Anaerobic digestion of microalgal biomass: Challenges, opportunities and research needs

•Microalgae biomass has a huge potential as feedstock for biogas production.•Novel pretreatments, reactor designs and operation parameters are addressed.•The microbial community carrying out anaerobic digestion is reviewed.•Research on intermediate compounds exploitation is discussed.•Integration of microalgae culture and anaerobic digestion is outlined. Integration of anaerobic digestion (AD) with microalgae processes has become a key topic to support economic and environmental development of this resource. Compared with other substrates, microalgae can be produced close to the plant without the need for arable lands and be fully integrated within a biorefinery. As a limiting step, anaerobic hydrolysis appears to be one of the most challenging steps to reach a positive economic balance and to completely exploit the potential of microalgae for biogas and fertilizers production. This review covers recent investigations dealing with microalgae AD and highlights research opportunities and needs to support the development of this resource. Novel approaches to increase hydrolysis rate, the importance of the reactor design and the noteworthiness of the microbial anaerobic community are addressed. Finally, the integration of AD with microalgae processes and the potential of the carboxylate platform for chemicals and biofuels production are reviewed.