Drainage of flooded paddy field soil triggers the exponential growth of iron-oxidizing bacterial community within a short duration

Gallionella-related iron-oxidizing bacteria (FeOB), most of which are microaerophiles, are involved in the ferrous iron (Fe[II]) oxidation in circumneutral freshwater environments, but the ecology has remained poorly understood in paddy field soils. The present study analyzed the dynamics of Gallionella-related FeOB community after drainage of a flooded paddy soil. A paddy soil was pre-incubated in glass syringes under flooded conditions, followed by drainage at 12, 30, and 37°C. The drainage from the bottom of syringes brought about only a slight decrease in the amounts of Fe(II) in the soils over a short duration. However, the copy number of 16S rRNA genes of Gallionella-related FeOB community exponentially increased by a factor of 87, 31, and 11 at 12, 30, and 37°C, respectively, in the soils during the short duration. These quick responses to drainage were unique to the Gallionella-related FeOB community and not observed for ammonia oxidizers and methane oxidizers in the soils. However, the growth responses varied depending on the temperatures. The copy number began to increase on day 1 and 3 after drainage at 37 and 30°C, respectively, and the increases ended within 3 days. On the other hand, the increase began after day 5 and continued until day 17 at 12°C. The amount of Fe(II) oxidized by the Gallionella-related FeOB community during the growth phase was calculated to be approximately 10−3–10−2 mg Fe(II) g−1 dry soil according to the Fe(II) oxidation rates estimated for the cells of Ferrigenium sp. strain NRt1 isolated from rice roots. Amplicon sequencing analysis of the V1–V9 region of bacterial 16S rRNA genes revealed that Ferrigenium spp. were the predominant members of Gallionella-related FeOB community in the soils, but the composition at the species level was different depending on the temperature and even among the three replicates at a certain temperature. The present study indicated that drainage of flooded paddy field soils triggered the rapid and exponential growth of Gallionella-related FeOB community within a short duration, while the development of favorable conditions for their growth was heterogenous and transient in the soils.