Molecular Insights of Phytohormone-Like Small Biomolecules for Microalgae-Mediated Heavy Metal Bioremediation

Environmental sustainability is seriously threatened by the discharge of wastewater containing hazardous heavy metals (such as Cr, Cd, As, Hg, etc.). The utilization of microalgae has recently come to light as a viable, environmentally acceptable method for removing heavy metals from contaminated sites. Certain small biomolecules that resemble phytohormones can be beneficial in microalgal biotechnology as they control biological processes and signal transduction to increase stress tolerance and simultaneously upregulate the production of beneficial metabolites. As a result, they make good candidates for bioremediation and an effective vector for removing heavy metal pollutants from the environment. Melatonin, γ-aminobutyric acid (GABA), polyamines, and glycine-betaine are small biomolecules that act as signaling molecules or regulators in microalgae. They play crucial roles in controlling cell development, metabolism, stress resistance, heavy metal accumulation, and redox homeostasis. The potential of phytohormone-like small biomolecules and their incorporation into microalgal systems has been immensely explored by researchers across the globe. However, most studies have reported compromised photosynthetic efficiency in the targeted microalgae and repressed metabolite accumulation. There is then the need for developing cultivation methods without compromising cell viability and photosynthetic efficiency. Therefore, there is a greater need to understand the underlying mechanisms controlling cell proliferation and heavy metal bioaccumulation through the application of phytohormone-like small biomolecules. The current review aims to explore the efficacy of phytohormone-like small biomolecules in the context of microalgal bioremediation of heavy metals alongside the enhancement of various algal metabolites.