## 1 Introduction  
Water pollution by heavy metals has become a serious environmental and public health problem due to the induced toxicity, persistence, and bioaccumulation [1, 2, 3]. Even though Pb, Hg, Cd, and Cr are naturally released into the environment through processes such as the weathering of rocks, volcanic action, and erosion of sedimentary deposits, human activities have artificially increased their levels in severe proportions that seriously threaten ecosystems and human health [4]. Heavy metal pollution mainly stems from industrial activities such as mining, smelting, and the manufacturing of electronics, while the use of fertilizers and pesticides in agricultural activities results in the leaching of metals into water systems [5]. Other contributors include tanneries, textiles, urban runoff, and coal combustion, adding to environmental contamination and enhancing the problem. These metals are non-biodegradable in nature and become bioaccumulated by aquatic organisms, where they are magnified through the food chain, posing serious health threats that include neurological damage, kidney failure, and cancer [6]. The conventional physicochemical techniques for heavy metal remediation include chemical precipitation, ion exchange, and adsorption. However, these techniques tend to be very costly, less effective, and cannot be viable for a long period [6]. At the same time, conventional techniques also create environmental problems, which include either the generation of secondary contaminants or the use of huge amounts of energy [7]. Adsorption--whereby metals are attached to the surface of materials such as activated carbon and biochar--is a cost-effective approach for small-scale applications. Chemical precipitation involves the transformation of dissolved metal ions into insoluble forms, which can then be more easily extracted; however, this process tends to produce large quantities of solid waste [8]. The physicochemical methods of coagulation and flocculation favor the aggregation of small size metal particles into big clusters, which could then easily be separated, but generally require chemical additives, which potentially modify the water chemistry itself [9]. Advanced membrane technologies, like reverse osmosis and electrodialysis, are very accurate but involve the production of concentrated brine wastes, creating disposal problems [10]. The appropriate remediation method is selected based on the cost, scalability, and environmental impacts, which calls for site-specific solutions and the integration of hybrid technologies to achieve optimal results [11]. Therefore, a dire need has emerged for the adoption of novel and cost-effective techniques capable of mitigating the heavy metal burden in an efficient manner. To this direction, various green remediation techniques were proposed for the removal of heavy metals by algae. Microalgae have emerged as efficient tools in heavy metal remediation because of their unique characteristics of absorbing and accumulating metals from contaminated water, being an environmentally friendly and economically viable option at the same time [1; 2; 3; 6]. Microalgae have the potential for a very innovative and green approach to the removal of heavy metals like Pb, Hg, Cd, Zn, and Ni from contaminated water [1; 3; 4; 6; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. Moreover, concerning circular economy, microalgae bioremediation helps not only to clean the environment, but it also provides value-added byproducts such as biofuels and other bio-based materials [23]. The application of microalga-based remediation to wastewater treatment systems was highlighted in several works as a promising tool [23]. In light of such advancements, it is important to explore the microalgae potential for the elaboration of environmentally enhancing processes for heavy metal control, wastewater treatment, and for sustainable biofuel production.