High methane yields in anaerobic digestion of the cyanobacterium Pseudanabaena sp

Microalgae are a novel biomass feedstock for the production of biogas through anaerobic digestion. However, anaerobic digestion of microalgal biomass often suffers from low methane yields due to their rigid cell walls. In contrast to eukaryotic microalgae, prokaryotic cyanobacteria have a weaker cell wall and might be more attractive as feedstock for anaerobic digestion. Nevertheless, feedstock biomass composition influences the anaerobic digestion process. In cyanobacteria, nitrogen is a key nutrient for which availability induces changes in the biomass composition. In this study, the cyanobacterium Pseudanabaena sp. CY14-1 was cultivated at different nitrogen concentrations (14, 42 and 56 mg N L-1) to achieve biomass with a variable carbohydrate content (61, 38 and 23% dry weight, respectively). Anaerobic digestion of these three types of Pseudanabaena sp. CY14-1 biomass was performed in batch experiments to evaluate the effect of biomass composition on methane yields. During the first days of the experiment, the methane yield of the biomass was comparable irrespective of the carbohydrate content. The final methane yield, however, was higher for the biomass with 23% carbohydrate content (251 mL CH4 g(-1) CODin) than for the biomass with 38 and 61% carbohydrates (226 and 215 mL CH4 g(-1) COD(in, )respectively). In contrast, 61% carbohydrate-rich biomass showed the highest hydrolysis constant (0.21 day(-1)) among the three types of Pseudanabaena sp. CY14-1 biomass. Methane yields achieved with these three biomasses are higher than those reported for some eukaryotic microalgae species, highlighting the potential of cyanobacteria as biomass feedstock for anaerobic digestion.