Formation of humic-like substances in mixed and pure cultures of aquatic microorganisms

A natural decay of plant and animal biomass in aquatic environments results in the release of different simple structured organic substances into water. In our laboratory experiments we investigated whether some of them, e.g., starch or peptone, and also a complex yeast biomass can be transformed into humic-like substances by natural assemblages and some pure cultures of aquatic microorganisms. After a 6 months incubation most of the cultural media enriched with those natural organic substrates turned dark in color and humic-like substances (HS) could be isolated. However, the original substrate organic carbon was mainly mineralized in microbial cultures, and only about 3 % C was converted into HS. Total yields of HS differed in dependence of the individual substrate used (peptone > yeast > starch), the origin of inoculum (river > lake > groundwater), and the incubation temperature (20°C > 10°C). According to their elemental composition, and their spectroscopical and electrophoretic characteristics the microbially formed HS resembled natural aquatic humic matter, but were higher in aliphatic constituents (carbohydrates, peptides) and lower in aromatic structures. HS-like substances were also obtained from pure cultures of a bacterium Bacillus sphaericus and a fungus Cladosporium cladosporioides. The 13 C and 15 N isotopic contents of the microbially produced HS differed in dependence on the microbial inoculum and the type of organic substrates used, but in general they indicated anabolic processes to be mainly responsible for the humification of the simple organic substrates used in our experiments.