Recent trends in anaerobic digestion of macroalgae for blue carbon derived biofuels – A review

Macroalgae, commonly known as seaweeds, are a major cultivated species in the aquaculture industry, and a key component in the food and pharmaceutical industries due to their hydrocolloid fraction. Recently, macroalgae have been revisited as marine-derived carbon donors or “blue carbon” donors due to their superior carbon sequestration potential and accessibility to human intervention in terms of climate change mitigation action. There is an ongoing debate on including macroalgae as “blue carbon” in addition to the conventional blue carbon sources, namely salt marshes, mangroves, and seagrass. Existing experimental evidence indicates that macroalgal carbon fixation is high next only to phytoplankton, and the fixed carbon is channeled via vertical and horizontal transport along with a considerable level of sink in sediments. Macroalgae are phylogenetically diverse, and based on their taxonomical position and morphological features, they are an attractive feedstock for biofuel production. This review assesses the potential of macroalgae as a feedstock for the production of biogas/biomethane. The role of the inherent biomass composition of biomethane in anaerobic digestion and other parameters influencing biogas yield are discussed. Commercial realization of macroalgal biomethane via life cycle assessment, techno-economic assessment, and exergy analysis, with future research outlook, is presented. Macroalgae present a potential marine-derived alternative compared to terrestrial carbon feedstock, and macroalgal biomethane is an untapped resource waiting to be explored.