Northern High-Latitude Organic Soils As a Vital Source of River-Borne Dissolved Iron to the Ocean

Export fluxes of dissolved, colloidal and particulate organic carbon, major and trace elements from the Ob River and its tributaries across seasons

Riverine export fluxes of organic carbon (OC), major and trace elements remain at the forefront of environmental research in Arctic and subarctic regions, due mostly to high sensitivity of river hydrochemical parameters to climate warming and, at the same time, global importance of OC, nutrients and toxicants delivered by rivers to the Arctic Ocean. In contrast to reliable information on export fluxes of carbon and metals from the mainland to the Arctic Ocean by large and mid-size Arctic rivers, the majority of these studies quantify either dissolved (< 0.45 µm) or particulate suspended matter (PSM) fluxes, without mechanistic analyses of element speciation in most labile, low molecular weight (LMW) or colloidal form. Here we assessed colloidal versus LMW (< 3 kDa) and suspended (> 0.45 µm) export of OC, major and trace elements in a large Siberian river (Ob) and its smaller tributaries, situated within a boreal taiga / wetland zone. The main differences between Ob and its small tributaries are (i) higher dissolved OC and dissolved and particulate Fe concentrations in tributaries, due to input from surrounding bogs, (ii) much higher PSM load (clays) in the large river (Ob) compared to smaller tributaries; and (iii) strong underground water input for the Ob River, especially during baseflow, which is less pronounced for its tributaries. These major environmental factors were largely responsible for specific features of colloidal vs particulate export for the Ob River and its tributaries. Annual export fluxes normalized to the watershed area (yields) were similar for most elements in dissolved (< 0.45 µm) fraction between the Ob and the tributaries, exempting Fe, Mn, Co and Zn which were higher in tributaries essentially due to input from wetlands. Export of Mo, V, W and U was higher in the Ob River due to pronounced groundwater influence. Colloidal (3 kDa - 0.45 µm) yields were also generally higher in the tributary compared to Ob, except for those soluble elements of groundwater input (Sr, Mo, W, U). The particulate suspended (> 0.45 µm) yield of all elements, except Mn, was much higher in the Ob River when compared to its smaller tributaries. Overall, elemental fluxes of small wetland-draining tributaries of the Ob River can be considered analogous to the small rivers of the permafrost-free portion of the Arctic coast. Such small rivers are more sensitive to processes occurring at the watershed level, hence being an efficient potential sentinel for environmental changes. Fluxes in large Arctic coast rivers are more conservative and are strongly controlled by the effects of PSM interaction with river water at high flow and groundwater loading at base flow. Global significance of our findings is that elemental dissolved (< 0.45 µm) yields of small Artic rivers draining coastal wetlands and boreal forests can be adequately (within a factor of 2 to 3, comparable to inter-annul variations and uncertainties) approximated by those currently available for mid-sized and large rivers.

Challenges in using 0.45 µm filters to assess potentially bioavailable trace elements in the dissolved fraction of river and peat bog waters of the Boreal Zone

Passing through a 0.45 µm filter can significantly alter the concentrations of dissolved trace elements (TEs) in organic-rich waters of the Boreal Zone. However, little evidence has been provided about the impacts of filtration on the size distribution of dissolved TEs. This limitation hinders our comprehension of how filtration affects the assessment of the potentially bioavailable forms of dissolved TEs (i.e. ions and small molecules). Using AF4-UV-ICP-MS, this study systematically investigates such artefacts and their influence on the concentrations of dissolved, primarily ionic, and colloid-associated TEs in river waters and peat bog waters of the Boreal Zone. In river waters (circumneutral pH), membrane filtration had a significant impact on TEs that are associated with inorganic colloids such as Al, Mn, Fe, As, the rare earth elements, Pb, and Th. Approximately 20-80 % of primarily ionic TEs (< 0.5 kDa) and 100 % of large inorganic (ca. 40-300 nm) TEs were excluded from the filtrates under clogged conditions. In contrast, little impact of filtration was observed for bog waters (pH 4). Similarly, cartridge filtration of river waters also led to a decrease in concentrations and size distributions of dissolved TEs. However, cartridge filtration demonstrated a higher efficiency in allowing the passage of ions and excluding colloids than membrane filtration. On average, the ratio of the dissolved concentrations between a cartridge and a membrane filtrate was 1.0 ± 0.2. For primarily ionic species, the average ratio was 1.4 ± 0.4, while for colloidal species, it was 0.4 ± 0.1. Therefore, despite having similar dissolved concentrations, filtration methods with the same nominal pore size can yield different concentrations of bioavailable forms of TEs. These findings may be important for studies of the environmental relevance of dissolved TEs in surface waters of the Boreal Zone, and organic-rich waters elsewhere.

Terrigenous humic substances regulate the concentrations of dissolved Fe and Cu (but not Al, Mn, Ni or Zn) in the Gaoping River plume

The small mountainous rivers of Oceania discharge a large fraction of their dissolved and particulate load of materials within a very small percentage of the time. As a result, the yearly inputs and physicochemical forms of dissolved metals exported to the ocean by these rivers are poorly quantified. We investigated the wet-season distribution patterns of metals and fluorescent organic substances in the surface waters of the Gaoping River plume, SW Taiwan, under both moderate (Sep 2020) and strong flow conditions (Aug 2021). The mixing behaviour of both soluble (<5 kDa) and colloidal (>5 kDa) metals and fluorescent components was examined over the salinity range 3.0-32.2 in 2020 and 5.8-31.1 in 2021. We detected two humic-like and one protein-like fluorescent components, the same on both surveys. The humic-like components, C1 and C3, originated from the Gaoping River and correlated strongly with Cu and Fe, respectively. Component C3 showed a greater enrichment relative to C1 in the colloidal (C3/C1 > 0.8) than in the soluble phase (C3/C1 = 0.4). The protein-like component, C2, came from both terrestrial and marine sources and displayed a more complex mixing behaviour than the other two. One striking result was that the effective zero-salinity concentrations of Fe (∼300 nM) and Cu (∼23 nM) did not change significantly in response to a 10-fold increase in river discharge between Sep 2020 and Aug 2021. Similarly to Fe and Cu, the distribution patterns of Components C1 and C3 did not change significantly between the moderate and the strong plume, and C3 and C1 correlated strongly with Fe and Cu, respectively. We conclude that subtropical mountainous rivers can provide soil-derived humic substances which facilitate and regulate the delivery of Fe and Cu to the ocean, provided mountain forests are preserved.

Immobilization of Agaricus bisporus Polyphenol Oxidase 4 on mesoporous silica: Towards mimicking key enzymatic processes in peat soils

HYPOTHESIS

The use of immobilized enzyme-type biocatalysts to mimic specific processes in soil can be considered one of the most promising alternatives to overcome the difficulties behind the structural elucidation of riverine humic-derived iron-complexes. Herein, we propose that the immobilization of the functional mushroom tyrosinase, Agaricus bisporus Polyphenol Oxidase 4 (AbPPO4) on mesoporous SBA-15-type silica could contribute to the study of small aquatic humic ligands such as phenols.

EXPERIMENTS

The silica support was functionalized with amino-groups in order to investigate the impact of surface charge on the tyrosinase loading efficiency as well as on the catalytic performance of adsorbed AbPPO4. The oxidation of various phenols was catalyzed by the AbPPO4-loaded bioconjugates, yielding high levels of conversion and confirming the retention of enzyme activity after immobilization. The structures of the oxidized products were elucidated by integrating chromatographic and spectroscopic techniques. We also evaluated the stability of the immobilized enzyme over a wide range of pH values, temperatures, storage-times and sequential catalytic cycles.

FINDINGS

This is the first report where the latent AbPPO4 is confined within silica mesopores. The improved catalytic performance of the adsorbed AbPPO4 shows the potential use of these silica-based mesoporous biocatalysts for the preparation of a column-type bioreactor for in situ identification of soil samples.

Photoinduced production of substances with humic-like fluorescence, upon irradiation of water samples from alpine lakes

Evidence is here provided that irradiation of some lake water samples can trigger the formation of fluorophores with humic-like properties, at the same time increasing water absorbance. This phenomenon is the opposite of photobleaching, which is often observed when natural waters are irradiated. The photoproduced humic-like fluorophores observed here would be of autochthonous rather than allochthonous origin, which marks a difference with the fraction of humic substances that derives from terrestrial sources. Photogeneration of humic-like compounds can be highlighted in water samples where the fluorescence signal of initially occurring humic substances is low, so that their photobleaching is minimised. Samples that are most likely to show photoinduced formation of humic-like fluorophores are in fact characterised by high values of protein-like vs. humic-like contribution ratios to fluorescence, as evidenced by parallel factor (PARAFAC) analysis. Mountain lakes in late summer appear to be suitable candidates to highlight the described phenomenon. In some cases, lake-water irradiation caused a decrease in the spectral slope of the absorbance that, together with increasing absorbance values, is consistent with an increase in molecular mass and aromaticity of organic matter. The absorbance increase triggered by irradiation might play a role in screening biologically harmful UV radiation, in mountain environments that would otherwise be characterised by very clear water that allows for easy transmission of UV light along the water column.

Macronutrients, iron and humic substances summer cycling over the extended continental shelf of the South Brazil Bight

We surveyed macronutrients and dissolved iron (DFe) concentrations and speciation in a transect over the shelf of the South Brazil Bight (SBB) at Santa Marta Grande Cape (SE Brazil) during a coastal downwelling episode. Driven by dominant NE winds, coastal downwelling is a common feature during the austral summer and force after water convergence, with contribution of internal wave breaking at the shelf edge, upwelling of macronutrients into the nutrient-depleted waters of the southbound Brazil Current at ~100 km from the coastline. As a result, we found a plume of high turbidity that reached the euphotic layer, a deepening of the silicate, nitrate, and phosphate isolines over the shelf and a bulging of the nitrate and phosphate isolines over the shelf edge and the slope. Our first measurements of DFe concentration and speciation in the area revealed that against prior findings in other coastal areas, macronutrients, DFe, and iron ligand cycles were disentangled. Higher DFe concentrations were often found at the surface indicating aerial deposition. Secondary DFe maxima over the sediment-water interface and in the upwelled plume indicated DFe fluxes from the sediment and from resuspended instable colloids. Iron ligand concentrations were higher than DFe concentrations in most stations with a clear land-to-ocean gradient. Subtraction of HS iron ligands revealed that except in upwelled water, the bulk of surface ligands was the result of local biological processes. The analysis of the concentrations of Fe-HS complexes showed that the contribution of HS to DFe was dominant in upwelled waters, significant in waters close to the coast, but nearly negligible in the rest of the studied area. We hypothesize that the injection of iron-humic complexes into the euphotic layer during summer upwelling episodes is the key to understanding the persistent high chlorophyll meanders found over the shelf edge of the SBB coast.

Photodegradation Kinetics and Deep Learning-Based Intelligent Colorimetric Method for Bioavailability-Based Dissolved Iron Speciation

Via the photodegradation of dissolved iron (dFe) complexes in the euphotic zone, released free Fe(III) is the most important source of bioavailable iron for eukaryotic phytoplankton. There is an urgent need to establish bioavailability-based dissolved iron speciation (BDIS) methods. Herein, an intelligent system with dFe pretreatment and a colorimetric sensor is developed for real-time monitoring of newly generated Fe(III) ions. According to the photodegradation kinetics of dFe, including kinetic constant and photogenerated time of free Fe(III) ions, 3 sources, 6 kinds, and 12 species of dFe are determined by our photocatalytic-assisted colorimetric sensor and deep learning model within 20.0 min. The algal dFe-uptake for 4 days can be predicted by BDIS with correlation coefficient 0.85, which could be explained by the hard and soft acids and bases theory (HSAB) and density functional theory (DFT). These results successfully demonstrate the proof-of-concept for photodegradation kinetics-based speciation and bioavailability assessments of dissolved metals.

The Novel Role of Tyrosinase Enzymes in the Storage of Globally Significant Amounts of Carbon in Wetland Ecosystems

Over the last millennia, wetlands have been sequestering carbon from the atmosphere via photosynthesis at a higher rate than releasing it and, therefore, have globally accumulated 550 × 10¹⁵ g of carbon, which is equivalent to 73% of the atmospheric carbon pool. The accumulation of organic carbon in wetlands is effectuated by phenolic compounds, which suppress the degradation of soil organic matter by inhibiting the activity of organic-matter-degrading enzymes. The enzymatic removal of phenolic compounds by bacterial tyrosinases has historically been blocked by anoxic conditions in wetland soils, resulting from waterlogging. Bacterial tyrosinases are a subgroup of oxidoreductases that oxidatively remove phenolic compounds, coupled to the reduction of molecular oxygen to water. The biochemical properties of bacterial tyrosinases have been investigated thoroughly in vitro within recent decades, while investigations focused on carbon fluxes in wetlands on a macroscopic level have remained a thriving yet separated research area so far. In the wake of climate change, however, anoxic conditions in wetland soils are threatened by reduced rainfall and prolonged summer drought. This potentially allows tyrosinase enzymes to reduce the concentration of phenolic compounds, which in turn will increase the release of stored carbon back into the atmosphere. To offer compelling evidence for the novel concept that bacterial tyrosinases are among the key enzymes influencing carbon cycling in wetland ecosystems first, bacterial organisms indigenous to wetland ecosystems that harbor a TYR gene within their respective genome (tyr⁺) have been identified, which revealed a phylogenetically diverse community of tyr⁺ bacteria indigenous to wetlands based on genomic sequencing data. Bacterial TYR host organisms covering seven phyla (Acidobacteria, Actinobacteria, Bacteroidetes, Firmicutes, Nitrospirae, Planctomycetes, and Proteobacteria) have been identified within various wetland ecosystems (peatlands, marshes, mangrove forests, bogs, and alkaline soda lakes) which cover a climatic continuum ranging from high arctic to tropic ecosystems. Second, it is demonstrated that (in vitro) bacterial TYR activity is commonly observed at pH values characteristic for wetland ecosystems (ranging from pH 3.5 in peatlands and freshwater swamps to pH 9.0 in soda lakes and freshwater marshes) and toward phenolic compounds naturally present within wetland environments (p-coumaric acid, gallic acid, protocatechuic acid, p-hydroxybenzoic acid, caffeic acid, catechin, and epicatechin). Third, analyzing the available data confirmed that bacterial host organisms tend to exhibit in vitro growth optima at pH values similar to their respective wetland habitats. Based on these findings, it is concluded that, following increased aeration of previously anoxic wetland soils due to climate change, TYRs are among the enzymes capable of reducing the concentration of phenolic compounds present within wetland ecosystems, which will potentially destabilize vast amounts of carbon stored in these ecosystems. Finally, promising approaches to mitigate the detrimental effects of increased TYR activity in wetland ecosystems and the requirement of future investigations of the abundance and activity of TYRs in an environmental setting are presented.

Spectroscopic Characteristics and Speciation Distribution of Fe(III) Binding to Molecular Weight-Dependent Standard Pahokee Peat Fulvic Acid

Peat-derived organic matter, as powerful chelators, is of great significance for the transport of Fe to the ocean and the enhancement of dissolved Fe. However, the iron binding capacity of molecular weight (MW)-fractionated dissolved organic matter is variable, due to its structure and composition heterogeneity. In this work, we used the standard Pahokee Peat fulvic acid (PPFA) as an example, and investigated the spectroscopy properties and Fe(III) binding ability of PPFA and different molecular weight fractions by UV−Vis absorbance and fluorescence spectroscopy and the Donnan Membrane Technique (DMT). The results showed binding sites for Fe(III) at the 263 nm and >320 nm regions in differential absorbance spectra. Upon increasing the iron concentration to 18.00 μmol·L−1, the critical binding capacity was exceeded, which resulted in a decrease in absorbance. Fe(III) was found to prefer to bind to humic-like components, and ultraviolet humic-like fluorophores displayed stronger binding strength. High molecular weight PPFA fractions (>10 kDa) possessed more aromatic and hydrophobic components, displayed a higher degree of humification, and exhibited higher metal binding potential. Furthermore, the speciation analysis and stability constant (cK) were calculated using Donnan membrane equilibrium. The correlation between cK values and PPFA spectral properties demonstrated that aromaticity, hydrophobicity, molecular weight and humification degree were crucial indices of PPFA−Fe(III) affinity. Significantly, the humification degree, represented by HIX, showed the strongest correlation (r = 0.929, p = 0.003), which could be used to estimate the binding strength. This study provides further understanding of the complexation mechanism of iron and DOM in the peat environment and identifies the considerable effect of molecular weight.

Ocean Aerobiology

Ocean aerobiology is defined here as the study of biological particles of marine origin, including living organisms, present in the atmosphere and their role in ecological, biogeochemical, and climate processes. Hundreds of trillions of microorganisms are exchanged between ocean and atmosphere daily. Within a few days, tropospheric transport potentially disperses microorganisms over continents and between oceans. There is a need to better identify and quantify marine aerobiota, characterize the time spans and distances of marine microorganisms’ atmospheric transport, and determine whether microorganisms acclimate to atmospheric conditions and remain viable, or even grow. Exploring the atmosphere as a microbial habitat is fundamental for understanding the consequences of dispersal and will expand our knowledge of biodiversity, biogeography, and ecosystem connectivity across different marine environments. Marine organic matter is chemically transformed in the atmosphere, including remineralization back to CO₂. The magnitude of these transformations is insignificant in the context of the annual marine carbon cycle, but may be a significant sink for marine recalcitrant organic matter over long (∼10⁴ years) timescales. In addition, organic matter in sea spray aerosol plays a significant role in the Earth’s radiative budget by scattering solar radiation, and indirectly by affecting cloud properties. Marine organic matter is generally a poor source of cloud condensation nuclei (CCN), but a significant source of ice nucleating particles (INPs), affecting the formation of mixed-phase and ice clouds. This review will show that marine biogenic aerosol plays an impactful, but poorly constrained, role in marine ecosystems, biogeochemical processes, and the Earth’s climate system. Further work is needed to characterize the connectivity and feedbacks between the atmosphere and ocean ecosystems in order to integrate this complexity into Earth System models, facilitating future climate and biogeochemical predictions.