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 wate r. In our laboratory experiments we investigated whether some of them, e.g. , starch or peptone, and also a complex yeast biomass can be transformed in to humic-like substances by natural assemblages and some pure cultures of a quatic microorganisms. After a 6 months incubation most of the cultural med ia enriched with those natural organic substrates turned dark in color and humic-like substances (HS) could be isolated. However, the original substra te organic carbon was mainly mineralized in microbial cultures, and only ab out 3 % C was converted into HS,Total yields of HS differed in dependence o f the individual substrate used (peptone > yeast > starch), the origin of i noculum (river > lake > groundwater), and the incubation temperature (20 de grees C > 10 degrees C). According to their elemental composition, and thei r spectroscopical and electrophoretic characteristics the microbially forme d HS resembled natural aquatic humic matter, but were higher in aliphatic c onstituents (carbohydrates; peptides) and lower in aromatic structures. MS- like substances were also obtained from pure cultures of a bacterium Bacill us sphaericus and a fungus Cladosporium cladasporioides. The C-13 and N-15 isotopic contents of the microbially produced HS differed in dependence on the microbial inoculum and the type of organic substrates used, but in gene ral they indicated anabolic processes to be mainly responsible for the humi fication of the simple organic substrates used in our experiments.