Novel iron-storage particles may play a role in aluminum tolerance of Cyanidium caldarium

Cyanidium caldarium (Tilden) Geitler, a unicellular red alga, has extraordinarily high aluminum (Al) tolerance. Algal cells cultured in the presence or absence of Al were subjected to transmission electron microscopy and energy dispersive X-ray analysis. Substantial changes to the thylakoid lumens were observed for the algal cells cultured in medium containing 200 mM Al, while other organelles were largely unaffected. Several spherical electron-dense bodies were found in the cytoplasm near the nucleus of both of the control and Al-treated cells. Although high levels of Fe and P were found in the bodies of control cells, immunocytochemical and morphological analysis data did not match the criteria established for Fe-accumulating substances like ferritin and phytate. In addition to these elements, Al was found in the bodies of the Al-treated cells. These results suggest that the electron-dense bodies function as an Fe-storage site under normal culture conditions, and that sequestration of Al in these bodies contributes to the high Al tolerance exhibited by C. caldarium.