B42B-02:
Evidence of a dynamic microbial community structure and predation through combined microbiological and stable isotope characterization
Thursday, 18 December 2014: 10:50 AM
Jennifer L Druhan1, Markus Bill2, Hsiao Chien Lim2, Cindy Wu2, Mark E Conrad3, Kenneth Hurst Williams3, Donald J DePaolo3 and Eoin Brodie3, (1)Stanford University, Geological and Environmental Sciences, Stanford, CA, United States, (2)Lawrence Berkeley National Lab, Berleley, CA, United States, (3)Lawrence Berkeley National Laboratory, Berkeley, CA, United States
Abstract:
The speciation, reactivity and mobility of carbon in the near surface environment is intimately linked to the prevalence, diversity and dynamics of native microbial populations. We utilize this relationship by introducing 13C-labeled acetate to sediments recovered from a shallow aquifer system to track both the cycling of carbon through multiple redox pathways and the associated spatial and temporal evolution of bacterial communities in response to this nutrient source. Results demonstrate a net loss of sediment organic carbon over the course of the amendment experiment. Furthermore, these data demonstrated a source of isotopically labeled inorganic carbon that was not attributable to primary metabolism by acetate-oxidizing microorganisms. Fluid samples analyzed weekly for microbial composition by pyrosequencing of ribosomal RNA genes showed a transient microbial community structure, with distinct occurrences of Azoarcus, Geobacter and multiple sulfate reducing species over the course of the experiment. In combination with DNA sequencing data, the anomalous carbon cycling process is shown to occur exclusively during the period of predominant Geobacter species growth. Pyrosequencing indicated, and targeted cloning and sequencing confirmed the presence of several bacteriovorous protozoa, including species of the Breviata, Planococcus and Euplotes genera. Cloning and qPCR analysis demonstrated that Euplotes species were most abundant and displayed a growth trajectory that closely followed that of the Geobacter population. These results suggest a previously undocumented secondary turnover of biomass carbon related to protozoan grazing that was not sufficiently prevalent to be observed in bulk concentrations of carbon species in the system, but was clearly identifiable in the partitioning of carbon isotopes. The impact of predator-prey relationships on subsurface microbial community dynamics and therefore the flux of carbon through a system via the microbial biomass pool suggests a diversity of processes that should be considered for inclusion in reactive transport models that aim to predict carbon turnover, nutrient flux, and redox reactions in natural and stimulated subsurface systems.
