Photosystem modulation and extracellular silicification in green microalgae: Key strategies for lead tolerance and removal

The escalating contamination caused by lead ions (Pb²⁺) and its harmful effects on all life forms has raised global concerns. Certain microalgae thrive in metal mining sites characterized by low pH and high concentrations of Pb²⁺, which are usually prohibitive for many microorganisms. Little is known about the mechanisms underlying the adaptation of such microalgae to these hostile conditions. In this study, we elucidated the adaptive strategies of the green microalga <i>Micractinium belenophorum</i> strain AUMW, isolated from a lead mining site, and its application for the removal of Pb⁺². Results revealed that strain AUMW can efficiently tolerate up to 200 ppm of Pb⁺² in an F/2 medium. Further experimental variables were optimized through response surface methodology (RSM), and 99.6 % removal of Pb²⁺ was achieved. Novel adaptive responses of strain AUMW to high levels of Pb²⁺ include: (i) activation of metal-protective response by modulation of quantum yield (<i>F</i> _<i>v</i> <i>/F</i> _<i>m</i> ) and non-photochemical quenching (NPQ) of photosystem II; (ii) extracellular silicification encapsulated cells of strain AUMW and altered cell morphology from oval to hexagonal; (iii) silicification prevented intracellular translocation of Pb⁺²; (iv) silicification boosted adsorption of Pb⁺², thus enhanced its removal. This study offers new insights into the protective role of silicification in green microalgae and its potential for the removal of metals from metal-polluted sites, waste from energy storage battery industries, and spent batteries. It also provides a solid base to explore the genetic and metabolic pathways involved in the adaptation of strain AUMW to elevated levels of Pb⁺².