Physiological and electron microscopical investigations on syntrophic dicyandiamide degradation by soil bacteria

Soil bacteria with high dicyandiamide (DCD) degrading capacity were isolated. In addition to pure cultures, two different consortia of soil bacteria were shown to degrade DCD under conditions of syntrophy. The single isolates of the consortia could not grow on minimal media or degrade DCD. These consortia turned out to be associations between Xanthomonas maltophilia and either Radiobacter sp. or Aureobacterium sp. Both types of consortia proliferate and degrade DCD rapidly. Supplementation experiments and microscopical investigations focusing on Xanthomonas maltophilia and Aureobacterium sp. showed that Aureobacterium sp. is probably auxotrophic for a yet unknown factor. Supplementation of minimal medium with minimum essential medium (MEM) vitamins enables this strain to grow with nitrate, but not with DCD or ammonia as single nitrogen sources. Xanthomonas maltophilia, however, is dependent on a substance which is not released into the medium by its companion. It cannot be provided either by the addition of vitamins or casamino acids or culture filtrates of proliferating consortia to the medium, but only by the presence of Aureobacterium sp. cells in the nutrient medium. Partially based on novel preparation techniques, microscopical investigations constantly showed a close contact between Xanthomonas maltophilia cells and its companion in enrichment medium, while Aureobacterium sp. is also found as single cells. The close contact appears to be of paramount importance to initiate growth and DCD degradation. The results demonstrate clearly the importance of investigations, including bacterial consortia in addition to pure culture studies when the degradability of agrochemicals has to be assessed.