Differential retention of proteins and bound divalent cations in dinoflagellate chromatin fixed under varied conditions: an x-ray microanalytical study.

X-ray microanalysis, with on-line computation of elemental mass fractions was used to determine the levels of chromatin-bound phosphorus (nucleic acid), sulphur (proteins) and divalent metals (Ca, Fe, Ni, Cu and Zn) in cells of GLenodinium foliaceum (Stein), fixed only in glutaraldehyde. The absence of osmium tetroxide as postfixative was necessary for quantitation (large contaminant Os peak), but resulted in poor fine structural preservation. Increasing the duration of the glutaraldehyde fixation by stages from 15 min to 24 h brought about a marked change in the ultrastructural appearance of the coagulated chromatin, which was correlated with changes in the level of sulphur-containing proteins. The level of phosphorus remained fairly constant. The mass fraction of total bound metals was highest under conditions of intermediate fixation, as also was the total number of metal atoms per 100 atoms of phosphorus. The microanalytical results suggest that divalent metal cations are associated both with high molecular weight (acid-insoluble) proteins and nucleic acids. Postfixation with osmium tetroxide only resulted in full stabilisation of chromatin fine structure when preceded by an aldehyde fixation that retained adequate levels of proteins and divalent cations.