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

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 10 kDa 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.

Screening of oleaginous algal strains from Chlamydomonas reinhardtii mutant libraries via density gradient centrifugation

Microalgae have a high potential to be utilized as feedstock for biofuels because they have high growth rates and do not compromise food production. Commercialized algae-based biofuel production relies on the development of strains with high lipid content. Based on the relatively low density of lipids compared to other cellular components, density gradient centrifugation was used to isolate high lipid content algal strains from Chlamydomonas reinhardtii mutant libraries. The correlation between cell density and lipid content was confirmed by analysis of Nile red fluorescence intensity, total lipids, and total fatty acid methyl ester content. A strain isolated by this screening method had 50% higher lipid content and 7% lower cell density than the parent wild-type strain. Consequently, we demonstrated that screening of algal strains with low cell density via continuous density gradient centrifugation allows simple, rapid, and inexpensive screening for high lipid content strains.