Seasonal hyporheic dynamics control coupled microbiology and geochemistry in Colorado River sediments

Abstract

Riverbed microbial communities play an oversized role in many watershed ecosystem functions, including the processing of organic carbon, cycling of nitrogen, and alterations to metal mobility. The structure and activity of microbial assemblages depend in part on geochemical conditions set by river‐groundwater exchange or hyporheic exchange. To assess how seasonal changes in river‐groundwater mixing affect these populations in a snowmelt‐dominated fluvial system, vertical sediment and pore water profiles were sampled at three time points at one location in the hyporheic zone of the Colorado River and analyzed by using geochemical measurements, 16S rRNA gene sequencing, and ecological modeling. Oxic river water penetrated deepest into the subsurface during peak river discharge, while under base flow conditions, anoxic groundwater dominated shallower depths. Over a 70 cm thick interval, riverbed sediments were therefore exposed to seasonally fluctuating redox conditions and hosted microbial populations statistically different from those at both shallower and deeper locations. Additionally, microbial populations within this zone were shown to be the most dynamic across sampling time points, underlining the critical role that hyporheic mixing plays in constraining microbial abundances. Given such mixing effects, we anticipate that future changes in river discharge in mountainous, semiarid western U.S. watersheds may affect microbial community structure and function in riverbed environments, with potential implications for biogeochemical processes in riparian regions.

Dissolved Organic Matter Chemistry and Transport Along an Arctic Tundra Hillslope

“…Greater DOM diversity in riparian soils could also be due to diverse microbial communities exploiting multiple redox niches (Ernakovich & Wallenstein, 2015;Vos et al, 2013), limited denitrification at depth (Harms & Ludwig, 2016), or selective physicochemical preservation driven by DOM redox reactivity and position within the landscape. Spatial heterogeneity of redox gradients has been linked to the activation of metabolic pathways utilizing alternate terminal electron acceptors to degrade DOM (Boye et al, 2017;Danczak et al, 2016), including siderophores and other chelating agents that are abundant in low-lying Arctic environments (Lipson et al, 2012). We observed relatively high concentrations of metabolites associated with fermentation pathways (including acetate, formate, acetyl phosphate, and methanol) in riparian soils.…”

“…These structures are highly oxidized and enriched in polycarboxylated fused-ring moieties that resist microbial turnover and accumulate in deep ocean DOM pools (Hertkorn et al, 2006). Spatial heterogeneity of redox gradients has been linked to the activation of metabolic pathways utilizing alternate terminal electron acceptors to degrade DOM (Boye et al, 2017;Danczak et al, 2016), including siderophores and other chelating agents that are abundant in low-lying Arctic environments (Lipson et al, 2012). We also observed a significant decrease in pore water C:N, suggesting DOM pools acquire an increasingly microbial signature during downslope export, independent of soil horizon.…”

Hyporheic Zone Microbiome Assembly Is Linked to Dynamic Water Mixing Patterns in Snowmelt‐Dominated Headwater Catchments

“…Nearest taxon index (ßNTI) for each location and each season. Seasonal values from Danczak et al () provided for comparison. Dashed red lines indicate selective pressure delineations; ßNTI > 2 = variable selection; ßNTI < −2 = homogenizing selection; and|ßNTI|< 2 = stochastic process domination.…”

Microbial and Reactive Transport Modeling Evidence for Hyporheic Flux‐Driven Cryptic Sulfur Cycling and Anaerobic Methane Oxidation in a Sulfate‐Impacted Wetland‐Stream System

“…In this study, we hypothesize that currently underconstrained “cryptic” S cycling and AOM serve as major biogeochemical processes in wetlands and, further, that the variable rates and mechanisms observed in past studies can be attributed to dynamic conditions of hyporheic zones. Hyporheic zone mixing of surface water and groundwater creates steep redox gradients and promotes hot spots and hot moments of fluctuating microbial activity (Boano et al, ; Briggs et al, ; Danczak et al, ; Feris et al, ; Krause et al, ; McClain et al, ; Vidon et al, ; Zarnetske et al, ), but little is known about how these dynamic fluxes impact “cryptic” S cycling and AOM in wetlands. We evaluated our hypotheses using a combination of hydrologic and geochemical field observations, reactive transport modeling, and microbiome analysis at a SO ‐impacted wetland‐stream system in northern Minnesota (USA).…”