Characterization of functional communities and metabolic pathways during methanogenesis in wastewater sludge by isotopic and molecular analysis

International audienceDescription of the subject. Investigation of the relationships between methanogenic community and metabolic pathway dynamics during methanogenesis. Objectives. The objective was to use an isotopic approach coupled with molecular analysis to identify metabolisms and to understand the dynamics of Archaea under different temperature conditions. Method. Sludge was incubated anaerobically under mesophilic and thermophilic conditions. Biogas production was monitored, together with stable isotopic signatures of produced CH4 and CO2. Results. Isotopic signature values for CH4 indicated a change in methanogenic metabolism with time and temperature. CH4 was predominantly produced from H2/CO2 and acetoclastic metabolism at the beginning of the mesophilic incubations and after acetate injection. A progressive shift towards an acetoclastic metabolism was observed at the end of mesophilic incubations. This period was associated with stability within archaeal communities as monitored by automated ribosomal intergenic spacer analysis (ARISA) and FISH with oligonucleotidic probes targeting specifically the Archaea 16S rRNA gene. At the beginning of the thermophilic incubations and after acetate injection, methane was generated mostly from H2/CO2. At the end of the thermophilic incubations, an acetoclastic metabolism was observed and intergenic spacer analysis (ARISA) and FISH showed important shifts in archaeal communities. Conclusions. Isotopic methods coupled with molecular analyses enabled us to better understand methanogenesis and the involvement of archaeal populations, which varied according to temperature. We also observed a preadaptation of archaeal communities in thermophilic conditions, which could be due to the fact that mesophilic microorganisms were progressively eliminated during the thermophilic incubation period.Description du sujet. Cet article traite des relations entre les communautés des archées méthanogènes et les voies métaboliques lors de l’étape de la méthanogenèse. Objectifs. L’objectif est d’utiliser une approche isotopique couplée à des analyses moléculaires afin d'identifier les métabolismes et de comprendre la dynamique des populations d’Archaea sous différentes conditions de températures. Méthodes. Des boues ont été incubées en anaérobiose dans des conditions mésophiles et thermophiles. La production de biogaz et les mesures isotopiques stables de CH4 et CO2 produits ont été analysées. Résultats. Les valeurs isotopiques du CH4 ont exhibé un changement des voies métaboliques de la méthanogenèse en fonction du temps et des températures d’incubation. Au début des incubations mésophiles et après injection de l'acétate, le CH4 a été principalement produit à partir de H2/CO2 et par le métabolisme acétoclastique. À la fin des incubations mésophiles, un changement progressif vers un métabolisme acétoclastique strict a été observé. Une stabilité remarquable de la communauté des archées a été observée dans les conditions mésophiles par les analyses ARISA et FISH. Au début des incubations thermophiles et après injection de l'acétate, le CH4 a été produit essentiellement à partir de H2/CO2. À la fin des incubations thermophiles, un métabolisme acétoclastique s’est installé. De plus, les analyses ARISA et FISH ont montré des changements au niveau des communautés des archées actives. Conclusions. Une méthode isotopique couplée à des méthodes moléculaires nous ont permis de mieux comprendre comment se déroule la méthanogenèse et les populations d’archées impliquées en fonction de la température. Nous avons également observé une pré-adaptation des communautés d’archées dans les conditions thermophiles qui pourrait être due au fait que les micro-organismes mésophiles sont progressivement éliminés

Assessment of the toxicity and the fertilizing power from application of gamma irradiated anaerobic sludge as fertilizer: Effect on Vicia faba growth

With increasing sludge produced every day by wastewater treatment plants, new methodologies have been developed to reduce sludge or to valorize them at lower cost and energy. Among these methodologies, agricultural manuring is of great interest as sludge could complement or replace chemical fertilizers. However, sludge pretreatment was necessary prior to its use in order to eliminate the degradable organic contaminants, heavy metals and diverse pathogenic microorganisms. In this study we performed γ-irradiation of anaerobic sludge coming from wastewater treatment plants at different doses (0.5, 1.5, 2.5, 3.5 and 4.5 kGy), followed by anaerobic digestion. We then used them in addition to soil for Vicia faba cultures. Heavy metal concentrations, toxicological evaluations, and the number of cultivable pathogenic bacteria were also investigated in irradiated and non-irradiated sludge samples to determine the effect of γ-irradiation on toxicity and pathogens present in the sludge. Results show that 4.5 kGy is the optimal dose, and γ-irradiation pretreatment of anaerobic sludge added to soil (2 mL/30 g) has significantly improved Vicia faba growth. Furthermore, by eliminating pathogens and by degrading chemical pollutants, γ-irradiation of anaerobic sludge provides promising insights for re-using sludge as a safe fertilizer