Seasonal biomass production, fermentable saccharification and potential ethanol yields in the marine macroalga Ulva sp. (Chlorophyta)

Marine macroalgae (seaweeds) can accumulate high amounts of starch and cellulose in their tissues, hence, offering an attractive potential source of feedstock for bioethanol production. In this study we determined the specific growth rates (SGRs) in the seaweed Ulva sp. (Chlorophyta) during a full year cultured in a land-based facility to estimate sustainable biomass yields. We used 40 and 600 L outdoor tanks that received high and low irradiances, running seawater, aeration and external nutrients. Daily SGRs in 40 L tanks averaged 12 ± 2.04, 15.3 ± 0.8, 19.3 ± 1.8 and 10.3 ± 1.9% biomass increase per day for winter, spring, summer and autumn, respectively, while in 600 L growth averaged 2.5 ± 0.4, 2.2 ± 0.3, 9.8 ± 0.5 and 1.4 ± 0.4% d⁻¹, respectively. In an up-scaled growth trial during summertime in larger culture units of 3400 and 27,000 L growth rates were 4.6 ± 0.8 and 8.7 ± 1.04% d⁻¹, respectively. Altogether, the total biomass potential would yield 1.3 ton FW m⁻² y⁻¹. We also optimized enzymatic hydrolysis of dried biomass by means of time-course incubations with cellulase, amyloglucosidase and α-amylase in order to maximize the yields of total reducing sugars (TRS). We measured 163.6 ± 0.3, 179.7 ± 0.1 and 223.9 ± 0.5 mg TRS g⁻¹ DW for summer, spring and winter months, respectively, with an average fermentation of the hydrolysate of 0.3 ± 0.06 g EtOH g⁻¹ TRS. This work underlines the feasibility of seaweed feedstock production in land-based cultivation with respect to ethanol production.