance over biomass that was oven dried at lower temperatures, giving the highest carbohydrate yield overall. Ulva quality and its impact on enzymatically derived sugar release (saccharification) were then assessed. Freeze-dried U. lacinulata samples were autoclaved at different temperatures prior to enzyme saccharification using in-house ulvan lyases and commercial mixed cellulases. Pre-autoclaved biomass treated at 90°C gave the maximum release of rhamnose and other reducing sugars by ulvan lyases, whereas 120°C was optimal for cellulase-driven glucose release. U. lacinulata was left in air at room temperature for 0–72 h, then autoclaved and enzymically degraded using these optimal pre-autoclave conditions. A significant difference in sugar yields were seen, with longer degradation times typically leading to lower quantities of reducing sugars released. Collectively, these findings contribute to the development of efficient methodologies for processing and utilizing Ulva biomass including that from blooms for further bioprocessing applications.