Microbial Biofertilizers for Salinity Stress Mitigation in Hydroponic Systems

Salinity accumulation is a critical abiotic constraint in hydroponic agriculture, particularly in recirculating systems, where limited leaching and nutrient cycling intensify ionic accumulation and increase the conductivity of nutrient solutions. Hydroponic crops are sensitive to osmotic and ionic stress, which leads to reduced water uptake, disrupted nutrient homeostasis, and yield loss. Traditional mitigation strategies, such as nutrient dilution, flushing, and water blending, provide temporary relief while increasing operational costs, nutrient discharge, and water consumption. Microbial biofertilizers, including plant growth-promoting bacteria, fungi, and microalgae, offer a sustainable approach for enhancing salinity resilience. These microorganisms influence root zone processes through mechanisms such as ion transport regulation, exopolysaccharide-mediated Na⁺ immobilization, osmolyte accumulation, antioxidant enhancement, phytohormonal modulation, and siderophore-mediated micronutrient mobilization. This review (i) summarizes the physiological, microbial, and system-level drivers of salinity stress in hydroponics, (ii) synthesizes evidence for microbial inoculation in saline solutions, and (iii) identifies research gaps related to formulation stability, disinfection compatibility, and commercial-scale validation. We address advances in hydroponic microbiology, emphasizing optimized delivery systems, including encapsulated formulations, consortium-based inoculation, and system-specific strategies to support microbial colonization in soilless environments.