Understanding how microalgae cells remediate heavy metals using a biophysical approach

VICTORIA PASSUCCI; CÉCILE FORMOSA-DAGUE; GUSTAVO CURUTCHET; MARÍA MAR ARECO

Simposio; The 7th International Symposium on Environmental Biotechnology and Engineering 2023 (7ISEBE); 2023

Microalgae have recently emerged as a promising biomass for the biosorption of heavy metals from aqueous environments, by offering an efficient and cost-effective alternative compared to conventional remediation techniques. However, to date, the mechanisms by which microalgae can remediate these pollutants is still unclear. In this work, we investigate the role of extracellular polymeric substances (EPS) of a green microalgae species, Parachlorella kessleri, isolated from the polluted Reconquista River in Argentina, in the biosorption of zinc. For that, we use an original approach based on atomic force microscopy (AFM) experiments. A first step in the study was to optimize a culture medium favoring the production of EPS by cells. It was showed that cells cultivated in the presence of nitrates could produce EPS while the cells had much less EPS when cultivated with ammonium as a nitrogen source. This was confirmed by nanomechanical experiments performed with AFM that showed that cells cultivated with nitrates were covered by a thick layer of a soft material (200 Pa approximately), corresponding to the EPS layer. To test the extent of the EPS on the biosorption of Zn, AFM force spectroscopy experiments were performed using zinc-functionalized AFM tips, this way allowing to directly probe the interactions between zinc and cell surfaces at the molecular level. The results obtained showed that zinc could interact with the EPS present at the surface of cells cultivated with nitrates with a force of up to 1 nN. Since no interactions were recorded when cells cultivated with ammonium were probed, this confirms that the EPS of P. kessleri can interact directly with and absorb zinc, thereby remediating it from the medium. Overall, these experiments provide a new understanding of biosorption mechanisms using an original biophysical approach, and show the potential of P. kessleri for heavy metal biosorption.