Microalgae-Driven Circular Agriculture: System Integration, Nutrient Recovery, and AI-Assisted Optimization.

With rising global pressures on resources and the environment, transitioning out of our traditional linear agricultural models is long overdue. By itself, circular agriculture seeks to close loops for nutrients, but it also has a future that is constrained by the fragmentation of process integration, lack of system integration and optimization, and poor adaptive decision-making under the often very variable circumstances of agricultural systems. Microalgae are a versatile photosynthetic platform with unique value in this context. They can recover key nutrients (nitrogen, phosphorus and carbon) from agricultural wastes simultaneously and also convert these vital nutrients into multipurpose biomass. Here, this review synthesizes the multifunction of microalgae towards sustainable agriculture, with a particular emphasis on nutrient recycling and the use of whole microalgal biomass. Downstream applications are manifold, ranging from agricultural outputs, such as biofertilizers and biostimulants, to different products of high value (HVPs). Realizing this potential requires practical challenges to be addressed in integrated system design, coupling and scaling up. AI-assisted modelling and optimization have already started emerging as important tools for this purpose. Reliable system optimization relies on defining objective functions and balancing resource recovery efficiency and economic output, which in turn enables robust multi-objective decision-making. Concluding this review, we propose a holistic vision from a central integral biorefinery concept. Our framework clearly demonstrates how to fully enhance competitiveness, sustainability and scalability of microalgae-based agricultural systems through co-integrated high-value utilization and nutrient cycling.