Yang et al. 2025 — Microalgal strategies against heavy-metal toxicity (Microorganisms narrative review)

Yang and colleagues review biochemical, additive, genetic, strain-selection, and immobilization strategies by which microalgae remove heavy metals (HMs) from contaminated water while continuing to produce biomass. The review covers two principal removal modes — passive biosorption (metabolism-independent, fast, dead-or-live biomass) and active bioaccumulation (metabolism-dependent, slow, live cells only) — and surveys exogenous chemical additives (organic acids, dissolved organic matter, sulphate, phosphate, nitric-oxide donors, salicylic acid, fulvic acid), transgenic strategies (metallothionein and metal-tolerance-protein over-expression, mercuric-reductase MerA expression, surface-displayed MerR), microalgal strain selection for acid-tolerance and metal-tolerance phenotypes, immobilization carriers (alginate, biopolymer films, pine sawdust, biochar-algae complexes, dielectrophoresis-assisted devices), and coupled HM-bioremediation/biofuel-production systems. The scope is wastewater bioremediation and algal biotechnology, not food or supply-chain contamination; HMI relevance is restricted to (a) mechanistic vocabulary for any future microalgae-and-metal microbiome page, and (b) the table inventory of microalgae × heavy-metal removal efficiencies, which restates primary-literature figures the chapter itself does not re-measure. Evidence tier C; cite as a leads document only.

Key numbers

The review restates removal-efficiency, IC50, and biosorption-capacity figures from primary references rather than reporting any author-derived measurements. Each value below is the review’s restatement; cross-check against the underlying paper before any quantitative use.

Microalgae removal-efficiency inventory (Table 1, p. 4–5)

Removal-efficiency values for common heavy metals across microalgal species. Values are % removal at a given initial concentration unless otherwise noted:

The review’s bottom-line generalization: clearance >80% is generally only achievable below ~3 mg/L HM; above 10 mg/L, microalgal growth is severely inhibited and removal efficiency typically falls below 50% (§3, p. 5).

Exogenous additive effects (Table 2, p. 7)

Selected entries from the review’s restated inventory of how chemical additives modulate algal growth or HM biosorption:

Additional review-narrative claims: Cr-VI biosorption by Chlorella vulgaris is maximized near pH 2 (acid conditions facilitate Cr-VI → Cr-III reduction; §4, p. 6, citing [64,71,72]); 2.0 g/L metabisulfite raised Cr-VI reduction in Rhodobacter sphaeroides SC01 from 50% to 91% at 96 h, 500 mg/L Cr-VI (the authors’ own prior work, [33]).

Genetic-modification inventory (Table 3, pp. 7–8)

Transgenic-strain effects restated from primary references:

Strain-selection inventory (Table 4, p. 9)

The review also frames the Zeraatkar et al. 2014 [53] meta-collection of pre-2014 biosorption capacities for 14 microalgal species: maximum biosorption capacity ranged from 0.6 mg/g for Ni-II up to 836.5 mg/g for Zn (§3, p. 4) under each study’s reported optimum conditions.

Immobilization and coupled-system numbers (§7–§8, pp. 9–10)

• Immobilization in general raises maximum sorption by 2.1–3.1× over free cells, per Zeraatkar et al. [53] (§7, p. 9).
• AlgaPol biofilm of Chlorella sorokiniana with renewable copolymers achieved >90% removal of 8 mg/L Cd²⁺ or Cu²⁺ from growth medium (§7, p. 9, citing [102,103]).
• Activated-carbon-derived biochar-alga complex (coconut-shell biochar + Chlorella) adsorbed up to 46.8 µg/g at 0.1 mg/L Hg (§7, p. 10, citing [107]).
• Dielectrophoresis-assisted device with Chlorella in 0.5 mg/L Cd²⁺ + Cu²⁺ mixture: 98% Cu and 96% Cd removal individually; up to 97% combined under electric-field optimization (§7, p. 10, citing [108]).
• Photosynthetic microbial fuel cell with mixed Chlorella vulgaris + C. sorokiniana in 50 mg/L Cu/Co: 94% Cu, 88% Co removal; lipid production increased 1.2× under Cu stress and 1.1× under Co stress (§7, p. 10, citing [14]).
• Coupled HM-bioremediation/biofuel cultivation declines unit energy cost by 20–25% relative to standalone algal biofuel and greatly reduces freshwater + nutrient consumption (§8, p. 10, citing [117–119]).

Methods (brief)

Narrative open-access review in Microorganisms (MDPI). No PRISMA, no inclusion criteria, no quality assessment, no formal extraction, no quantitative pooling. ~130 references cited. Structure: §1 introduction (HM toxicity in water, microalgae as primary producers, biosorption-versus-bioaccumulation framing); §2 mechanisms of HM removal by microalgae (cell-wall biosorption via -OH/-COOH/-NH2/-PO4 active groups; EPS adsorption; intracellular GSH/phytochelatin/metallothionein chelation; vacuolar sequestration; Figure 1 mechanism map); §3 microalgae remove HMs efficiently at low concentrations (Table 1 species inventory); §4 exogenous chemical additives (organic acids, DOM, sulphate, phosphate, NO donors, salicylic acid, fulvic acid; Table 2); §5 genetic manipulation (metallothionein, P5CS, CrMTP4, MerA, MerR over-expression; Table 3); §6 microalgal strain selection (Cd-tolerant, Cr-tolerant, U-tolerant, low-pH-tolerant strains; Table 4); §7 immobilization methods (alginate, pine-sawdust biocarriers, AlgaPol biofilm, biochar-alga complex, dielectrophoresis-assisted device, photosynthetic microbial fuel cell, microalgal-bacterial-symbiosis biofilm reactors, fungi-cyanobacteria symbiotic systems); §8 coupling HM bioremediation and biofuel production (economic argument, large-scale-cultivation contamination challenge); §9 conclusions. Eight authors, all affiliated with institutions in Sichuan, China (Sichuan Agricultural University; Sichuan Normal University). Academic editors Zivan Gojkovic and Lilan Zhang. MDPI Microorganisms is an open-access journal with rapid peer review; review-article peer-review depth is generally lighter than primary-research-article peer review at the same journal.

Limitations

C-tier review with no primary data. Notable internal weaknesses:

• Removal-efficiency figures in Table 1 are not normalized for biomass dose, contact time, pH, or temperature; values are not directly comparable across rows without consulting the underlying primary references. The review acknowledges no standard reporting framework across the inventoried studies.
• Speciation of arsenic is not preserved: Scenedesmus almeriensis “As 40.7%” (Table 1) is reported without specification of As-III vs As-V, and the review’s narrative does not clarify. iAs vs tAs distinction is absent throughout.
• Hg in the genetic-modification table (Table 3) is reported as elemental “Hg” without methylmercury-versus-inorganic-mercury distinction; the MerA pathway acts on Hg²⁺ specifically, while MerB handles organomercurial substrates — the review does not consistently flag which species the modified strains act on.
• The chromium chemistry section conflates Cr-VI and Cr-III at points (e.g., “reduced Cr-III may form an organic-metal complex through ion exchange” — §4, p. 6); reproducible interpretation requires reading the primary references.
• Units across Table 1 are mixed (mg/L, ppm, µg/L) and one row reports a metal without unit (“As 40.7% removal of 12 mg/L” is clear, but “B 38.6% removal of 60 mg/L” treats boron — a metalloid, not a heavy metal — without comment on classification). Boron is not within HMI scope.
• Thiomonas spp. tolerance is stated as “up to 6” without unit; the underlying figure cannot be reconstructed from the review text alone. (Note: this attribution applies to the related extremophiles literature surveyed in §2 background; see Aishwarya et al. 2024 for a parallel case.)
• The review does not address food, supply-chain, or human-exposure outcomes anywhere. All figures are in the bioremediation/wastewater-treatment register.
• Rare-earth elements (Ce, Gd, La) are included in Table 1 without contextual discussion of whether they fall within HM scope by the IUPAC or environmental-engineering definitions.
• The conclusion explicitly notes that “microalgae-based biofuel production coupled with HM removal is not economically feasible” yet — i.e., the review frames the field as a forward-looking research agenda, not a synthesis of demonstrated large-scale practice.

Implications

This source has minimal direct value for the Heavy Metal Index in the food-and-supply-chain register. The wiki’s primary scope is heavy-metal occurrence in food, beverage, supplement, and personal-care matrices; this review covers wastewater bioremediation by microalgal cultures and algal biotechnology. Two narrow utilities:

Mechanistic-vocabulary leads: the review systematically catalogues the cell-wall and intracellular HM-binding chemistry in microalgae — extracellular polymeric substances (EPS) composed of glycoproteins and polysaccharose; cell-wall mannans, xylans, sulfated galactans, alginates; -OH/-COOH/-NH2/-PO4 surface groups; ion-exchange and electrostatic mechanisms; intracellular glutathione (GSH), phytochelatins (PCs), metallothioneins (MTs); vacuolar sequestration. Useful background for any future microalgae-and-metal microbiome page (Part 22 WikiBiome federation prep) and for cross-linking from chromium-hexavalent, cadmium, and mercury pages when bioremediation methods are referenced.

Genus inventory leads: the review names the principal HM-removing microalgal genera and species (Chlorella vulgaris, C. pyrenoidosa, C. sorokiniana; Scenedesmus spp.; Desmodesmus spp.; Chlamydomonas reinhardtii, C. moewusii, C. CPCC 121; Botryococcus sp. NJD-1; Parachlorella kessleri R-3; Coelastrella sp. PCV; Dunaliella salina; Dyctiosphaerium chlorelloides). Restate primary references — not this review — when citing specific removal-efficiency or IC50 figures.

The review does not provide primary contamination data on any food matrix, ingredient, product, or regulation. No contamination_profile synthesis is triggered. No HMTc threshold implication. No regulatory event documented. This is a research-agenda framing piece for algal biotechnology, retained as C-tier mechanistic background.

Wiki pages this source may touch

• chromium
• chromium-hexavalent
• cadmium
• copper
• nickel
• zinc
• lead
• mercury
• arsenic

Verification notes

• Frontmatter metals: uses tAs (not As) and tHg (not separate Hg + MeHg) per Part 14 speciation discipline: the review discusses arsenate, arsenite, and elemental mercury without speciated iAs or MeHg measurement, and Table 3’s Hg entries cover both Hg²⁺ (MerR-targeted) and broader Hg (MerA-targeted) without consistent species labels. Cr and Cr-VI both retained because §4 and Table 1 explicitly discuss Cr-VI biosorption and Cr-VI → Cr-III reduction.
• Frontmatter matrices: uses [microalgae, algal-biomass, wastewater-remediation]. microalgae is precedented (see raab2024-arsenolipids-chlamydomonas). algal-biomass and wastewater-remediation are descriptive matrix slugs analogous to existing slugs (bacterial-cell in aishwarya2024-extremophiles-bioremediation-review; chromium-wastewater/wastewater-remediation in yang2023-amidoxime-carbon-felt-crvi-removal and wu2023-hybrid-membranes-lead-copper-adsorption); they should be confirmed against the matrices controlled vocabulary on the next Karen review.
• ingredients and products are empty by design: this is a methods-and-bioremediation review with no food or supply-chain measurement. An advisory routing_malformed entry for empty products/ingredients is intentional and precedented for this paper class (see macias2023-polymer-inclusion-membranes-seawater-metals, wu2023-hybrid-membranes-lead-copper-adsorption audit-queue notes).
• jurisdictions: [CN] reflects author affiliation (all eight authors at institutions in Sichuan, China). The review’s underlying primary references are global; the jurisdictional tag captures the synthesis venue, not the underlying contamination evidence.
• Brand firewall (Part 12): no brand names appear in the review’s content; commercial-microalgal-strain identifiers (e.g., “JSC-7”, “CNW11”, “CPCC 121”, “ZBS1”, “TU5”, “NJD-1”, “MAS1”, “MAS3”, “ChlSG”, “PCV”, “R-3”) are strain designations from primary culture collections or research-isolate labels, not consumer brands. Retained per Exception 2 (scientific-method/material names — see verification-checklist.md §Firewall checks).
• Part 2 firewall: no HMTc threshold proposal, no consumer translation, no synthesis claims relative to the broader literature.
• Audit subagent (2026-06-02, ingest-next-manual-fetch-pdf v2 skill) flagged that the Table 2 Chlorella pyrenoidosa / Cu row originally compressed three additive rows into “87–88%”; verified against PDF p. 7 Table 2 — citric acid 81%→87%, fulvic acid 81%→87%, humic acid 81%→88%. Split into three rows for full fidelity.

Page history

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