Visualization of the Dynamic Rhizosphere Environment: Microbial and Biogeochemical Perspectives

Zoe G Cardon1, Elizabeth Sullivan Forbes2, Francois Thomas3, Patrick M. Herron4, Daniel J Gage5, Suzanne Michelle Thomas1, Morten Larsen6, Catalina Arango Pinedo7, Stefan Manfred Sievert8 and Anne E Giblin1, (1)Marine Biological Laboratory, Ecosystems Center, Woods Hole, United States, (2)Marine Biological Laboratory, Woods Hole, MA, United States, (3)University of Lorraine Nancy, Nancy Cedex, France, (4)Mystic River Watershed Association, Arlington, MA, United States, (5)University of Connecticut, Storrs, CT, United States, (6)University of Southern Denmark, Department of Biology, Odense, Denmark, (7)Saint Joseph's University, Philadelphia, PA, United States, (8)Woods Hole Oceanographic Institution, Biology, Woods Hole, MA, United States
Abstract:
The rhizosphere is a hotbed of nutrient cycling fueled by carbon from plants and controlled by microbes. Plants also strongly affect the rhizosphere by driving water flow into and out of roots, and by oxygenating saturated soil and sediment. Location and dynamics of plant-spurred microbial growth and activities are impossible to discern with destructive soil assays mixing microbe-scale soil microenvironments in a single”snap-shot” sample. Yet data are needed to inform (and validate) models describing microbial activity and biogeochemistry in the ebb and flow of the dynamic rhizosphere.

Dynamics and localization of rapid microbial growth in the rhizosphere can be assessed over time using living soil microbiosensors. We used the bacterium Pseudomonas putida KT2440 as host to plasmid pZKH2 containing a fusion between the strong constituitive promoter nptII and luxCDABE(genes coding for light production). High light production by KT2440/pZKH2 correlated with rapid microbial growth supported by high carbon availability. Biosensors were used in clear-sided microcosms filled with non-sterile soil in which corn, black poplar or tomato were growing. KT2440/pZKH2 revealed that root tips are not necessarily the only, or even the dominant, hotspots for rhizosphere microbial growth, and carbon availability is highly variable in space and time around roots.

Roots can also be sources of oxygen (O2) to the rhizosphere in saturated soil. We quantified spatial distributions of O2 using planar optodes placed against the face of sediment blocks cut from vegetated salt marsh at Plum Island Ecosystems LTER. Integrated over time, Spartina alterniflora roots were O2 sources to the rhizosphere. However, “sun-up” (light on) did not uniformly enhance rhizosphere O2 concentrations (as stomata opened and O2 production commenced). In some regions, the balance of O2 supply (from roots) and O2 demand (root and microbial) tipped toward demand at sun-up (repeatedly, over days). We speculate that in these regions, carbon produced during photosynthesis was released from roots and stimulated microbial O2 demand in the light. In situ, such dynamics in O2 and carbon availability around plant roots will influence interlinked sulfur, nitrogen, and carbon cycling in salt marsh rhizosphere.

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