Soil Microorganisms

Whether fungi isolated from organic matter become a substrate for the fungivorous astigmatid mite Tyrophagus similis was evaluated. The suitability of fungus species to sustain the population growth of T. similis differs among fungus strains, which indicates that this mite consumes fungi in a species-specific manner. Species prediction with the rDNA internal transcribed spacer (rDNA-ITS) sequence showed that fungi with a sequence identical to that of Mucor racemosus, Rhizopus oryzae, Fusarium proliferatum, and Penicillium olsonii were suitable for mite amplification. These funfus species may serve as nutrients for T. similis and possibly support spinach herbivores. On the other hand, fungus species isolated from rice husk, which is good for mite amplification, were not suitable for mite amplification. This result indicates that the involvement of fungus species on mite amplification differs depending on the type of organic matter. Fungus species classified into Penicillium sp. showed various rates of mite amplification, indicating that the mite differently propagates on fungus species that have a close genetic relationship.

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The abundance of rDNA from each of 3 domains (Bacteria, Eukarya, and Archaea) in 30 samples of agricultural soil was estimated with dot-blot hybridization. The abundance of the domains Bacteria, Eukarya, and Archaea in all soils tested was estimated around 90%, 5%, and 3%, respectively, although the soil types and field management of the sampling sites were different. To reveal the behavior of the domain Archaea, containing ammonia-oxidation archaea and methanogen, in agricultural environments, soil factors affecting the abundance of archaeal rDNA in the soils were analyzed. Based on the results obtained from 30 soils sampled, soil pH was the most effective factor. In upland field soils, those with higher pH showed a greater abundance of archaeal rDNA; in contrast, in paddy field soils, those with higher pH showed lower abundance of archaeal rDNA. An amendment of organic matter, such as tree leaves or compost, to the soils also affected the copy number and abundance of archaeal rDNA in the soils. The soil amended with organic matter showed a higher copy number and lower abundance of archaeal rDNA than unamended soil. It was suggested that in such nutrient-rich environment, the archaeal rDNA copy number increased according to the increase of cell numbers, while the abundance of archaeal rDNA decreased by competition with other organisms that grow faster.

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