Engineering symbiotic networks in microalgae-bacteria consortia for sustainable biohydrogen production: mechanisms,...

Microalgae-bacteria consortia show better H₂ productivity compared with axenic microalgae, but the quantitative understanding of the underlying mechanisms is still limited. This review summarizes the available quantitative knowledge of the shift from mono-cultivation to consortium, with a focus on symbiotic network engineering. The co-culture of Chlamydomonas reinhardtii and Microbacterium forte is capable of producing 11.27 g·L⁻¹ biomass production and 185.8 mL·L⁻¹ H₂ titre, reaching up to 10 times higher H₂ yield compared with axenic controls. A three-stage developmental model, comprising metabolic cooperation, spatial self-organization, and functional specialization, is proposed as a conceptual framework rather than an established trajectory. Mechanisms such as cross-feeding, direct interspecies electron transfer, gas signaling, and quorum sensing are considered quantitatively. Important design principles include C/N ratio between 20 and 30 for carbohydrate accumulation, microaerobic oxygen concentration between 2 and 4% for protection of hydrogenases, and over 70% COD, N, and P removal simultaneously. The multi-agent systems paradigm is proposed for the explanation of emergent properties like substrate flexibility and robustness. There are still major challenges with low consortium stability, lack of evidence for DIET, and the absence of performance metrics. Priorities include AI-assisted optimization and extremophile chassis engineering.