The Strategies Microalgae Adopt to Counteract the Toxic Effect of Heavy Metals

Overview

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 Arsenolipids in the green microalga Chlamydomonas reinhardtii: first report of arsenosugar phospholipid (AsSugPhytol)). algal-biomass and wastewater-remediation are descriptive matrix slugs analogous to existing slugs (bacterial-cell in Role of Extremophiles in the Removal of Heavy Metal and E-Waste; chromium-wastewater/wastewater-remediation in Asymmetric Alternative Current Electrochemical Method Coupled with Amidoxime-Functionalized Carbon Felt Electrode for Fast and Efficient Removal of Hexavalent Chromium from Wastewater and Adsorption Performance of Heavy Metal Ions under Multifactorial Conditions by Synthesized Organic-Inorganic Hybrid Membranes); 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 On the Use of Polymer Inclusion Membranes for the Selective Separation of Pb(II), Cd(II), and Zn(II) from Seawater, Adsorption Performance of Heavy Metal Ions under Multifactorial Conditions by Synthesized Organic-Inorganic Hybrid Membranes 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.

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