1 2 Anaerobic digestion of microalgae as a necessary step to make 3 microalgal biodiesel sustainable 4 5

13 14 The potential of microalgae as a source of biofuels and as a technological solution for 15 CO2 fixation is subject to intense academic and industrial research. In the perspective of 16 setting up massive cultures, the management of large quantities of residual biomass and the 17 high amounts of fertilizers must be considered. Anaerobic digestion is a key process that can 18 solve this waste issue as well as the economical and energetic balance of such a promising 19 technology. Indeed, the conversion of algal biomass after lipid extraction into methane is a 20 process that can recover more energy than the energy from the cell lipids. Three main 21 bottlenecks are identified to digest microalgae. First, the biodegradability of microalgae can 22 be low depending on both the biochemical composition and the nature of the cell wall. Then, 23 the high cellular protein content results in ammonia release which can lead to potential 24 toxicity. Finally, the presence of sodium for marine species can also affect the digester 25 performance. Physico-chemical pretreatment, co-digestion, or control of gross composition 26 are strategies that can significantly and efficiently increase the conversion yield of the algal 27 organic matter into methane. When the cell lipid content does not exceed 40 %, anaerobic 28 digestion of the whole biomass appears to be the optimal strategy on an energy balance basis, 29 for the energetic recovery of cell biomass. Lastly, the ability of these CO2 consuming 30 microalgae to purify biogas and concentrate methane is discussed. 31 32 33