Microalgae can pull valuable and critical elements out of dilute waste streams, but a new review argues that the field has been measuring success the wrong way. A narrative Mini Review published in Frontiers in Plant Physiology on 28 October 2026 examines how pH, redox conditions, ligand complexation, ionic competition, and the chemistry of the phycosphere—the microenvironment surrounding algal cells—control which forms of an element are actually accessible to cells, and how those constraints interact with biosorption, membrane transport, intracellular homeostasis, sequestration, and stress responses. Its central warning is blunt: total dissolved concentration does not necessarily represent the bioavailable pool, and element content associated with harvested biomass does not establish intracellular, metabolically available, or readily recoverable accumulation.
The review, authored by researchers at the Universidad Autónoma de Nuevo León in Mexico, distinguishes three endpoints that are routinely conflated in the literature: removal of an element from water, association of that element with biomass, and actual downstream recovery into a usable stream. Removal from water, including precipitation or nonspecific binding, is explicitly not an overall recovery yield. A mass-balance perspective separates dissolved-element loss from target mass in harvested biomass and target mass obtained downstream, with recovery yield defined as downstream recovered target mass divided by initial target mass within a defined system boundary, accounting for volume changes, sampling, residual liquids, and solids.
To make this concrete, the authors propose a conceptual process metric combining harvested dry-biomass productivity, target-element concentration in that biomass, and the downstream recoverable fraction. Expressed as PR ≈ PB × CE × fR, the metric multiplies biomass productivity in grams of dry biomass per liter per day by target concentration in milligrams of element per gram of dry biomass and by the dimensionless fraction of that biomass-associated target recovered into a defined downstream stream, yielding milligrams of element per liter per day. The authors stress that this bookkeeping approximation is not a validated kinetic or predictive model, does not establish economic feasibility, and requires all terms to share the same operating conditions, biomass basis, time interval, and volume reference, with background loading accounted for. Selectivity remains a separate criterion, assessed through preferential target capture or recovery relative to competing elements under stated feed conditions.
The chemical case for skepticism begins with speciation. Dissolved metals occur as free ions, hydrolyzed species, and inorganic or organic complexes, and redox-active elements also differ in oxidation state; these forms differ in reactivity and accessibility. Photosynthetic activity can raise phycosphere pH and oxygen levels, modifying local speciation relative to the bulk medium, and light-driven pH and oxygen changes can accelerate oxidation of Fe(II), Mn(II), and Cu(I), with particularly pronounced modeled effects in freshwater phycospheres. Yet the review cautions that oxygen concentration is not a direct measurement of Eh, that oxidation need not cause precipitation, and that the direction and magnitude of pH and Eh effects are element-specific and matrix-dependent. Neither increasing pH nor increasing Eh implies universally lower bioavailability, and Figure 1 in the review presents a conceptual framework rather than a Pourbaix diagram or universal thermodynamic stability fields.
Competition and complexation further complicate predictions. The free-ion paradigm assumes that complexation lowers the fraction of metal available for biological interaction, but the review shows this relationship is not universal. In the green alga Chlamydomonas reinhardtii, calcium, magnesium, samarium, and europium ions reduced neodymium uptake through competitive interactions. In the presence of citric or malic acid, however, neither free Nd³⁺ nor total dissolved Nd accurately predicted internalization, because small Nd–organic complexes also contributed to the bioavailable pool. Medium constituents are chemically active in other ways too: phosphate can constrain rare earth element availability through LaPO₄ precipitation, while complexation and supersaturation can delay the expected phase change. Synthetic media, natural waters, and waste-derived matrices may therefore produce different relationships between concentration, speciation, and uptake.
At the cell surface, biosorption—the association of ions with surface functional groups of living or non-living biomass, distinct from membrane crossing—depends on electrostatic interactions, ion exchange, and coordination, with affinity governed by protonation and elemental species. Extracellular polymeric substances expand this reactive interface with additional metal-binding sites; cyanobacterial EPS can contribute substantially to rare earth element biosorption, with strain-dependent contributions to total adsorption. Partitioning can differ dramatically even within one organism: in Chlamydomonas reinhardtii, 54.6 ± 2.0 percent of accumulated copper was associated with the cell surface, whereas 74.3 ± 3.0 percent of accumulated zinc was intracellular, and EPS removal affected copper accumulation far more strongly than zinc accumulation. Because EPS composition and production are strongly influenced by cultivation conditions, metal sorption outcomes shift with how cultures are grown.
The review is equally careful about what analytical fractions actually mean. Operationally, harvested-biomass content may combine surface-bound material, extracellular precipitates, and intracellular pools. Studies that compared EDTA/NaCl-washed and NaCl-washed pellets followed by nitric-acid digestion, or used EDTA treatment before digestion to estimate intracellular cadmium, produced fractions that depend on washing and extraction conditions and do not independently establish metabolically available metal. Total digestion quantifies retained element without resolving localization. Selective desorption provides an operational estimate rather than proof of exclusive surface localization: insufficient washing can leave externally bound material, while aggressive extraction may alter cells or mobilize additional pools. The authors call for protocol descriptions, cell-integrity checks, abiotic controls, and recovery of wash fractions, and recommend reporting unseparated biomass-associated content when fractionation is unavailable rather than assigning an intracellular pathway by assumption.
Inside the cell, transport specificity is incomplete. Natural resistance-associated macrophage protein transporters handle divalent cations, while transcriptomic evidence in Auxenochlorella protothecoides implicates several transporter families in cadmium responses; proposed entry routes should not be treated as directly validated flux pathways. Similar neodymium, samarium, and europium uptake kinetics in Chlamydomonas reinhardtii are consistent with a shared uptake site, without identifying its molecular transporter. For essential trace elements such as iron, manganese, copper, and zinc, microalgae maintain homeostasis through regulated trafficking, compartmentalization, storage, and metal-sparing or recycling mechanisms, and cellular metal quota is not equivalent to intracellular metal availability: total content may increase through luxury uptake or compartmentalization without a proportional increase in the metabolically available fraction. For excess or non-essential elements, retention is better described as sequestration than nutrient-demand regulation, and responses are element-specific—elevated copper in Chlamydomonas promoted oxidative stress and greater EPS-mediated surface binding, whereas high zinc exposure was associated with glutathione synthesis and intracellular detoxification.
On the recovery side, the review draws a hard line between desorption and recovery. Surface-bound elements may be released by acid or chelator desorption, whereas intracellular pools may require disruption and leaching. One cited study reported 91.7 percent Pr³⁺ desorption from dry Spirulina biomass with 0.1 M HNO₃, but the review emphasizes that this establishes release of the sorbed pool, not 91.7 percent recovery of the initial feed or a purified product. Similarly, in saline culture, cobalt removal by the alga Dunaliella sp. FACHB-558 demonstrates element-specific tolerance and depletion, but not a purified recovered product. Ash content receives the same treatment: a flocculating consortium in municipal secondary effluent contained 46.6 ± 0.5 percent ash on a dry-weight basis in its third outdoor cycle, and 82.8 ± 0.5 percent refers to acid solubility of that ash fraction—neither value is a target-element recovery yield, and high bulk inorganic content in readily harvested biomass does not establish selective target enrichment.
The practical upshot is that growth and capture optima may diverge. Galdieria sulphuraria showed poor rare earth element association at its growth-optimal pH of 1.5–2.5 but substantially higher association at pH 5 without phosphate, while phosphate-containing media introduce precipitation and kinetic complications—suggesting that separating cultivation from capture may be preferable to maximizing both in one stage. The authors note that mechanistic partitioning and rare earth capture are more frequently demonstrated in the literature than closed-loop recovery, that rare earth evidence is not sufficient to establish lithium selectivity where abundant competing ions and weak intrinsic affinity constrain capture, and that synthetic-media performance cannot be transferred directly to real wastewaters. Repeated capture–release cycles must quantify retention of capacity, physiological performance where living biomass is reused, and cumulative recovery losses, and residual biomass requires contaminant testing before reuse or managed disposal. Real-matrix, repeated-cycle studies with mass balances and techno-economic assessment, the review concludes, are needed to test whether microalgal biomass—promising as an intermediate concentrator—can deliver scalable, economically viable recovery across critical-mineral classes.
Subject of Research: How chemical speciation and cellular processes govern microalgal recovery of critical and valuable elements from dilute waste streams
Article Title: Chemical drivers of mineral bioavailability and accumulation in microalgae: from speciation to cellular uptake and resource recovery
Article References: Guajardo-Barbosa, C., Quiñones-Gutiérrez, Y., López-Chuken, U. J., Gallegos-López, J. A., & Beltrán-Rocha, J. C. (2026). Chemical drivers of mineral bioavailability and accumulation in microalgae: from speciation to cellular uptake and resource recovery. Frontiers in Plant Physiology, 4, Article 1994915. https://doi.org/10.3389/fphgy.2026.1994915
Image Credits: AI Generated
DOI: 10.3389/fphgy.2026.1994915
Keywords: microalgae, critical minerals, rare earth elements, speciation, biosorption, bioavailability, resource recovery, phycosphere, mass balance, wastewater, metal uptake, biomass
News Source: Bethany Barker. (October 28, 2026). Microalgae Turn Dilute Waste Streams Into Critical-Metal Sources—If the Chemistry Cooperates. Scienmag.
