Chemical drivers of mineral bioavailability and accumulation in microalgae: from speciation to cellular uptake and resource recovery

Critical and valuable elements are increasingly sought from dilute waste streams, but successful microalgal recovery depends on more than dissolved concentration. This narrative Mini Review examines how pH, redox conditions, ligand complexation, ionic competition, and phycosphere chemistry control elemental speciation and accessibility, and how these constraints interact with biosorption, membrane transport, intracellular homeostasis, sequestration, and stress responses. Essential and toxic trace-metal studies provide mechanistic evidence, while rare earth element studies more directly address uptake, association with biomass, and downstream release. Transferability between elements requires chemical and physiological justification. Total concentration does not necessarily represent the bioavailable pool, and biomass-associated content does not establish intracellular, metabolically available, or readily recoverable accumulation. Comparative synthesis distinguishes removal, biomass association or enrichment, and downstream recovery. Removal from water, including precipitation or nonspecific binding, is not an overall recovery yield. A conceptual process metric combines harvested dry-biomass productivity, target-element concentration, and downstream recoverable fraction; selectivity remains a separate criterion. Microalgal biomass can function as an intermediate concentrator, but practical performance depends on matrix chemistry, strain tolerance, harvesting, repeated operation, and downstream processing. This framework identifies evidence requirements for evaluating element-specific recovery from complex aqueous streams.