Specific rhizosphere bacterial and fungal groups respond differently to elevated atmospheric CO2

Soil community responses to increased atmospheric CO2 concentrations are expected to occur mostly through interactions with changing vegetation patterns and plant physiology. To gain insight into the effects of elevated atmospheric CO2 on the composition and functioning of microbial communities in the rhizosphere, Carex arenaria (a non-mycorrhizal plant species) and Festuca rubra (a mycorrhizal plant species) were grown under defined atmospheric conditions with either ambient (350 p.p.m.) or elevated (700 p.p.m.) CO2 concentrations. PCR-DGGE (PCR-denaturing gradient gel electrophoresis) and quantitative-PCR were carried out to analyze, respectively, the structure and abundance of the communities of actinomycetes, Fusarium spp., Trichoderma spp., Pseudomonas spp., Burkholderia spp. and Bacillus spp. Responses of specific functional groups, such as phloroglucinol, phenazine and pyrrolnitrin producers, were also examined by quantitative–PCR, and HPLC (high performance liquid chromatography) was employed to assess changes in exuded sugars in the rhizosphere. Multivariate analysis of group-specific community profiles showed disparate responses to elevated CO2 for the different bacterial and fungal groups examined, and these responses were dependent on plant type and soil nutrient availability. Within the bacterial community, the genera Burkholderia and Pseudomonas, typically known as successful rhizosphere colonizers, were significantly influenced by elevated CO2, whereas the genus Bacillus and actinomycetes, typically more dominant in bulk soil, were not. Total sugar concentrations in the rhizosphere also increased in both plants in response to elevated CO2. The abundances of phloroglucinol-, phenazine- and pyrrolnitrin-producing bacterial communities were also influenced by elevated CO2, as was the abundance of the fungal genera Fusarium and Trichoderma