ASSESSMENT OF EXOMETABOLITES BIOSYNTHESIZED BY SOIL MICROORGANISMS AS THE MAIN SOURCE OF HUMIC PRECURSORS

The presence of humus enhances and stimulates the growth of micro- and macro-level biodiversity of flora and fauna. It also appears consisting of organic residues and by-products which contain microbial type bio-synthesized substances as prehumic substances. Prehumic compounds are made up of carbohydrates, lipids, proteins, amino acids, DNA, etc. Their importance appears well highlighted while the balance between formation and microbial decomposition of humus is disrupted, their concentration being strongly influenced by the dynamics of soil processes mediated by microorganisms. To assess exometabolites of the edaphic microflora, as the main source of biosynthetic precursors of humus, 66 microorganisms were initially isolated to set up a base wide enough for analysis and selection. Microorganisms belonging to 11 genera have been identified on the basis of morphological and biochemical tests. Strains producing colouring exometabolites were selected and isolated on PDA and Topping media (yellow, yellow-green by bacterial isolates of genera Streptomyces, Azotobacter and Xanthomonas, or green-orange, red by bacterial isolates of genera Pseudomonas and Streptomyces). Isolates 7 and 5 of Bacillus spp. had the highest content of brown compounds. Indole test performed on bacterial isolates belonging to the genera Streptomyces, Bacillus, Pseudomonas, Azotobacter, and Xanthomonas were positive for the 3 bacterial isolates of Azotobacter (10, 11 and 12). Isolates belonging to the genus Pseudomonas (8 and 9) had nitrate reductase activity, those belonging to the genera Bacillus and Azotobacter had a variable response on this activity, and isolates of genera Streptomyces and Xanthomonas showed no nitrate reductase activity. The highest content of polysaccharide compounds was synthesized by isolates 9 of Pseudomonas sp., 12 of Azotobacter sp. and 5 of Bacillus sp. Polyphenolic compounds were excreted in large quantities by isolates 6 (Bacillus sp.), 8 (Pseudomonas sp.) and 11 (Azotobacter sp).  Among isolates tested, the isolate 9 of Pseudomonas sp. was the most active in biosynthesis of protein compounds. Understanding the complexity of soil microorganisms extracellular matrix will allow better management for humus regeneration, the biologically-mediated nutrient circuit and soil health.