Organic matter transformations are disconnected between surface water and the hyporheic zone

Abstract

Abstract. Biochemical transformations of organic matter (OM) are a primary driver of river corridor biogeochemistry, thereby modulating ecosystem processes at local to global scales. OM transformations are driven by diverse biotic and abiotic processes, but we lack knowledge of how the diversity of those processes varies across river corridors and across surface and subsurface components of river corridors. To fill this gap we quantified the number of putative biotic and abiotic transformations of organic molecules across diverse river corridors using ultra-high-resolution mass spectrometry. The number of unique transformations is used here as a proxy for the diversity of biochemical processes underlying observed profiles of organic molecules. For this, we use public data spanning the contiguous United States (ConUS) from the Worldwide Hydrobiogeochemical Observation Network for Dynamic River Systems (WHONDRS) consortium. Our results show that surface water OM had more biotic and abiotic transformations than OM from shallow hyporheic zone sediments (1–3 cm depth). We observed substantially more biotic than abiotic transformations, and the numbers of biotic and abiotic transformations were highly correlated with each other. We found no relationship between the number of transformations in surface water and sediments and no meaningful relationships with latitude, longitude, or climate. We also found that the composition of transformations in sediments was not linked with transformation composition in adjacent surface waters. We infer that OM transformations represented in surface water are an integrated signal of diverse processes occurring throughout the upstream catchment. In contrast, OM transformations in sediments likely reflect a narrower range of processes within the sampled volume. This indicates decoupling between the processes influencing surface water and sediment OM, despite the potential for hydrologic exchange to homogenize OM. We infer that the processes influencing OM transformations and the scales at which they operate diverge between surface water and sediments.

Determining the biogeochemical transformations of organic matter composition in rivers using molecular signatures

“…Overall our results offer a glimpse into biochemical assemblage of environmental metabolomes and clearly demonstrate a need to further assess the entire WHONDRS environmental metabolomes (e.g., using transformation dendrograms) for a complete biogeochemical picture. Despite these limitations, the MCD assemblage and PBT composition results presented here are consistent with previous research regarding surface water and sediment biotic and abiotic transformations (Stegen et al, 2022). Regardless of stream order, seasonal variation likely has a predominant effect on the environmental metabolome assemblage (Guarch-Ribot and Butturini, 2016;Coble et al, 2019), evidenced by similar assemblage patterns across low and high stream orders.…”

“…Further analysis of potential ecological drivers was evaluated using meta-metabolome ecology methods as described by Danczak et al (2020) and Stegen et al (2022). Briefly, to quantify deterministic versus stochastic processes responsible for driving putative biochemistry across stream orders and sample type, we applied mean nearest taxon distance (MNTD; ses.mntd; R "picante" package v. 1.8.2; Kembel et al, 2010) and nearest taxon index for sample beta diversity (βNTI; comdistn) to a subset of the FT-ICR-MS dataset (Stegen et al, 2012(Stegen et al, , 2013Danczak et al, 2020), specifically analyzing the elemental groups categorized as "protein" and "protein and lipid" with the identified molecular formula and elemental atoms present per metabolite, as identified through molecular formula assignment described in Danczak et al (2020).…”

“…Biogeochemical transformations of DOM are shaped by its reactivity and transport, which are controlled by biotic and abiotic processes in localized habitats and along a watershed (Stegen et al, 2022). During riverine transport, aromatic DOM compounds are especially susceptible to photochemical modification and degradation, producing carbon dioxide and new inorganic and organic compounds (Stubbins et al, 2010).…”

Quantifying Stochastic Processes in Shaping Dissolved Organic Matter Pool with High-Resolution Mass Spectrometry

“…61,5,62 On this basis, Danczak et al integrated an ecological null model to unravel the spatiotemporal organization of DOM molecules, which shares common themes with the neutral model presented here. 15,18,17 However, the null model employs random permutations of HRMS data to generate statistical expectations, thereby inferring the underlying mechanisms of molecular assembly by comparing the observed patterns with statistical expectations. 63 The neutral model, on the other hand, serves as a mechanistic framework that incorporates the fundamental dynamics of molecular production, degradation, and dispersal.…”

“…16 When deterministic processes are spatiotemporally inconsistent or insufficient to overcome random effects like hydrologic mixing, stochasticity prevails in DOM molecular dynamics. 17,18 Considering the inherent stochastic nature of molecular dynamics, it is necessary to incorporate stochasticity into the mechanistic comprehension of the spatiotemporal variability of organic matter. 19,20 The interplay between deterministic and stochastic factors in shaping DOM molecular assemblages bears resemblance to ecological processes in the microbial community assembly.…”