Cyanobacterial biomass pretreatment strategies enhance biogas production and alter the anaerobic microbial community

Cyanobacterial blooms pose severe environmental challenges in surface waters on a global scale. Although mechanical harvesting is an effective bloom mitigation strategy, sustainable utilization of the harvested biomass remains a major challenge. Anaerobic digestion (AD) offers an attractive valorisation alternative; however, relatively low methane yields limit its practical application. This study evaluated the effects of four pretreatment methods (grinding, ultrasonication, thermal, and thermochemical) on the AD performance of two bloom-forming cyanobacteria ( Aphanizomenon flos-aquae and Microcystis aeruginosa ), focusing on cell morphology, methane production and microbial community composition. Biomethane potential assays showed that ultrasonication produced the highest methane yields for both species (308.42 ± 37.67 and 376.95 ± 13.96 mL CH 4 g −1 VS for A. flos-aquae and M. aeruginosa , respectively), while grinding achieved comparable performance for A. flos-aquae , reflecting its filamentous morphology. In contrast, the colonial structure and glycocalyx of M. aeruginosa reduced the effectiveness of grinding while favouring ultrasonication and thermal pretreatments. Lower methane yields were observed at the higher organic loads assessed, suggesting substrate overloading under the tested experimental conditions. Optical microscopy demonstrated distinct species-dependent morphological responses to pretreatment, while microbial analysis revealed shifts in the relative abundance of bacterial and archaeal taxa associated with AD. These microbial observations indicate correlations between pretreatment, community composition and methane production. Overall, pretreatment enhanced methane yields by up to 94.8% compared with untreated biomass, demonstrating the importance of selecting pretreatment strategies according to cyanobacterial biomass characteristics to improve AD performance.