Field validation of DNA-based biosensor for rapid detection of ultra-trace mercury(II) in natural waters

Mercury (Hg) remains a significant pollutant of global concern. In particular, contamination of surface water and groundwater by Hg poses severe threats, unrecognized in many cases, to the drinking water safety of numerous, often economically challenged, communities in the world. While the speciation of aqueous Hg varies depending upon the environmental factors, inorganic Hg(II) represents a primary regulator of its fate and bioavailability in natural waters. In this regard, both the public sectors and private families are soliciting water quality sensors able to detect aqueous Hg(II) down to the ultra-trace level (below the drinking water limit) sensitively, reliably and quickly. Here, we present a summary of the development of DNA-based biosensors for Hg(II) that incorporates recent advance in the field deployment of two forms of the DNA functionalized biosensing tools. The first form is the DNAfunctionalized hydrogel sensor that can be readily applied through direct immersion in solution and water. The second setup is the DNA-DGT sensor that integrates the DNA-functionalized hydrogel with the diffusive gradients in thin films (DGT) technique to unlock more versatile applications in water, soils and aquatic sediments. These two types of Hg(II) sensors were tested with hydrochemically diverse ground and surface waters from the Datong Basin, northern China and the Great Lakes region, North America. The results indicate that the DNA-functionalized hydrogel sensor was able to measure total dissolved Hg(II) quickly (within few hours), yet inapplicable to Hg(II) concentrations below 10 nM especially in the presence of interfering components (e.g., Cl- and natural dissolved organic matter). In contrast, the DNA-DGT sensor could detect variably ultra-trace Hg(II) (even <1 nM) depending upon the deployment time. In combination with equilibrium species calculations, the DNA-DGT sensor shows the capacity to differentiate the partitioning of Hg(II) between various aqueous species and to calibrate the interferences by water temperature and natural dissolved organic matter. It reveals that the bioavailability of Hg(II), even at the ultra-trace levels, to the water organisms varies significantly depending upon the environmental conditions. Furthermore, the sensor measurements together with results of hydrochemical analyses suggest that the transformation of Hg(II) is linked to the biogeochemical cycling of sulfur in the groundwaters. Overall, our DNA-based sensors represent ultrasensitive, field-deployable detection methods that can unravel the mobility of Hg(II) in natural waters and early warning of Hg pollution to the drinking water.This research was undertaken thanks, in part, with support from the Global Water Futures Program funded by the Canada First Research Excellence Fund (CFREF). Additional funding funding was also provided by the NSERC SPG biosensor project awarded to Juewen Liu., the GWF Winter Soil Processes in Transition project awarded to Fereidoun Rezanezhad., and the CERC program awarded to Philippe Van Cappellen

Effects of riboflavin and desferrioxamine B on Fe(II) oxidation by O2

Flavins and siderophores secreted by various plants, fungi and bacteria under iron (Fe) deficient conditions play important roles in the biogeochemical cycling of Fe in the environment. Although the mechanisms of flavin and siderophore mediated Fe(III) reduction and dissolution under anoxic conditions have been widely studied, the influence of these compounds on Fe(II) oxidation under oxic conditions is still unclear. In this study, we investigated the kinetics of aqueous Fe(II) (17.8 μM) oxidation by O2 at pH 5‒7 in the presence of riboflavin (oxidized (RBF) and reduced (RBFH2)) and desferrioxamine B (DFOB) as representative flavins and siderophores, respectively. Results showed that the addition of RBF/RBFH2 or DFOB markedly accelerates the oxidation of aqueous Fe(II) by O2. For instance, at pH 6, the rate of Fe(II) oxidation was enhanced 20‒70 times when 10 μM RBFH2 was added. The mechanisms responsible for the accelerated Fe(II) oxidation are related to the redox reactivity and complexation ability of RBFH2, RBF and DFOB. While RBFH2 does not readily complex Fe(II)/Fe(III), it can activate O2 and generate reactive oxygen species, which then rapidly oxidize Fe(II). In contrast, both RBF and DFOB do not reduce O2 but react with Fe(II) to form RBF/DFOB-complexed Fe(II), which in turn accelerates Fe(II) oxidation. Furthermore, the lower standard reduction potential of the Fe(II)-DFOB complex, compared to the Fe(II)-RBF complex, correlates with a higher oxidation rate constant for the Fe(II)-DFOB complex. Our study reveals an overlooked catalytic role of flavins and siderophores that may contribute to Fe(II)/Fe(III) cycling at oxic-anoxic interfaces

The Cold Region Critical Zone in Transition: Responses to Climate Warming and Land Use Change

Global climate warming disproportionately affects high-latitude and mountainous terrestrial ecosystems. Warming is accompanied by permafrost thaw, shorter winters, earlier snowmelt, more intense soil freeze-thaw cycles, drier summers, and longer fire seasons. These environmental changes in turn impact surface water and groundwater flow regimes, water quality, greenhouse gas emissions, soil stability, vegetation cover, and soil (micro)biological communities. Warming also facilitates agricultural expansion, urban growth, and natural resource development, adding growing anthropogenic pressures to cold regions' landscapes, soil health, and biodiversity. Further advances in the predictive understanding of how cold regions' critical zone processes, functions, and ecosystem services will continue to respond to climate warming and land use changes require multiscale monitoring technologies coupled with integrated observational and modeling tools. We highlight some of the major challenges, knowledge gaps, and opportunities in cold region critical zone research, with an emphasis on subsurface processes and responses in both natural and agricultural ecosystems