Ultrastructure and X-ray microanalysis of Euglena gracilis (Euglenophyta) under chromium stress

Abstract

The effect of chromium on the biology of Euglena gracilis was studied. The ultrastructural modifications caused by this metal and its location within the cell were analyzed by TEM and EDXA, respectively. The effects of chromium on protein, pigment, and lipid contents were also studied in order to evaluate the metabolic responses to metal exposure. Two strains of Euglena gracilis, UTEX 753 (from the Culture Collection of Texas University) and MAT (isolated from the Matanza River), were used in this research. Both were grown in photoauxotrophic and photoheterotrophic conditions and exposed to different metal concentrations. In all treated cells, increases in total protein and lipid contents, changes in chlorophyll amount, and alterations in fine structure were observed, especially with the higher concentration tested. In photosynthetic treated cells, assays showed chloroplast thylakoid disorganization, the presence of cytoplasm lipid globules, and several vacuoles with electron-dense inclusions and remnants of membranes inside. Nuclei presented lobulations, and eventually total fragmentation in some cells treated with the highest chromium concentration was seen, suggesting that chromium cytotoxicity leads to cellular death. The EDXA spectrum showed well-defined Cr and S peaks in the vacuoles containing electron-dense inclusions and remnants of membranes from autotrophic MAT samples. These results indicate that the different defense mechanisms against chromium depend on strain type and culture conditions. The S peak detected in MAT would suggest that sulfur-rich proteins groups play an important role in the detoxification system inducing metal-complex accumulation into vacuoles.