Different Bacterial Populations Associated with the Roots and Rhizosphere of Rice Incorporate Plant-Derived Carbon

Microorganisms associated with the roots of plants have an important function in plant growth and in soil carbon sequestration. Rice cultivation is the second largest anthropogenic source of atmospheric CH4, which is a significant greenhouse gas. Up to 60% of fixed carbon formed by photosynthesis in plants is transported below ground, much of it as root exudates that are consumed by microorganisms. A stable isotope probing (SIP) approach was used to identify microorganisms using plant carbon in association with the roots and rhizosphere of rice plants. Rice plants grown in Italian paddy soil were labeled with (CO2)-C-13 for 10 days. RNA was extracted from root material and rhizosphere soil and subjected to cesium gradient centrifugation followed by 16S rRNA amplicon pyrosequencing to identify microorganisms enriched with C-13. Thirty operational taxonomic units (OTUs) were labeled and mostly corresponded to Proteobacteria (13 OTUs) and Verrucomicrobia (8 OTUs). These OTUs were affiliated with the Alphaproteobacteria, Betaproteobacteria, and Deltaproteobacteria classes of Proteobacteria and the "Spartobacteria" and Opitutae classes of Verrucomicrobia. In general, different bacterial groups were labeled in the root and rhizosphere, reflecting different physicochemical characteristics of these locations. The labeled OTUs in the root compartment corresponded to a greater proportion of the 16S rRNA sequences (similar to 20%) than did those in the rhizosphere (similar to 4%), indicating that a proportion of the active microbial community on the roots greater than that in the rhizosphere incorporated plant-derived carbon within the time frame of the experiment.