Microbial uptake kinetics of dissolved organic carbon (DOC) compound groups from river water and sediments

Abstract

Dissolved organic matter (DOM) represents a key component of carbon (C) cycling in freshwater ecosystems. While the behaviour of bulk dissolved organic carbon (DOC) in aquatic ecosystems is well studied, comparatively little is known about the turnover of specific DOC compounds. The aim of this study was to investigate the persistence of 14 C-labelled low molecular weight (LMW) DOC at a wide range of concentrations (0.1 µM to 10 mM), in sediments and waters from oligotrophic and mesotrophic rivers within the same catchment. Overall, rates of DOC loss varied between compound groups (amino acids > sugars = organic acids > phenolics). Sediment-based microbial communities contributed to higher DOC loss from river waters, which was attributed, in part, to its greater microbial biomass. At higher DOC compound concentrations, DOC loss was greater in mesotrophic rivers in comparison to oligotrophic headwaters. A lag-phase in substrate use within sediments provided evidence of microbial growth and adaptation, ascribed here to the lack of inorganic nutrient limitation on microbial C processing in mesotrophic communities. We conclude that the higher microbial biomass and available inorganic nutrients in sediments enables the rapid processing of LMW DOC, particularly during high C enrichment events and in N and P-rich mesotrophic environments.

Warming and ocean acidification may decrease estuarine dissolved organic carbon export to the ocean

“…This release of DOC in the dark was attributed to enhanced microbial coupling in the sediments under warmer temperatures (Maher and Eyre, 2010), yet here, and in previous reports, DOC uptake suggests that bacteria not only intercepted DOC produced from within the pore waters (potentially satisfying up to 60 % of total mean bacterial production, Boto et al, 1989), but also took up available DOC from the water column to satisfy its metabolic requirements (Boto et al, 1989;Brailsford et al, 2019), effectively acting as a DOC sink.…”

“…As such, the efflux of DOC in the dark at Δ-3 °C suggests that heterotrophic bacterial productivity, and therefore DOC uptake, was reduced by lowered temperatures (Raymond and Bauer, 2000), resulting in a failure to intercept all DOC produced in the pore waters. This failure to intercept DOC may be compounded if nutrient supply is limited (Brailsford et al, 2019), as it is common for heterotrophic bacteria to rely on refractory DOC under such conditions (Chróst, 2017).…”

Nutrient enrichment induces a shift in dissolved organic carbon (DOC) metabolism in oligotrophic freshwater sediments

“…5). The concentration of glucose added was such that glucose would be available in excess to the microbial population of the sediment without fully saturating the system, based on previously observed glucose uptake in sediments from the same upland peat sites(Brailsford et al 2019). The amount of C added was approximately 4 orders of magnitude higher than the baseline concentrations of C present as total free carbohydrates and 5 orders of magnitude higher than concentration in overlying river waters (0.61 ± 0.08 mg C kg wet sediment -1 and 0.09 ± 0.02 mg C L-1 respectively; Brailsford et al 2019).Cumulative 14 CO 2 respiration over the duration of the experiment for the upland river sediments was an order of magnitude lower than rates previously observed for lowland agricultural soils(Hill et al 2008;Rousk et al 2014).…”

Complex interactions of in-stream dissolved organic matter and nutrient spiralling unravelled by Bayesian regression analysis

“…The uptake was calculated using a Bayesian non-linear model and solved with a Markov chain Monte Carlo (MCMC) algorithm as provided in the R package brms (Bürkner, 2017) relying on stan (Carpenter et al, 2017). We used the Bayes factor (BF; Goodman, 1999a, b) for hypothesis testing and model comparisons.…”