Effect of ulvan extraction and process temperature on dark fermentation of the marine macroalgae Ulva

• Process temperature and pretreatment were investigated to enhance H 2 yield from Ulva. • Impact of ulvan extraction on the biorefinery potential of Ulva was investigated. • Ulvan-extracted Ulva achieved a maximum H 2 yield of 15.3 mL H 2 /g VS. • Highest VFA produced from fermentation of ulvan-extracted Ulva was 783.7 mg HAc eq /L. • Lyophilised Ulva returned the highest carbon recovery as sugars, i.e. 82%. Investigation of circular bioeconomy renewable fuel systems is currently a key area of research due to climate change. Biohydrogen production through dark fermentation of biomass wastes offers a pathway to renewable fuel, with numerous pretreatment techniques enabling the enhancement of this biological process. The macroalgal species Ulva offers significant potential as it contains the polysaccharide ulvan within the cell wall, which has uses in the medical industry due to its antiviral, antibacterial and anticancer activities. This study aims to investigate the impact of ulvan extraction and lyophilisation on the biohydrogen potential of Ulva biomass at different process temperatures (25, 37 and 55 °C). Among the investigated conditions, ulvan-extracted Ulva was the most promising biomass in terms of H 2 production, achieving 12.8 ± 0.2, 15.0 ± 1.2, and 15.3 ± 1.5 mL H 2 /g VS at 25, 37, and 55 °C, respectively. The return of H 2 at thermophilic conditions was negligibly higher than 25 °C and 37 °C. Thus, it is more energy-beneficial to operate at lower temperatures as the investment of energy to elevate the temperature does not equate to the return of H 2 obtained. In terms of volatile fatty acids, a peak concentration of 783.7 ± 31.8 mg HAc eq /L was observed at the end of the incubation of ulvan-extracted Ulva at 55 °C. On the other hand, the highest concentration of carbohydrates was obtained with lyophilised Ulva at 25 °C 1381.5 ± 141.1 mg/L. This data indicates that the removal of the polysaccharide ulvan did not hinder the biohydrogen potential of Ulva due to the effect of the pretreatment, both physical and chemical, intrinsic to the ulvan extraction process. Furthermore, this study highlights the biorefinery potential of Ulva biomass , aiming at maximising the bioproduct recovery from this macroalgae species, such as carbohydrates and volatile fatty acids.