Impact of organic matter molecular weight on hexavalent chromium enrichment in green microalgae

Abstract

In lightly polluted water containing heavy metals, organic matter, and green microalgae, the molecular weight of organic matter may influence both the growth of green microalgae and the concentration of heavy metals. This study elucidates the effects and mechanisms by which different molecular weight fractions of fulvic acid (FA), a model dissolved organic matter component, facilitate the bioaccumulation of hexavalent chromium (Cr(VI)) in a typical green alga, Chlorella vulgaris. Findings show that the addition of FA fractions with molecular weights greater than 10kDa significantly enhances the enrichment of total chromium and Cr(VI) in algal cells, reaching 21.58%-31.09% and 16.17%-22.63%, respectively. Conversely, the efficiency of chromium enrichment in algal cells was found to decrease with decreasing molecular weight of FA. FA molecular weight within the range of 0.22 μm-30 kDa facilitated chromium enrichment primarily through the algal organic matter (AOM) pathway, with minor contributions from the algal cell proliferation and extracellular polymeric substances (EPS) pathways. However, with decreasing FA molecular weight, the AOM and EPS pathways become less prominent, whereas the algal cell proliferation pathway becomes dominant. These findings provide new insights into the mechanism of chromium enrichment in green algae enhanced by medium molecular weight FA. Environmental implicationHexavalent chromium (Cr(VI)) presents a notable hazard in water environments due to its toxicity, even at low concentrations. In water with slight pollution levels containing dissolved organic matter (DOM), Cr(VI), and green microalgae, the molecular weights of DOM play a crucial role in Cr(VI) bioaccumulation in microalgae cells. However, the precise mechanisms underlying this process remain poorly understood. This study established a ternary system consisting of "Chlorella-DOM fractions-Cr(VI)" to investigate how medium molecular weight DOM enhances Cr(VI) enrichment in Chlorella, providing insight into the regulatory role of organic matter in the enrichment of heavy metals in green algae.