## 1 Introduction  
Heavy metals or trace metallic elements are the main hazardous chemical pollutants contaminating water, posing significant threats to marine ecosystems, seafood safety, and human health. They accumulate persistently within the food chain affecting various biological levels from cells to tissues [1; 2; 3; 4]. They are generally toxic, even at very low concentrations, ranging from micromolar levels to even lower concentrations for heavy metals such as lead (Pb), mercury (Hg), cadmium (Cd), chromium (Cr), and arsenic (As), which induce oxidative stress [5]. Cd, Pb and Cr are among the most persistent metallic elements, posing significant health risks, including carcinogenic effects through dermal contact, ingestion, and inhalation [6]. These heavy metals are commonly present in effluents from various sources, such as industrial wastewater from electroplating, metal finishing, battery manufacturing, and electronics production [7]. Mining operations, particularly those involving metal ore extraction and processing, also contribute to heavy metal contamination in water bodies [8]. Additionally, effluents from the textile and dyeing industries and tannery operations often contain substantial amounts of Cr [9]. Traditional methods for treating water polluted by these metals include reverse osmosis, ion exchange, chemical precipitation and membrane filtration. These methods can produce secondary pollutants and are energy-intensive [10; 11]. In contrast, bioremediation techniques, such as those utilizing microalgae, offer a more sustainable and cost-effective alternative for heavy metal removal. Many studies have focused on the removal of heavy metals using dead or immobilized microalgae biomass [12; 13]. However, utilizing living microalgae provides a more dynamic and comprehensive assessment of bioremediation capabilities, allowing for a better understanding of their physiological responses to metal stress. In this context, several studies have demonstrated the effectiveness of living microalgae strains, particularly from _Chloroyhvae_, in removing heavy metals [14]. For instance, _Scenedesmus_ species have demonstrated removal efficiencies of 25% to 78% for Cr and Zn, with initial concentrations of 2 mg/L for Cr and treatment over 10 days [15]. Furthermore, _Chlorella_ species have shown high efficiency, achieving 62% to 83% removal for Pb, Cd and Cu, with an initial concentration of 0.5 mg/L and a treatment period of 14 days [15]. In addition, microalgae strains affiliated with _Dunaliella_ and _Chlamydomonas_ genus have exhibited exceptional potential in removing Cr, and Cd, respectively [16]. In particular, _Dunaliella_ achieved a removal rate of approximately 90% at concentrations of 4 mg/L and 8 mg/L for Cr over 14 days. Meanwhile, _Chlamydomonas_ reached a similar removal efficiency toward Cd at 5 mg/L over 5 days of treatment [17]. The effectiveness of microalgae in heavy metal removal is largely attributed to a combination of extracellular and intracellular mechanisms, including biosorption, bioaccumulation and biotransformation [18]. These mechanisms are further supported by the production of antioxidants, the accumulation of protective pigments, and the synthesis of various bioproducts, all of which play a vital role in detoxification and enhance the microalgae's tolerance to different heavy metals [1; 19]. _Chlorococcum_ species are well-known for their efficiency in heavy metal bioremediation [20; 21; 22; 23]. Found in both freshwater and marine habitats, these green microalgae have spherical or slightly oblong cells that can exist individually or in clusters [24]. They produce different carotenoids, and during the carotenogenesis process, microalgae cells accumulate lipids [24]. This process significantly contributes to their ability to survive under stressful conditions [20].