May 5, 2017
Feature

Modeling Microbial Ecology in the Hyporheic Zone

Biogeographical patterns and processes that govern microbial assembly

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In the world's rivers, groundwater and surface water mix in hyporheic zones - sometimes one more than the other. This mixing creates regions of enhanced biogeochemical activity that control microbial respiration.

That's important. Globally, hyporheic zone microbiomes are responsible for as much as 95 percent of riverine ecosystem respiration. More broadly, hyporheic zones strongly influence a river's carbon, nutrient, and contaminant dynamics.

Despite the importance of microbial ecology within hyporheic zones, it remains poorly understood. In turn, this knowledge gap impedes the development of models to simulate microbial function and seasonal variability.

A new paper, led by Pacific Northwest National Laboratory post-doc and microbial ecologist Emily B. Graham, starts to close that knowledge gap with a comprehensive analysis of biogeographical patterns in microbiomes that are both attached and waterborne. And it outlines the ecological processes governing the composition and function of subsurface microbiomes through space and time.

To arrive at their results, the researchers investigated biogeographical patterns across three physicochemically distinct, hydrologically connected zones: inland hyporheic, nearshore hyporheic, and river. They also investigated groups of organisms that correspond to deterministic changes in the environment, and how these groups correlate to hyporheic metabolism.

Results showed a pronounced hydrologic connectivity throughout the hyporheic zone, which suggests there is strong potential for the dispersal of microorganisms. Yet the researchers also found that ecological selection associated with changes in water chemistry governs microbiome composition within local environments. Using statistics, they identified one cluster of nearshore organisms associated with increases in biomass and respiration that correspond to seasonal changes in hydrology.

Based on their results, the researchers proposed a conceptual model for metabolism in the hyporheic zone that pivots on two basic observations: When surface water intrudes into the hyporheic zone, there are comparatively high levels of microorganisms associated with heterotrophic organic carbon metabolism. And during periods of groundwater discharge, the predominant microorganisms are autotrophic.

What's Next? The study provides new opportunities to develop microbially explicit ecosystem models that incorporate the hyporheic zone and its influence over riverine ecosystem function.

Acknowledgements

Sponsor: The Department of Energy's Office of Science, Office of Biological and Environmental Research, supported this research as part of the Subsurface Biogeochemical Research Program's Scientific Focus Area (SFA) at PNNL.

Reference: E.B. Graham, A.R. Crump, C.T. Resch, S. Fansler, E. Arntzen, D.W. Kennedy, J.K. Fredrickson, J.C. Stegen, "Deterministic influences exceed dispersal effects on hydrologically-connected microbiomes." Environ Microbiol. 2017 Apr;19(4):1552-1567. doi: 10.1111/1462-2920.13720.

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Pacific Northwest National Laboratory draws on its distinguishing strengths in chemistry, Earth sciences, biology and data science to advance scientific knowledge and address challenges in energy resiliency and national security. Founded in 1965, PNNL is operated by Battelle and supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit the DOE Office of Science website. For more information on PNNL, visit PNNL's News Center. Follow us on Twitter, Facebook, LinkedIn and Instagram.

Published: May 5, 2017