A stress-response-recovery framework integrating microbial life-history strategies for stressed anaerobic digestion

Anaerobic digestion (AD) is a mature biotechnology; however, its operational stability remains constrained by the limited predictability of microbial responses to environmental stress. In this review, stressed AD systems are defined as microbial ecosystems in which one or more environmental stressors disrupt microbial functioning, reduce microbial growth and survival, and drive deterministic and/or stochastic shifts in the AD microbiome, thus impacting process performance. For stressed AD, this review proposes a unifying stress-response-recovery framework integrating microbial life-history strategies. Specifically, stress acts as an ecological filter that suppresses stress-sensitive microorganisms while selecting stress-tolerant ones. During the response phase, system resilience can be achieved through multiple forms of microbial plasticity, including spatial reorganization (e.g., granulation), metabolic rerouting (e.g., shifts from acetoclastic methanogenesis to syntrophic acetate oxidation), and alternative electron-transfer pathways such as direct interspecies electron transfer. Importantly, recovery does not necessarily ensure a return to the original community structure; instead, it is governed by functional redundancy, microbial memory, and ecological hysteresis, often resulting in alternative stable states with reconfigured metabolic networks while preserving methane production. Collectively, these insights provide complementary ecologically informed management strategies, highlighting the potential of strategy-based monitoring, microbiome engineering, and targeted interventions to enhance system resilience. By integrating ecological theory with engineering practice, this review advances the understanding of AD stress and provides a framework for designing more robust and adaptable anaerobic systems.