Recent research on the archaea community in aerobic granular sludge (AGS) has attracted considerable attention. This review summarizes the existing literature on composition, distribution, and related functions of archaea community in AGS. Furthermore, the effects of granulation, substrate, temperature, process types, and aeration models on the archaea community were discussed. Significantly, the layered structure of AGS facilitates the enrichment of archaea, including methanogenic archaea and ammonia-oxidizing archaea. Archaea engage in metabolic interactions with other microorganisms, enhancing the ecological functionalities of AGS and its tolerance to adverse conditions. Future investigations should focus on minimizing greenhouse gas emissions and exploring the roles and interactive mechanisms of archaea and other microorganisms within AGS.

Methanogenic archaea are a part of the commensal gut microbiota responsible for hydrogen sink and the efficient production of short-chain fatty acids. Dysbiosis of methanogens is suspected to play a role in pathogenesis of variety of diseases, including inflammatory bowel disease (IBD). Unlike bacteria, the diversity of archaea seems to be higher in IBD patients compared to healthy subjects, whereas the prevalence and abundance of gut methanogens declines in IBD, especially in ulcerative colitis. To date, studies focusing on methanogens in pediatric IBD are very limited; nevertheless, the preliminary results provide some evidence that methanogens may be influenced by the chronic inflammatory process in IBD. In this review, we demonstrated the development and diversity of the methanogenic community in IBD, both in adults and children.

In-depth understanding of the molecular mechanisms and physiological consequences of oxidative stress is still limited for anaerobes. Anaerobic biotechnology has become widely accepted by the wastewater/sludge industry as a better alternative to more conventional but costly aerobic processes. However, the functional anaerobic microorganisms used in anaerobic biotechnology are frequently hampered by reactive oxygen/nitrogen species (ROS/RNS)-mediated oxidative stress caused by exposure to stressful factors (e.g., oxygen and heavy metals), which negatively impact treatment performance. Thus, identifying stressful factors and understanding antioxidative defense mechanisms of functional obligate anaerobes are crucial for the optimization of anaerobic bioprocesses. Herein, we present a comprehensive overview of oxidative stress and antioxidant mechanisms of obligate anaerobes involved in anaerobic bioprocesses; as examples, we focus on anaerobic ammonium oxidation bacteria and methanogenic archaea. We summarize the primary stress factors in anaerobic bioprocesses and the cellular antioxidant defense systems of functional anaerobes, a consortia of enzymatic and nonenzymatic mechanisms. The dual role of ROS/RNS in cellular processes is elaborated; at low concentrations, they have vital cell signaling functions, but at high concentrations, they cause oxidative damage. Finally, we highlight gaps in knowledge and future work to uncover antioxidant and damage repair mechanisms in obligate anaerobes. This review provides in-depth insights and guidance for future research on oxidative stress of obligate anaerobes to boost the accurate regulation of anaerobic bioprocesses in challenging and changing operating conditions.

Recycling waste into new materials and energy is becoming a major challenge in the context of the future circular economy, calling for advanced methods of waste treatment. For instance, microbially-mediated anaerobic digestion is widely used for conversion of sewage sludge into biomethane, fertilizers and other products, yet the efficiency of microbial digestion is limited by the occurrence of antibiotics in sludges, originating from drug consumption for human and animal health. Here we present antibiotic levels in Chinese wastewater, then we review the effects of antibiotics on hydrolysis, acidogenesis and methanogenesis, with focus on macrolides, tetracyclines, β-lactams and antibiotic mixtures. We detail effects of antibiotics on fermentative bacteria and methanogenic archaea. Most results display adverse effects of antibiotics on anaerobic digestion, yet some antibiotics promote hydrolysis, acidogenesis and methanogenesis.