『Abstract
　Bacteria, actinomycetes, and fungi are the dominant components of the soil microflora and some of their species can perform denitrification. The aim of this study was to investigate the interactions of three kinds of denitrifiers in mix-culturing systems. Three denitrifying strains, i.r., one bacterial strain (strain B5), one actinomycete strain (strain A2), and one fungal strain (strain F1), were isolated from a rice paddy soil. Denitrifier interactions were examined by analyzing the population dynamics and metabolic substance in the mix-culturing systems with two and three strains and by estimating the effects of cell-free culture filtrates on the strains. Results showed that the growth of B5 was enhanced by F1 and A2, respectively, and nitrate removal proportions in the culture systems increased from 52％ (B5) to 64％ (B5+F1) and 67％　(B5+A2), and the nitrate removal was further enhanced in the three strain mix-culturing system (74％, A2+F1+B5). Strain B5 stimulated the cell growth of A2 directly and indirectly. The existence of A2 was lethal for cell growth of F1, while A2 was also suppressed by F1. The suppressive interaction reduced nitrate removal rates from the single systems of 12.8 (F1) and 11.5 mg L^-1 day^-1 (A2) to 8.75 mg L^-1 day^-1 (A2+F1). Likewise, F1 was inhibited by B5. The results also showed that the cell-free culture filtrates of other strains suppressed the cell growth of B5 and F1, respectively, but enhanced the cell growth of A2. In addition to the direct effect of cell-free culture filtrates, other indirect relationships could affect the denitrifier spatial distributions and balance of the suppression or promotion effects, which were beneficial to maintain the microbial structure and function stability with a low nitrous oxide emission in the soil.

Keywords: Bacteria; Actinomycetes; Fungi; Denitrifier; Nitrate removal』

1. Introduction
2. Materials and methods
　2.1. Strain isolation and denitrifier identification
　2.2. Phylogenetic analysis
　2.3. Mix-culturing systems
　2.4. Determination of bacterial, fungal, and actinomycete biomass
　2.5. Determination of nitrate and nitrite concentrations
　2.6. Effect of cell-free culture filtrates on strains
3. Results
　3.1. Denitrifying actinomycete, fungal, and bacterial strains
　3.2. Population dynamics in mix-culturing systems
　　3.2.1. Nitrate removal in mix-culturing systems
　　3.2.2. Nitrite accumulation in mix-culturing systems
　　3.2.3. Growth dynamics in mix-culturing systems
　3.3. Effect of culture filtrate on the strains
　　3.3.1. Effect on the nitrite accumulation of the strains
　　3.3.2. Effect on the growth of other strains
4. Discussion
　4.1. Effect on strain B5
　4.2. Effect on strain A2
　4.3. Effect on strain F1
　4.4. Overall relationships
5. Conclusions
Acknowledgments
References