Electron Microscopy of Stable Electrophoretic Fractions of Natural Humic Acids as a Key to the Understanding of Their Structural Organization

Objective: Natural humic substances (HS) make up a significant part of the organic carbon of the biosphere. Considering the numerous significant ecological functions of HS, the lack of consensus either on the mechanisms of their formation or on the main features of their structure is somewhat paradoxical. Methods: Transmission electron microscopy (TEM) with contrast staining by uranyl acetate solution was used first time to study the morphological differences between soil humic acids (HAs) and fractions obtained by coupling preparative low-pressure size-exclusion chromatography and analytical polyacrylamide gel electrophoresis. Results and Discussion: Significant morphological differences between the fractions were observed and could be associated with their composition, established previously using several physicochemical methods. The C_ar (165–108 ppm)/C_alk (108–0 ppm) ratio, or aromaticity index, calculated from the ¹³C NMR data, could be one of the indicators of the formation of various morphological structures. Conclusions: The obtained TEM results clearly confirm the supramolecular organization of soil HAs.