## 1 Introduction
Innovative approaches of using microalgae could pave ways toward more sustainable and efficient paths to reduce the harmful effect of heavy metal pollution and enable a clean and healthy environment [23]. Consequently, microalgae remediation is a promising biological and environmentally friendly method of removing heavy metals from contaminated water. Their cell wall functional groups (e.g., carboxyl, hydroxyl, and amine groups) are mainly responsible for effective biosorption. Non-living algal biomass is especially efficient in passive adsorption and is therefore fit for extreme conditions, since it is resistant to metal toxicity and requires very low maintenance. This approach is considered as a single-use one, unless the biomass is subsequently processed for metal recovery [6]. In contrast, living microalgae offer dual functionality due to the complementarity of passive biosorption with metabolism-drivenuptake, allowing intracellular sequestration of metals and the regeneration of binding sites through growth [11]. This active method is enabled by photosynthesis and assists in removing metals while promoting water quality by the production of oxygen to counteract eutrophication and nurture aquatic life [24]. Apart from their bio-treatment possibilities, microalgae are highly prized for high-value co-products, such as biofuels [25; 26; 27], fertilizers [28], and in the food industry [29; 30], including animal feed [31]. Following this, their recovery gives further evidence to circular economy perspectives [32; 33]. Despite the challenges, such as sensitivity to high metal concentration and the need for special cultivation conditions, microalgae are a promising tool in heavy metal removal and restoring the ecosystem due to their adaptive features, multi-functionality, and sustainable technology [34]. Microalgae have developed various self-protection mechanisms to counteract the toxicity of heavy metals, which can be beneficial at low concentrations but harmful at high levels. These mechanisms include adsorption or bioaccumulation. Adsorption occurs when metal ions bind to the surface of microalgae cells through physical and/or chemical processes such as ion exchange and electrostatic interactions without their incorporation into the cellular structure. On the other hand, bioaccumulation is the active uptake of metal ions into the cells, where they can become integral parts of the cell's metabolic machinery, either by substituting for essential elements in enzymes or being stored in vacuoles for detoxification [1; 7; 14; 35; 36; 37]. Chelation and complexation with active groups on cell walls, where negatively charged functional groups bind heavy metals, and ion exchange on cell walls, where ions like Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\) and Mg\({}^{2+}\) are replaced by toxic heavy metal ions through physical forces, also enhance metal binding. Additionally, extracellular polymeric substances (EPSs) enhance adsorption on the cell surface and prevent the accumulation of heavy metals within the cell. Microalgae also synthesize antioxidant enzymes, such as superoxide dismutase and catalase, to mitigate heavy metal stress. Heavy metals accumulate in cytoplasmic vacuoles, chloroplasts, and mitochondria, while biotransformation processes include reduction, methylation, and conversion from inorganic to organic states. These processes ensure the survival and growth of microalgae in environments with varying levels of metal toxicity [6; 38]. _Arthrospira platensis_ (_A. platensis_), formerly _Spirulina platensis_, is a resourceful microalgae that is highly valued for its high protein, amino acid, vitamin, and mineral content, making it a popular supplement in the food, pharmaceutical, and cosmetic industries. One of its environmental applications is heavy metal removal, with its cell walls, rich in functional groups like carboxyl and phosphate, able to effectively bind metals. Dried _A.