PNNL

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.