Microalgae cultivation in aqueous phases of hydrothermal biomass processing: Composition, constraints, and integration into circular biorefineries

Abstract Hydrothermal biomass processing, notably hydrothermal carbonization, hydrothermal liquefaction and hydrothermal gasification, generates an aqueous phase enriched in dissolved organic carbon, nitrogen, phosphorus, and inorganic ions. Although often regarded as a problematic effluent due to high chemical oxygen demand and the presence of inhibitory organics, this aqueous phase also represents a concentrated nutrient stream suitable for biological upgrading. Microalgae cultivation in hydrothermal aqueous phases has therefore emerged as a promising strategy to couple wastewater remediation with biomass production in integrated circular biorefineries. This review synthesizes current knowledge on how feedstock composition and hydrothermal process severity govern aqueous-phase chemistry, with particular emphasis on the formation and fate of short-chain organic acids, ammonium and nitrogenous organics, phenolics, and furanic intermediates. Microalgae cultivation studies in hydrothermal aqueous phases are critically evaluated, with particular focus on strain-specific tolerance, dilution strategies, nutrient and organic-carbon removal performance, and biochemical responses associated with mixotrophic growth. Across the literature, successful cultivation is most frequently achieved at moderate aqueous-phase loadings, where inhibitory compounds are sufficiently attenuated while nutrient availability remains adequate to sustain growth comparable to standard culture media. Under these conditions, systems commonly achieve high removals of nitrogen and phosphorus alongside substantial reductions in organic load, while producing biomass suitable for downstream valorisation. Key barriers to scale-up include variability in aqueous-phase composition, residual toxicity from phenolics, furans, and nitrogen heterocycles, and operational constraints in light-limited, high-ionic-strength media. Emerging solutions focus on process optimization and reactor design strategies that enable efficient coupling of detoxification and biomass production. Overall, this review links hydrothermal operating conditions to aqueous-phase chemistry and microalgal physiology, providing design guidance for integrated hydrothermal–microalgal platforms targeting nutrient recovery, effluent polishing, and sustainable biomass generation.