Saturday, 15 July 2006

The Role of Microbial Chitinoclastic Complex in Soil.

Natalia A. Manucharova1, Alexeys M. Yaroslavtsev1, Elvira V. Belova1, and Alexei L. Stepanov2. (1) Moscow Lomonosov State Univ, Dept of Soil Science, Leninskie gori, Moscow, Russia, (2) Dept of Soil Science, Moscow State Univ, Vorobiovi gori, Moscow, Russia

Chitin is one of the most abundant organic polymer in soils, coming only second after cellulose among polysaccharide. This organic substance is included in fungal cell walls, exoskeletons of arthropods and insects. Microbial chitin degradation in soil, involves hydrolysis of the glycosidic bonds by the action of exo- and endo-chitinases. Chitinase genes can be found in a range of bacteria, actinomycetes and fungi. Chitinoclastic microorganisms could be used against plant fungi diseases. Despite of the great importance of chitin in natural ecosystems there is a lack of information about the contribution of different groups of organisms to its degradation. The purpose of this research was studying soil microbial cells and chitinoclastic biomass dynamics complex under aerobic and anaerobic conditions in 3 soil types: gray forest, chernozem and chestnut soils. Two series of experiments were performed: one part of soil samples was incubated under aerobic conditions and another part - anaerobic ones for 1 month at ambient temperature (25oC). Anaerobic conditions above soil samples were created by substitutions of an atmospheric gas phase with nitrogen (N2). The samples of native soils (without chitin application) were taken as control. The amount of cells and biomass of soil chitinolytic microorganisms were evaluated by direct luminescent-microscopic method. The carbon dioxide and methane emission were measured by gas chromatography method. Chitin was intensively decomposed in soils under aerobic and anaerobic conditions. Under aerobic conditions chitin degradation was carried out by three groups of microorganisms: fungi, actinomycetes and unicellular bacteria. The study of the structure of chitinolytic complex in soils demonstrated, that the cell amount and the biomass of all groups of microorganisms (bacteria, fungi and actinomycetes) increased in soil enriched with chitin, as compared with control samples. However, the calculation of prokariote part from total microbial biomass showed increasing prokariote biomass from 16% to 35% as compared with eucariot under chitin decompose in soil. This peculiarity indicates a liding prokaryotes role in chitin decompose under aerobic conditions. Using of the successional analysis of the chitinolytic complex let to discover the domination of the actinomycetes on the beginning and the final studies of the succession initiating by wetting and adding chitin into the different types of soils. Under anaerobic conditions, soil chitin degradation is carried out by the prokaryotes group of microorganisms: actinomycetes and unicellular bacteria. Some researchers proposed anaerobic pathway of chitin decomposition. The actinomycetes biomass was most actively increased among other microbial organisms. Actinomycetes mycelium and biomass was longer in chitin content variant in 2,5 times above, than in the comparison template. For bacteria the similar ratio was only 1,2 times. The length of a fungi mycelium and biomass in a sample with chitin was less in comparison with the control samples. It may be explained by activity of fungal chitininolitic complex, which supresses fungi growth. Such type of chitin transformation was found in all soils studied. Thus, anaerobic chitin degradation in soils is carried out by prokaryotic microorganisms, among prokatiotes - actinomycetes. The emission of carbon dioxide (under aerobic conditions) and methane (under anaerobic conditions) from soils enriched with chitin excides the samples without organic matter (chitin). The high level of carbon dioxide and methane emission was found from soils with low organic matter contents. It was shown that in soils under aerobic conditions the peak of CO2 emission was observed on 1-2 weeks of incubation, under anaerobic conditions CH4 emission reached the maximal level at 3-4 weeks of incubation, according to the changes in quantity and biomass of chitinoclastic microorganisms in soils. Thus, chitin is one of the most significant structure elements in transformation carbon and nitrogen content substances in soil; and all the studied groups of microorganisms participate in it's decomposition. All using methods (luminescent-microscopic, succession and gas chromatography methods) allowed to describe the process of chitin decomposition. We have shown that actinomycetes play the major role in chitin transformation in soil.

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