Interplay of humic acid and Cr(VI) on green microalgae: metabolic responses and chromium enrichment

Dissolved organic matter (DOM) present in aquatic environments can significantly influence microalgal metabolism and the enrichment of heavy metals. However, the specific mechanism through which typical DOM affects the enrichment of the heavy metal chromium (Cr) in green algae remains unclear. This study investigates the impacts of varying concentrations of humic acid (HA), selected as a representative DOM in water, on the growth, metabolism, and Cr enrichment in Chlorella vulgaris, a typical green alga. The results indicated that low concentrations of HA were capable of enhancing Cr enrichment in C. vulgaris, with the highest Cr enrichment rate recorded at 41.50 % at TOC = 10 mg/L. The enrichment of Cr in algal cells primarily occurred through cell proliferation and complexation reduction of extracellular polymeric substances (EPS). In the presence of HA, C. vulgaris predominantly removed Cr through extracellular adsorption, accounting for 79.76-85.88 % of the total Cr removal. Furthermore, carboxyl complexation and hydroxyl reduction within EPS facilitated both the enrichment of Cr (18.72-21.49 %) and the reduction of Cr(VI) (63.93-74.10 %). These findings provide valuable insights into strategies for mitigating heavy metal pollution and managing associated risks in aquatic environments.