The molecular diversity of dissolved organic matter in forest streams across central Canadian boreal watersheds.

Small headwater streams can mobilize large amounts of terrestrially derived dissolved organic matter (DOM). While the molecular composition of DOM has important controls on biogeochemical cycles and carbon cycling, how stationary landscape metrics affect DOM composition is poorly understood, particularly in relation to non-stationary effects from hydrological changes across seasons. Here, we apply a combination of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and absorbance spectroscopy to characterize stream DOM from 13 diverse watersheds across the central Canadian boreal forests and statistically relate DOM compositional characteristics to landscape topography and hydrological metrics. We found that watershed runoff across different surface physiographies produced DOM with distinctly different chemical compositions related to runoff pH. Specifically, streams in sandy soil watersheds contained more abundant aromatic, nitrogenated and sulfurized fractions of DOM, likely due to a combination of lower soil capacity to absorb DOM than other soil types and high conifer forest coverage that generated acidic litterfall in more sandy watersheds. In contrast, streams with more neutral pH in watersheds with shallow soils had DOM resembling low oxidized phenolic molecules mainly due to increased brush/alder and deciduous vegetation coverage in relatively steeper watersheds. However, as precipitation and flows increased in the fall, the overall water chemistry of streams became more similar as runoff pH increased, the overall chemical diversity of DOM in streams decreased, and stream DOM resembled fresher, lower molecular weight lignin material likely originating from freshly produced leaf litter. Together, our findings show that during hydrologically disconnected periods, pH and landscape characteristics have important controls on the mobilization of aromatic DOM but that many landscape-specific characteristics in the Canadian boreal forest are less influential on DOM processing during wetter conditions where chemically similar, plant-derived DOM signatures are preferentially mobilized. These findings collectively help predict the composition of DOM across diverse watersheds in the Canadian boreal to inform microbial and contaminant biogeochemical processes in downstream ecosystems.

An untargeted metabolomic approach for the putative characterization of metabolites from Scenedesmus obliquus in response to cadmium stress.

Cadmium (Cd) is a widespread contaminant in aquatic systems and has a variety of toxicological implications on freshwater microorganisms. In this study, the green algae Scenedesmus obliquus was exposed to increasing Cd concentrations that inhibited growth by 20% (12.6 µmol L-1), 30% (39.8 µmol L-1) and 40% (83.2 µmol L-1) and the metabolite profiles of released and cellular biomolecules were explored using an untargeted direct infusion high resolution Fourier transform ion cyclotron resonance mass spectrometry approach. In Cd untreated cultures, intrinsic differences in composition existed between released biomolecules and freeze-dried cells. Based on putatively characterized compound groups, a greater proportion of Cys-GSH isomers and carboxyamides were present in exudates whereas sugar isomers and phosphonic acids comprised most cellular metabolites. In cultures exposed to 83.2 µmol L-1 Cd, an overall shift in metabolomic response across both released biomolecules and cellular components resulted in an increase of lipid-based esters, and Cys-GSH isomers. These two important metabolites are used in antioxidant defense mechanisms and reactive oxygen species prevention during cellular stress. The diversity of metabolites also decreased as Cd concentrations increased when compared to untreated cultures, suggesting that overall metabolites specialize upon metal stress. We show systemic shifts from sugar and carboxylic isomers to specialized proteins and lipid isomers to help S. obliquus cope with stress. These findings highlight the potential use of this green algae as a potential biosorbent and sheds light into the metabolomics of Cd toxicology and insights into microbial metal adaptation.

Linking molecular and optical properties of dissolved organic matter across a soil-water interface on Akimiski Island (Nunavut, Canada).

Dissolved organic matter (DOM) plays a crucial role in terrestrial and aquatic carbon and biogeochemical cycles; however, molecular transformations between aquatic and terrestrial systems remain poorly understood due to the complexity and heterogeneity of DOM. In this study, we investigated the molecular diversity of aquatic DOM and adjacent soil derived water extractable organic matter (WEOM) from seven locations on Akimiski Island, Nunavut using a combination of absorbance spectroscopy and Fourier transform ion cyclotron mass spectrometry (FT-ICR-MS). Assigned elemental formula and Van Krevelen compositional analysis reveal compositional similarities in river, inland ponds and coastal pool sites for aquatic DOM and WEOM. More aromatic, oxygenated polyphenolic carbon rich molecules were found in aquatic DOM whereas WEOM was abundant in highly unsaturated aliphatic material. A total of 276 phenolic, unsaturated aliphatic, and vascular plant-derived polyphenolic molecules were identified as being conserved between WEOM and aquatic DOM at one river and two inland pond locations suggesting similar CHO sources from adjacent soils. Moreover, contributions of polyphenolic compounds in aquatic DOM and WEOM were greater at inland ponds than coastal pools, congruent with a greater aromaticity at inland sites. Our results highlight the similarities and differences in WEOM to aquatic DOM composition and how they range across surrounding watersheds that provide insight into the biogeochemical dynamics across a Canadian subarctic terrestrial-aquatic-continuum.

Assessing cadmium and vanadium accumulation using diffusive gradient in thin-films (DGT) and phytoplankton in the Churchill River estuary, Manitoba.

Diffusive gradient in thin films (DGT) and phytoplankton communities were evaluated for the measurement of Cd and V at environmentally relevant concentrations in laboratory settings and in the Churchill River estuary (Manitoba, Canada) during an annual spring melt. Despite rapid changes in hydrology and water quality, DGT samplers and intracellular Cd and V concentrations were positively correlated (0.79 < r(2) < 0.99), suggesting comparable accumulation trends between both DGT-labile and intracellular monitoring techniques. The largest accumulated concentrations of both Cd and V by DGT and phytoplankton accumulation methods were found later into the river discharge period. In controlled settings, accumulated Cd and V concentrations by the diatom Attheya septentrionalis displayed a strong correlation with metals accumulated by DGTs (r(2) > 0.99). Principal component analysis (PCA) reinforced similarities between both metal monitoring techniques and assessed how changing environmental variables during the river discharge period influenced each monitoring technique. Cd accumulation was influenced by DOC concentrations and protein-like DOM whereas ionic strength (i.e. conductivity) and humic-like DOM influenced V accumulation. The present findings suggest that (1) DGT is a versatile tool for monitoring bioaccumulation of Cd and V in highly dynamic environmental systems and (2) DOC concentration, DOM composition, conductivity, pH, and river discharge influence the bioavailability of Cd and V in estuarine and riverine waters.