Purple phototrophic bacteria (PPB) are thought to be crucial in the nutrient cycling of rice field. However, it remains unclear how PPB would respond to the climate change associated with the projected atmospheric CO2 in the future. A factorial design of field experiments was set up with two levels of atmospheric CO2 concentration (350 and 550 μmol mol−1) and N application rate (150 and 250 kg N ha−1) to investigate the abundance and composition changes of PPB in rhizospheric and bulk soils in response to the rising atmospheric CO2 concentration. Based on denaturant gradient gel electrophoresis (DGGE) analysis of pufM gene encoding the M subunit of anoxygenic PPB light reaction center, elevated CO2 appeared to enhance the biodiversity of PPB in flooded paddy soils. This was further supported by canonical correspondence analysis (CCA) of DGGE fingerprinting pattern of pufM genes in paddy soils as well as Shannon diversity indices. Real-time quantitative PCR analysis of pufM gene further indicated that PPB abundance was stimulated by elevated CO2 in bulk soil, while the contrasting result was observed in rhizospheric soil. Our result for the first time demonstrated that elevated CO2 enhanced the biodiversity of PPB within α and β subdivisions of Proteobacteria.