Bacterial consortia in iron-deposited colonies formed on paddy soil surface under microaerobic conditions

Abstract

The present study observed bacterial consortia in iron-deposited colonies formed on the surface of paddy soils. Plow layer soils under reduced condition, which were collected from a paddy field in Japan and paddy fields located at two sites in Mekong Delta, Vietnam, were incubated at 25 and 30°C in test tubes under a nitrogen (N2) atmosphere with sleeve stoppers made from a synthetic rubber. The sleeve stopper enabled the establishment of microaerobic conditions because of gradual diffusion of oxygen (O2) into the tube (29.2 nmol mL−1 day−1). Many colonies were observed on the soil surface in the test tubes after 2–3 weeks of incubation from both Japanese and Vietnam paddy soils. Bright orange and blood red colonies, which had 0.5–1-mm diameters and circular shapes, were observed. Micro X-ray fluorescence analysis of the colonies showed deposition of iron (Fe) in the colonies, indicating bacteria were enriched in the Fe oxidation. Microscopic observation showed acicular, floc and concentric crystal structures of Fe oxides in the colonies. Long- and short-rod bacterial cells mainly existed inside (or interspace) and outside surface of the colonies. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and clone library analysis of bacterial 16S rRNA genes showed that most of the obtained sequences belonged to Betaproteobacteria, in which bacteria related to “Sideroxydans lithotrophicus” and “S. paludicola” in Gallionellales, known as microaerobic, neutrophilic Fe-oxidizers, were predominant. The sequences related to Thiobacillus spp. and Dechloromonas spp., some strains of which are known to be anaerobic, nitrate-dependent neutrophilic Fe-oxidizers, were also obtained. The present study showed proliferation of bacteria related to neutrophilic Fe-oxidizers in paddy soils under microaerobic condition.