Interaction of chromium with resistant strain Aspergillus versicolor:: Investigation with atomic force microscopy and other physical studies

The interaction of chromium and a chromate resistant Aspergillus versicolor strain has been studied by atomic force (AFM) and transmission electron (TEM) microscopies. The nanomechanical properties such as cell wall rigidity and elasticity were measured by force spectroscopy and found to be 0.6.1 +/- 0.08 N/m, and 20.5 +/- 2.1 MPa, respectively. On chromium binding, ultrastuctural changes of the cell wall along with the formation of layered structures on the cell wall were observed. TEM and AFM micrographs demonstrate the accumulation of chromium on the cell wall, which were rough and irregular compared with the smooth pristine mycelia. The surface roughness, cell wall rigidity and elasticity increased to 35.5 +/- 3.5 nm, 0.88 +/- 0.05 N/m, and 62.5 +/- 3.5 MPa, respectively, from the corresponding values of 5.2 +/- 0.68 nm, 0.61 +/- 0.02 N/m, and 20.5 +/- 2.1 MPa for the pristine mycelia. X-ray photoelectron spectroscopy and Fourier transform infrared studies suggest that bound chromium was reduced to its trivalent state by the cell wall components. The reduced chromium species on the cell surface further electrostatically bind chromate ions forming layered structure on the cell wall.