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

Abstract

Besides biomass production, some microalgae have been used to treat wastewater contamination. However, in general, high concentrations of heavy metals significantly inhibit algal growth. We thus need to find ways to promote the resistance of microalgae to heavy metals, increase their growth rate under stress, and achieve coupling of heavy metal removal and biomass production simultaneously. In this review, mechanisms for removal of heavy metals by microalgae are proposed. Effects of exogenous chemical additives (dissolved organic matters, formaldehyde, sulphate, phosphate, nitric oxide donors, etc.) on algal biosorption to heavy metals are summarized. Genetic manipulation and microalgal strain selection strategies are also introduced, especially for the acid-tolerant strains with high biosorption efficiencies to Cr(VI) and Cd²⁺ at low pH conditions. Recent advances in (semi)continuous heavy-metal-bioremediation and biomass-production coupled system with immobilized microalgae, as well as challenges and solutions to the commercialization and industrialization of the coupled system were discussed.

Keywords: biomass production; biosorption; coupled algal system; heavy metal removal; microalgae.

Figure 1

Biochemical mechanisms for removal of heavy metals by microalgae. Seven biosorption or bio-accumulation processes are summarized: (1) microalgal extracellular polymeric substances (EPS) contain polysaccharides, proteins, lipids, and alginates, which adsorb heavy metal (HM) ions through non-polar interactions, such as van der Waals forces and hydrogen bonds; (2) metal ions on the cell membrane, such as calcium, sodium, and potassium, can undergo ion exchange reactions with HM ions, allowing them to enter the cell; (3) the abundant negatively charged chemical groups such as -OH, -COOH, and -NH₂, contained on the cell membrane and cell walls of microalgae attract positively charged HM ions through electrostatic attraction; (4) the adsorbed HM ions can undergo chemical reactions with negatively charged ions on the cell membrane, forming biological precipitates such as lead phosphate, cadmium phosphate, and cadmium sulfide, which may accumulate in the periplasmic space; (5) HM ions enter cells through active transport or passive diffusion on the cell membrane, producing a large amount of ROS. To cope with these oxidative damages, microalgae produce a large amount of glutathione (GSH) to eliminate ROS; (6) intracellular phytochelatin (PC) and metallothionein (MT) bind with HM ions to form protein-HM complexes, reducing their toxicity; (7) HM ions may be sequestered into the microalgal vacuoles, where organic acids, proteins, and other substances can also bind with HM ions to achieve detoxification.