A Comprehensive Review of Heavy Metal Toxicity and Tolerance in Microalgae

In the past decades, the uncontrolled growth of industrialization led to a steady increase in heavy metals (HMs) in aquatic environments, especially coastal ones, causing negative impacts on them and serious health risks to humans. Indeed, since HMs are non-degradable compounds, they rapidly accumulate in environmental matrixes, such as sediments, and in marine organisms. Microalgae HM tolerance and sorption ability varies by species and metal type, with higher concentrations adversely affecting physiological processes and impairing cell morphology. Below certain thresholds, they can be considered promising candidates for HM removal, offering a viable alternative to conventional methods - often relying on costly chemicals, and producing hazardous by-products. Indeed, these microorganisms exhibit a high surface-to-volume ratio and consequent sorption capacity, which together with rapid growth rates and high biomass yields make them suitable candidates for bioremediation. HM sequestering can employ either dead biomass, which passively adsorbs metals via cell- wall interactions, or living organisms that actively transport HMs into cells. The present review provides an overview of microalgae-based HM removal capability, and HM-driven stress to which they are subjected in case of serious HM pollution.