Acidogenic fermentation of Ulva in a fed-batch reactor system: tubular versus foliose biomass

The present study proposes a biorefinery of the macroalgae Ulva, focusing on evaluating two different morphologies of the species (foliose and tubular) during acidogenic fermentation in fed-batch reactors. Stage 1 of the study evaluates lyophilised foliose and tubular Ulva, whilst Stage 2 analyses the impact of ulvan extraction on volatile fatty acids yield and changes in carbohydrate availability. Acetic, propionic, and butyric acids were produced from each substrate, with peak concentrations of total VFAs recorded at 2179.5 mg HAc/L (foliose Ulva) and 2029.3 mg HAc/L (tubular Ulva) when ulvan was present. After ulvan extraction, the acidogenic fermentation of the foliose morphotype was negatively affected, reaching at most 315.3 mg HAc/L. In contrast, the extraction showed no influence on the tubular morphotype, peaking at 2165.0 mg HAc/L. Additional variations were noted in the availability of carbohydrates in each substrate during the acidogenic fermentation process. The ulvan-extracted tubular morphotype exhibited the highest peak in carbohydrate concentration (9.8 g glucose/L), whilst the ulvan-extracted foliose morphotype yielded up to 8.5 g glucose/L. This study highlights the biorefinery potential of Ulva biomass, proposing a multiple cascading approach linking multiple energy and biomolecule applications to maximise the valorisation of the biomass.