Mechanism of HMs removal by living microalgae: in-depth characterization of two marine strains used as biosorbents.

In the past decades, the uncontrolled growth of industrialization led to a steady increase in heavy metal contamination in seawater environments, especially coastal ones. Apart from conventional methods for HM removal, such as novel environmentally-sustainable approaches based on the exploitation of aquatic or-ganisms, such as marine microalgae, is attracting a great deal of attention. In general, both living and non-living organisms can be used for HMs removal. Biosorption of heavy metals in non-living micro-algal consists in the bond of metal ions onto cell walls, while, in living organisms, metal sequestration generally foresees an addi-tional step, in which metal ions are actively transported across the cell membrane and enter into the cell. The main advantage of employing non-living biomasses is the possibility to re-use sev-eral times these natural biosorbents, while, the use of living bio-mass assures a more quantitative removal, in reason of the differ-ent removal mechanism. In order to assess what is more conven-ient to use, a distinction between the amount of HM adsorbed on cell wall and the fraction internalised in microalgal cells should be distinguished. On this account, in this work a deep characterization of different microalgae strains (such as S. pseudocostatum, T. convolutae, etc.) exposed to specific culture conditions (metal-free controls, a sublethal and a lethal dose) has been performed, aimed at dis-criminating HM quotas and HM distribution among the cell wall and the compartmentalized fraction. In particular, freeze-dried samples of microalgae exposed to both Cu and Cd have been characterized through FT-IR, CHNS-O, TGA-DSC techniques, while the concentration of HMs in the whole matrix and in the surface of the cell has been established by XRF and XPS analysis, respectively. Notably, both S. pseudocostatum and T. convolutae are capable of removing both heavy metals from the culture medium, alt-hough the removal process is more efficient for S. pseudocosta-tum. Furthermore, Cd appears to be absent on the cells surface, while in the case of Cu removal, a precise correlation between the Cu amount detected in surface and bulk of the cell has been detected.