Effects Of Environmental Factors On Geobacteraceae And Anaeromyxobacter Spp. Communities In Paddy Soils

Geobacteraceae spp. and Anaeromyxobacter spp., genera affiliating to delta-proteobacteria, widely perch in various anaerobic sedimentary environments and are characterized by deriving energy from microbial Fe(Ⅲ) reduction. They are capable of oxidating heavy metal of variant valene, immobilizing radioactive metal element and degrading of organic halide, which make them be significant in environmental bioremediation and harvesting energy. Geobacteraceae spp. and Anaeromyxobacter spp. are characterized by acetate metabolism and capable of competing electron donor with methanogen through microbial Fe(Ⅲ) reduction and thus depress the acetate-dependent methanogenesis in paddy soils. Recently, most research are focusing on the community structures and microbial abundances of methanogen, ammonia oxidizing bacteria and Archaea in subsurface environment, while few information about the dynamic change of Fe(Ⅲ) reducers in paddy soils is available. Combining the rich paddy soil sources in China, to study the community structures and abundances of Geobacteraceae spp. and Anaeromyxobacter spp. in these soils, including the influence of environmental factor on them, will not only deepen the understang of microbial ecology in paddies, but also be significant in clarifying the mechanism of microbial Fe(Ⅲ) reduction and the successions of Fe(Ⅲ) reducers.Anaerobic incubation under constant temperature was conducted to investigate how Geobacteraceae spp. and Anaeromyxobacter spp. will response to flooding in paddy soils from different rice planting areas in order to disscuss the correlation between microbial community, Fe(Ⅲ) reduction and soil property respectively. The dynamic changing of abundance and community structure were investigated by SSCP and realtime PCR, considering the concentration variances of Fe(Ⅱ) and sulfate, we discussed the relation of Geobacteraceae and Anaeromyxobacter with Fe(Ⅲ) and sulfate reduction. By 16S rRNA -RFLP, bacterial community succession and diversity changing under different oxygen levels and temperature were observed, as well as the comparison of the abundances and community structures of Geobacteraceae spp. and Anaeromyxobacter spp. on both DNA and RNA level. SSCP, sequencing and phylogenetic analysis was performed to identify the dominant genera of Bacteria and Geobacteraceae spp. in slurries with different oxygen levels and incubating temperatures. How initial pH and the addition of straw and grain will influence Geobacteraceae spp. and Anaeromyxobacter spp. was also covered in this study through determining the pH value, Fe(Ⅱ) concentration and the abundance of Geobacteraceae and Anaeromyxobacter.The major results and conclusions are listed below:(1) Soil samples from 4 patterns of rice planting aeras were incubatied by inmitating the natural flodding process with anaerobic condition and constant temperature. Fe(Ⅲ) reduction potential varied in different paddy soils and could be ranked declining trend that correlated with their geographical distributions from the north to the south. Geobacteraceae spp. and Anaeromyxobacter spp. in Jilin and Guangxi soils showed no sensitivity to 12h flooding; however, Geobacteraceae spp. and Anaeromyxobacter spp. were sensitive to flooding in the soils of Zhejiang and Tianjin.(2) Anaerobic flooding incubation under 16~50°C with soil sample from colder aera was conducted. Diversities of metabolicly active communities of Geobacteraceae spp. and Anaeromyxobacter spp. were both the highest at 30°C; community diversity of Geobacteraceae spp. increased from 16°C to 36°C while Anaeromyxobacter spp. decreased, indicating their difference of survival strategies. In general, DNA-based diversity was higher than RNA-based ones for both Geobacteraceae spp. and Anaeromyxobacter spp.. Temperature caused significant impact on the dominant communit structure of Anaeromyxobacter while no similar influence was observed on Geobacteraceae. The abundanece of Geobacteraceae and Anaeromyxobacter raised with the increasing temperature, being the highest under 30°C and the lowest under 50°C. 16S rRNA gene copies numbers of Geobacteraceae and Anaeromyxobacter under 50°C were significantly lower than other treatments whereas the Fe(Ⅲ) reducing potential and Vmax was on a moderate level, suggesting the existence of thermophilic iron reducers in anaerobic paddy slurry. Most of the Geobacteraceae clones in paddy soils observed under different temperature were highly affiliated to the Geobacteraceae isolates from contaminated enviroments. This emphasized the function of Geobacteraceae in paddy slurry and its ecological significance.(3) Oxygen concentration significantly influenced the abundance and community structures of Geobacteraceae spp. and Anaeromyxobacter spp.. The abundances of Geobacteraceae spp. and Anaeromyxobacter spp. in slurry with oxygen supply above the aquatic layer were remarkably higher than the others, which reflected the growth of Geobacteraceae in aerobic paddy. In general,the abundance of Geobacteraceae spp. was always being higher than Anaeromyxobacter spp. especially in the early stage, indicating the good adaptability of Geobacteracea to environment. For both DNA- and RNA- based diversities, Geobacteraceae spp. and Anaeromyxobacter spp. in micro-aerobic slurry were significantly higher than the ones in anaerobic slurry, showing therir character of proliferatering new communities to survive oxygen exposure. Geobacter bremensis and Geobacter bemidjiensis were the dominant strains in slurry at ifferent oxygen levels.(4) Adjusting initial pH would not influence the variety of pH in paddy slurry and Fe(Ⅲ) reducing potential which depends on the amount of ferrihydrite in the paddies. Adjusting initial pH to an over-low level would continuously depress the 16S rRNA gene copies numbers of Geobacteraceae spp. and Anaeromyxobacter spp., consequently leading to significant retarding of the active reaction period of Fe(Ⅲ) reduction. Moderate adjustment of initial slurry pH enlarged the copy numbers of Geobacteraceae spp. and Anaeromyxobacter spp., especially in the late period. This observation may draw a conclusion that we can control the growth of Geobacteraceae spp. and Anaeromyxobacter spp. by adjusting the initial pH of paddy slurry.(5) Adding organic matter caused no influence on Fe(Ⅲ) reduction in anaerobic slurry whereas it significantly depressed the copies numbers of Geobacteraceae spp. and Anaeromyxobacter spp. while increasing the copy number of total Bacteria, suggesting that the bacterial community involved in degrading organic matter may also contributed to Fe(Ⅲ) reduction.By our study, we extended and deepened the knowledge about the abundance and community structure of Geobacteraceae and Anaeromyxobacter in different paddy soils under different environmental condition, discussed the correlation among soil property, environmental factor and microbial Fe(Ⅲ) reduction. This work provided a theoretical basis for the further understanding of the distribution, phylogenetic classification and ecological function, as well as discussing the difference of Fe(Ⅲ) reduction between various paddy soils.