Influences of organic carbon speciation on hyporheic corridor biogeochemistry and microbial ecology

Abstract

The hyporheic corridor (HC) encompasses the river–groundwater continuum, where the mixing of groundwater (GW) with river water (RW) in the HC can stimulate biogeochemical activity. Here we propose a novel thermodynamic mechanism underlying this phenomenon and reveal broader impacts on dissolved organic carbon (DOC) and microbial ecology. We show that thermodynamically favorable DOC accumulates in GW despite lower DOC concentration, and that RW contains thermodynamically less-favorable DOC, but at higher concentrations. This indicates that GW DOC is protected from microbial oxidation by low total energy within the DOC pool, whereas RW DOC is protected by lower thermodynamic favorability of carbon species. We propose that GW–RW mixing overcomes these protections and stimulates respiration. Mixing models coupled with geophysical and molecular analyses further reveal tipping points in spatiotemporal dynamics of DOC and indicate important hydrology–biochemistry–microbial feedbacks. Previously unrecognized thermodynamic mechanisms regulated by GW–RW mixing may therefore strongly influence biogeochemical and microbial dynamics in riverine ecosystems.

Non‐random processes determine the colonization of groundwater sediments by microbial communities in a pristine porous aquifer

“…We are aware that our study consists of only a relatively limited number of observations and therefore the results should be interpreted with the necessary caution. Nevertheless, our results fit observations on assembly processes for communities in the hyporheic zone (Graham et al ., ; ; Stegen et al ., ; ) as well as biofilms in surface water streams (Besemer et al ., ; Veach et al ., ), suggesting that selection not only plays a determining role in the assembly of surface‐attached microbial communities in those dynamic environments but also in pristine groundwater aquifers, despite the comparatively more stable environmental conditions, which have been shown to promote the effect of stochastic over deterministic processes in other environments (Ofiţeru et al ., ; Stegen et al ., ; Wang et al ., ; Zhou et al ., ). Mineral composition has previously been demonstrated to be a driving factor for microbial community composition and assembly (Grösbacher et al ., ; Stegen et al ., ; Jones and Bennett, ).…”

“…In contrast, much insight has been gained over the past years into the assembly of sediment‐attached communities in groundwater‐surface water mixing zones (hyporheic zone). In these studies, the assembly of planktonic communities generally tended to be more subject to stochastic effects and shifts in assembly processes related to changes in water chemistry, whereas selection had a relatively more pronounced effect on the assembly of sediment‐attached communities which, at the same time, was less affected by hydrochemical changes (Graham et al ., ; ; Stegen et al ., ; ). Compared with the hyporheic zone, pristine groundwater environments (in the absence of surface water impacts) are more stable and only experience little environmental changes (Griebler and Lueders, ), which may promote the effect of stochastic processes on community assembly compared with more dynamic environments (Ofiţeru et al ., ; Stegen et al ., ; Wang et al ., ; Zhou et al ., ).…”

“…We used 16S rRNA amplicon sequencing data to study changes in alpha and beta diversity of the sediment‐attached communities incubated at the two sites as well as differences in community composition between sediment‐attached and planktonic communities in the surrounding groundwater over the course of the succession. To explore the influence of deterministic and stochastic processes on microbial community assembly and succession, we applied the null model approach developed by Stegen and colleagues (, ), which has previously also been used in studies on community assembly in the hyporheic zone (Graham et al ., ; ; Stegen et al ., ; ) as well as biofilms in other environments (Langenheder et al ., ; Brislawn et al ., ), and thus allows us to compare our results to those previous findings.…”

Exploring the Sources of Unexpected High Methane Concentrations and Fluxes From Alpine Headwater Streams

“…Conrad (1999), however, suggests that in freshwater environments rich in OM, the ratio of acetoclastic to CO 2 reduction pathway is about 7:3. This discrepancy could be attributed to the overall low C content (FBOM: 53.4 ± 68.2 g AFDM·m -2 ; pore water DOC: 51.4 ± 36.6 μmol·L -1 ; Figure S1) in the streambed of our Alpine streams; the low amounts of bioavailable DOC and FBOM may be preferentially metabolized by heterotrophic bacteria further up in the OM degradation cascade and the C quantity might simply be too low to provide enough energy (Stegen et al, 2018) for the methanogens. Therefore, methanogenesis in streams low in OC might be subsidized by inorganic C from weathering and or soil respiration.…”

Unification of Environmental Metabolomics with Metacommunity Ecology

“…We propose 40 that applying this framework to a broad range of ecosystems will reveal generalizable principles 41 that can advance our predictive capabilities regarding NOM dynamics. 42 43 44 45 46 47 48Environmental metabolomics enables the investigation of the metabolic processes and interactions 49 occurring within an ecosystem and can provide deep insight into ongoing biogeochemical cycles 50 (Graham et al 2018;Stegen et al 2018; Sengupta et al 2019; Garayburu-Caruso et al 2020). This 51 knowledge has been collected using high-resolution mass spectrometric techniques, such as like 52Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and Orbitrap, which 53 have allowed researchers to investigate the individual carbon compounds that constitute natural 54 organic matter (NOM).…”