Thermophilic anaerobic conversion of raw microalgae: Microbial community diversity in high solids retention systems

The potential of microbial communities for efficient anaerobic conversion of raw microalgae was evaluated in this work. A long-term operated thermophilic digester was fed with three different Organic Loading Rates (OLR) (0.2, 0.3 and 0.4 g.L-1.d(-1)) reaching 32-41% biodegradability values. The microbial community analysis revealed a remarkable presence of microorganisms that exhibit high hydrolytic capabilities such as Thermotogae (similar to 44.5%), Firmicutes (similar to 17.6%) and Dictyoglomi, Aminicenantes, Atribacteria and Planctomycetes (below similar to 5.5%) phyla. The suggested metabolic role of these phyla highlights the importance of protein hydrolysis and fermentation when only degrading microalgae. The ecological analysis of the reactor suggests the implication of the novel group EM3 in fermentation and beta-oxidation pathways during microalgae conversion into methane. Scenedesmus spp. substrate and free ammonia concentration strongly shaped thermophilic reactor microbial structure. Partial Least Square Discriminant Analysis (PLS-DA) remarked the resilient role of minor groups related to Thermogutta, Armatimonadetes and Ruminococcaceae against a potential inhibitor like free ammonia. Towards low-cost biogas production from microalgae, this study reveals valuable information about thermophilic microorganisms that can strongly disrupt microalgae and remain in high solids retention anaerobic digesters.