Microalgae-based living hydrogels as engineered soil biogeochemical reactors: bridging synthetic microbiomes, climate resilience, and sustainable remediation

Microalgae-based living hydrogels represent an emerging class of engineered living materials (ELMs). They combine biological activity with the physicochemical properties of hydrated polymer networks to form a multifunctional platform that is potentially useful in soil remediation. This review critically evaluates the extent to which this potential is supported by current evidence, with particular emphasis on distinguishing soil-specific findings from mechanisms extrapolated from aquatic and other controlled systems. The conceptual basis for using microalgae–hydrogel composites as localized biogeochemical reactors is examined, with a focus on contaminant sorption, nutrient assimilation, biological transformation, water regulation, and microenvironmental control. Available studies indicate that hydrogel matrices can improve microalgal immobilization, water retention, and local control of mass transfer. Microalgae can also contribute to nutrient assimilation and contaminant interactions. However, the relative contributions of the living component and the hydrogel matrix, particularly under soil conditions, remain insufficiently resolved. The review further addresses plant–soil–hydrogel interactions, spatially organized microbial systems, and emerging material strategies, such as stimuli-responsive and hybrid hydrogels. These approaches offer opportunities for context-dependent regulation of biological and physicochemical processes. Nevertheless, many remain at the proof-of-concept or conceptual stage and require validation under realistic soil conditions. Similarly, proposed benefits for climate resilience, carbon dynamics, and ecosystem services remain insufficiently quantified at the field scale. Microalgae-based living hydrogels represent a promising but still developing platform for soil remediation. Their translation beyond controlled experiments is constrained by several factors such as limited long-term and multi-season field evidence, uncertain contaminant fate, incomplete understanding of plant–soil–microbiome interactions, and the absence of standardized performance metrics. Future research should prioritize comparative experiments involving free microalgae, hydrogel-only, and microalgae-hydrogel treatments. Long-term field validation across contrasting soils and climates is also needed. In addition, mechanistic assessment of contaminant removal, ecological risk evaluation, and integrated techno-economic and life-cycle assessments should be undertaken. These efforts are necessary to determine whether and under which conditions microalgae-based hydrogels provide measurable environmental and functional advantages over existing remediation approaches.