Bacterial bioaugmentation for improving methane and hydrogen production from microalgae

Background: The recalcitrant cell walls of microalgae may limit their digestibility for bioenergy production. Considering that cellulose contributes to the cell wall recalcitrance of the microalgae Chlorella vulgaris, this study investigated bioaugmentation with a cellulolytic and hydrogenogenic bacterium, Clostridium thermocellum, at different inoculum ratios as a possible method to improve CH4 and H-2 production of microalgae. Results: Methane production was found to increase by 17 similar to 24% with the addition of C. thermocellum, as a result of enhanced cell disruption and excess hydrogen production. Furthermore, addition of C. thermocellum enhanced the bacterial diversity and quantities, leading to higher fermentation efficiency. A two-step process of addition of C. thermocellum first and methanogenic sludge subsequently could recover both hydrogen and methane, with a 9.4% increase in bioenergy yield, when compared with the one-step process of simultaneous addition of C. thermocellum and methanogenic sludge. The fluorescence peaks of excitation-emission matrix spectra associated with chlorophyll can serve as biomarkers for algal cell degradation. Conclusions: Bioaugmentation with C. thermocellum improved the degradation of C. vulgaris biomass, producing higher levels of methane and hydrogen. The two-step process, with methanogenic inoculum added after the hydrogen production reached saturation, was found to be an energy-efficiency method for hydrogen and methane production.