Carboxyl-rich Alicyclic Molecules Research Articles

Tidal Effects on Dissolved Organic Matter Dynamics in a Brackish Water Front Adjacent to Yangtze River Estuary

A brackish water front, where river water meets seawater, is a hotspot for biogeochemical processes. In this study, we examined the quantity and composition of dissolved organic matter (DOM) over a 24 h tidal cycle at a brackish water front near the Yangtze River estuary. Utilizing elemental analysis, fluorescence and ultraviolet spectroscopy, and ultra-high-resolution mass spectrometry, we observed rapid fluctuations in DOM throughout the tidal cycle. The dissolved organic carbon (DOC) and total nitrogen (TN) concentrations ranged from 0.70 to 1.5 mg/L and 0.43 to 0.94 mg/L, respectively. Water samples during low tide exhibited a higher fractional abundance of CHON (17.2 ± 0.1% vs. 14.6 ± 0.4%), CHOS (14.6 ± 4.5% vs. 9.1 ± 3.1%), and CHONS (1.6 ± 0.5% vs. 0.5 ± 0.3%) formulas, and a higher aromatization and average molecular weight, which is consistent with a stronger terrestrial influence. In contrast, at high tide, the water samples contained a greater abundance of CHO compounds (75.7 ± 3.8% vs. 66.5 ± 4.1%), a humic-like fluorescent C1 component, and carboxyl-rich alicyclic molecules (CRAMs), indicating a greater release of refractory DOM from resuspended sediments. However, variations in the DOC concentrations and several optical spectral parameters did not correlate with the changes in the salinity and tidal height. The results of the principal component analysis revealed different controls on specific fractions of DOM that are related to variable DOM sources or biogeochemical processes. The complexity of DOM dynamics underscores the necessity of elucidating DOM compositions at varying levels to enhance our understanding of carbon cycling in estuarine and coastal ecosystems.

Network-Based Methods for Deciphering the Oxidizability of Complex Leachate DOM with •OH/O3 via Molecular Signatures.

In landfill leachates containing complex dissolved organic matter (DOM), the link between individual DOM constituents and their inherent oxidizability is unclear. Here, we resolved the molecular signatures of DOM oxidized by •OH/O3 using FT-ICR MS, thereby elucidating their oxidizability and resistance in concentrated leachates. The comprehensive gradual fragmentation of complex leachate DOM was then revealed through a modified machine-learning framework based on 43 key pathways during ozonation. Specifically, humic substances like humic acid (HA) and fulvic acid (FA) were measured to be the dominant DOM fractions in concentrated leachates, accounting for 35.9-51.7% of the total organic carbon, which was consistent with the observation by three-dimensional fluorescence spectroscopy. According to FT-ICR MS, carboxyl-rich alicyclic molecules (CRAMs) or lignin-like substances were the most abundant components, comprising 40.2-54.5% of all substances. The machine learning modeling showed that molecular weight was the most important structural factor for DOM resistance to •OH and O3 degradation (SHAP value 0.84), followed by (DBE-O)/C (0.32), S/C (0.31), and H/C (0.08). During •OH and O3 attacking, unsaturated and reduced compounds were the dominant precursors. For the molecular transformation of CRAMs-DOM, oxygen addition reactions were found to be the predominant O3-attacking process, along with the dealkyl and carboxylic acid reactions during •OH oxidation that often resulted in more complete degradation of DOM. This study proposed a new framework integrating molecular signatures and machine learning for unraveling DOM's inherent reactivity in complexity, which informs strategies for managing concentrated leachates.

Evaluating the role of recalcitrant dissolved organic matter in bacterial community dynamics in urbanized freshwater ecosystems

Dissolved organic matter (DOM) and recalcitrant dissolved organic matter (RDOM) play distinct roles in shaping microbial communities. However, characterizing these roles is difficult, especially in ecosystems subjected to varying degrees of anthropogenic influence. This study investigated the molecular compositions and ecological impacts of DOM and RDOM in the Fen River, Shanxi Taiyuan, comparing pristine upstream regions with highly urbanized downstream areas. Using 16S rRNA gene sequencing and LC-MS-based metabolomics, we observed significant shifts in microbial community composition, diversity, and metabolic functions. Upstream communities, characterized by higher diversity, were dominated by Bacteroidota, Proteobacteria, and Cyanobacteria, while downstream communities, influenced by pollution, exhibited increased expression of genes related to amino acid metabolism. Fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that upstream DOM contained higher proportions of complex, high molecular weight compounds, including significant proportions of carboxyl-rich alicyclic molecules (CRAM) and island of stability (IOS) compounds, which play key roles in long-term carbon storage and microbial carbon sequestration. In contrast, downstream DOM was characterized as having lower aromaticity and more saturated compounds, with reduced proportions of CRAM and IOS, reflecting the impact of anthropogenic activities. These findings underscored the critical roles of CRAM and IOS in regulating DOM stability and microbial communities, further highlighting the need for targeted pollution control strategies to preserve ecosystem function in urbanized water bodies.

Molecular fingerprints of sedimentary dissolved organic matter in mangroves: Importance to blue carbon sequestration

Mangroves are important organic carbon (OC) reservoirs that can mitigate climate changes. Although sedimentary dissolved organic matter (SDOM) has been established as a dominant OC component within these ecosystems, its molecular composition, origin, and fate remain largely unknown. This study examined SDOM molecules from one of the largest peri-urban mangroves and adjacent tidal flats in China by using techniques like absorption fluorescence spectroscopy and ultrahigh-resolution mass spectrometry. We found that SDOM in mangroves differs significantly from that in non-mangrove ecosystems. The presence of mangroves created higher levels of N- and S-containing SDOM molecules with depth and exhibited higher lability. However, the suboxic mangrove sediments may limit microbial activity, promoting labile SDOM preservation. Mangrove litter also contributed to the formation of carboxyl-rich alicyclic molecules in sediments, potentially transforming into biologically refractory SDOM that acts as OC sinks. As the understanding of molecular fingerprints of SDOM in mangroves is still at early stages, these findings provide molecular-level evidence that mangrove sediments promote the OC sequestration through preservation and transformation processes. This study offers crucial insights into the role of SDOM in carbon sequestration and the method could be applied into other blue carbon ecosystems.

Molecular characteristics of dissolved organic matter in the porewater of "Challenger" Deep sediments, Mariana Trench

Hadal trenches (> 6,000 m water depth) have been revealed as hotspots of organic carbon deposition and mineralization. Here, we present the molecular compositions of porewater dissolved organic matter (DOM) at the “Challenger” Deep (Site MT02; 10,954 m water depth) sediments of the Mariana Trench and the adjacent abyssal plain sediments (Site MT04; 5,800 m water depth) using ultra high-resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). The "Challenger" Deep sediments are characterized by higher abundance of recalcitrant compounds, mainly composed of highly unsaturated compounds (79.7%) and carboxyl-rich alicyclic molecules (57.1%), compared to the abyssal plain sediments (68.3%&51.5%). Principal component analysis suggests that TOC content in the sediments exerts an important control on the molecular characteristics of porewater DOM. It is likely that higher TOC content triggers elevated microbial-mediated organic matter mineralization, thereby forming more refractory organic matter compounds. These results improved the knowledge of the poorly-understood DOM compositions and microbial organic matter degradation in the ultra-deep, extreme ocean environment.

Characterisation of dissolved organic matter in two contrasting arsenic-prone sites in Kandal Province, Cambodia

Aquifers throughout Asia are impacted by the release of geogenic arsenic (As) into groundwater by microbial reduction of As-bearing Fe(III) (oxy)hydroxide minerals, severely impacting water quality. Groundwater dissolved organic matter (DOM) is likely key to As release, mainly as electron donor or electron shuttles. This study used optical analyses and ultra-high resolution mass spectrometry to examine the sources and composition of groundwater DOM in the As-prone aquifers of Kandal Province, Cambodia, at boreholes with differing host lithology (clay- and sand-dominated). Groundwater and surface water DOM composition were related to As concentrations, to infer the potential role of DOM in promoting As release. Optical and molecular-level analyses indicated an overall dominance of terrestrial-derived DOM in the groundwater samples, with higher freshness index and relative abundance (RA) of aliphatic compounds in clay compared to sand-dominated lithology. Compared to surface water, groundwater DOM had relatively lower O/C ratios and nominal oxidation state of carbon (−0.19 to −0.13 compared to 0.04 for ground and surface water, respectively), with a lower %RA of aliphatic compounds and higher %RA of carboxyl-rich alicyclic molecules, suggesting microbial processing of DOM since percolation into the aquifer. Concentrations of As across both sites were negatively correlated with DOM tryptophan:fulvic-like fluorescence and the %RA of aliphatics, potentially indicating microbial degradation of biolabile DOM in connection with As release, which is consistent with its role as an electron donor source. Together these data support DOM composition as an important control on microbial mediated As release.

Molecular insights into the degradation of organic matter from secondary swine wastewater effluent: A comparative study of advanced oxidation processes

Advanced oxidation processes (AOPs) are promising for the treatment of secondary swine wastewater effluents, however, the molecular-level understanding of effluent organic matter (EfOM) removal and transformation during AOPs is limited. This study employed molecular-level characterization based on Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and bulk characterizations to investigate these processes in various AOPs, including Cu-Fenton, UV-Cu-Fenton, Fenton, UV-Fenton, and UV/H2O2 treatments. Our findings revealed that the removal rates of dissolved organic carbon and EfOM molecules follow the sequence of UV-Fenton > Fenton > UV-Cu-Fenton > UV/H2O2 > Cu-Fenton, correlating with the rates of H2O2 decomposition during reactions. AOPs with faster H2O2 decomposition, indicative of higher reactive oxygen species generation, predominantly mineralize rather than transform EfOM. Linkage analysis highlighted oxygen addition and deamination as the primary transformation reactions, with variations in the dominance levels of these reactions across different AOPs. Recalcitrant molecule, particularly CHNO and CHO types, including low-molecular-weight carboxyl-rich alicyclic molecules, pose challenges in treatment. To enhance the efficacy of secondary effluent treatment, strategies focusing on the targeted removal of such recalcitrant EfOM should be developed. This study provided new insights into the selection and optimization of AOPs for secondary swine wastewater effluent treatment.

Long-Term Photochemical and Microbial Alterations Lead to the Compositional Convergence of Algal and Terrestrial Dissolved Organic Matter.

Lakes are expected to become more active in processing dissolved organic matter (DOM), but the fate of DOM with different origins remains poorly constrained. We conducted long-term incubation experiments (∼1 year) with sole light, sole microbial, and combined light and microbial treatments using DOM from algal and terrestrial sources (DOMa and DOMt, respectively). Fourier transform ion cyclotron resonance mass spectrometry and 16s rRNA were used to analyze the DOM molecular composition and bacterial community, respectively. We observed that DOMa and DOMt converged toward a similar composition under the combined light and microbial treatment, driven by the removal of source-specific compositions along with the production of refractory, carboxylic-rich alicyclic molecules (CRAM). For CRAM enrichment, microbial processes played a greater role for DOMa, while phototransformation was more important for DOMt. The co-occurrence patterns between DOM molecules and bacteria showed that DOM molecular composition influenced the bacterial community. More complex DOM-bacteria interactions were observed for DOMt compared to DOMa, suggesting that greater bacterial cooperation was necessary for transforming DOMt. Collectively, these findings offer new insights into the mechanisms underlying the uniformity of DOM from various sources through prolonged environmental transformations in lakes.

Cycling of labile and recalcitrant carboxyl-rich alicyclic molecules and carbohydrates in Baffin Bay

Marine dissolved organic matter (DOM) is an important, actively cycling carbon reservoir (662 GtC). However, the chemical structure and cycling of DOM within rapidly warming, polar environments remains largely unconstrained. Previous studies have shown rapid surface cycling of carbohydrates as biologically-labile DOM (LDOM). Conversely, carboxyl-rich alicyclic molecules (CRAM) are often used as examples of biologically-recalcitrant DOM (RDOM). Traditional DOM isolation methods (e.g., ultrafiltration (10–30% of DOM) and solid-phase extraction (40–60% of DOM) induce chemical-, size- and compositional-bias – complicating inferences to total DOM cycling. Here, we use a total DOM proton (1H) nuclear magnetic resonance (NMR) spectroscopy method to show carbohydrates and CRAM have high concentrations in the surface ocean and low concentrations at depth in Baffin Bay. Between 21–43% of surface CRAM is removed at depth. These results suggest both CRAM and carbohydrates are major LDOM constituents – contradicting the existing CRAM cycling paradigm and further constraining the long-term persistence of deep ocean DOM.

Changes in plant resource inputs lead to rapid alterations in soil dissolved organic matter composition in an old-growth tropical forest

Alterations in plant resource inputs to soil affect soil organic matter (OM) dynamics. However, it remains unclear how to alter soil dissolved OM (DOM) composition. Here, we used UV/fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry to analyze soil DOM’s optical and molecular characteristics after eight months of detritus input and removal in an old-growth tropical forest. Changes in plant inputs significantly altered soil DOM’s optical properties, and the most pronounced changes were observed in the humification index and fluorescent components. In litterfall removal and no-input plots, molecular characteristic values increased greatly, such as O/C, double-bond equivalent, aromaticity index, and proportion of carboxyl-rich alicyclic molecules, while biolabile compounds decreased. The abundance of lignin-like and tannin-like compounds was more than 20 % higher in litter removal plots than in no-input plots. Our findings indicate that changes in plant resource inputs can lead to rapid alterations in soil DOM composition.

Multi-tracer evidence of hydrology and primary production controls on dissolved organic matter composition and stability in the semi-arid aquatic continuum

Autochthonous dissolved organic matter (Auto-DOM) produced by a biological carbon pump using dissolved inorganic carbon (DIC) from carbonate weathering plays an important role in carbon cycling within inland waters. However, little is known regarding how environmental conditions impact the composition and fate of organic matter, especially in surface waters of the semi-arid Loess Plateau, which is enriched in DIC by significant carbonate weathering. To obtain novel insight, we combined hydrochemistry, isotopic composition (δ2H, δ18O, and δ13CDIC), Rayleigh fractionation model, optical spectroscopy (absorbance and fluorescence), and Fourier transform ion cyclotron resonance mass spectrometry measurements to elucidate how primary production and hydrology impact the composition of DOM and the stability of the resulting Auto-DOM throughout the river–reservoir–wetland aquatic continuum of the Bahe River in the carbonate-mineral-rich semi-arid Loess Plateau where carbonate weathering is significant. The Rayleigh fractionation model results indicated that watershed DIC is primarily consumed through aquatic primary production rather than CO2 degassing. Further investigation revealed that primary production and evaporation co-occurred in this watershed. With the enrichment of the stable water isotope δ2H, the relative abundances of the allochthonous compounds decreased and the relative abundances of the autochthonous substances increased, suggesting that the terrestrial signal of riverine DOM decreased while autochthonous production increased along the flow pathway. In addition, associations between optical and molecular characteristics among DOM samples from different water bodies revealed that the stability ratio (Fmax(C2/(C2 + C4))) of Auto-DOM to the ratio of carboxylic-rich alicyclic molecules showed a consistent trend, suggesting that phytoplankton-derived and biomineralized C2 compounds are potentially recalcitrant DOM in inland waters. We conclude that hydrology and primary production affect the source, composition, and, potentially, the stability of DOM in DIC-enriched surface waters of the semi-arid Loess Plateau, which may lead to a more humic-like DOM composition in inland water and export this lower bioavailability DOC to the ocean in the long term.

Microbial regulation on refractory dissolved organic matter in inland waters

The production of refractory dissolved organic matter (RDOM) is complex and closely related to microbial consortia in aquatic ecosystems; however, it is still unclear how microorganisms regulate the production of RDOM and its molecular composition in inland waters. Therefore, we conducted a large-scale survey of inland waters and analyzed the optical and mass spectrometric characteristics of DOM, the microbial community and functional genes, as well as related environmental parameters, to understand the abovementioned issues. Here, the RDOM production was found mainly regulated by microbial (e.g., phylogeny and community assembly) rather than other environmental factors in inland waters. Biostatistical analyses and carbon isotopic evidence indicated that the successive microbial processing from labile DOM to RDOM (i.e., carboxyl-rich alicyclic molecules, CRAMs) was widely present in inland waters, involving the microbially mediated carbon skeleton turnover and heteroatom conversion. There was a significant empirical relationship between CRAMs and the ratio of Proteobacteria to Actinobacteria, highlighting the intraspecific interaction of bacteria more important than other microbial groups (i.e., archaea, eukaryotes, and fungi) for the RDOM production. This study demonstrated a fundamental role of microbial regulation in RDOM production within the inland waters, thereby facilitating future estimation of carbon sequestration potential in inland aquatic ecosystems.

Microbial transformation mechanisms of particulate organic carbon to mineral-associated organic carbon at the chemical molecular level: Highlighting the effects of ambient temperature and soil moisture

Soil dissolved organic matter (DOM), an important component of active soil organic carbon (C), plays a crucial role in controlling C transformation. However, the role of DOM in mediating the conversion of particulate organic C (POC) into mineral-associated organic C (MAOC) at the molecular level remains unclear. In this study, the microbial conversion of POC to MAOC at the molecular level was investigated at different ambient temperatures and soil moisture levels. An increase in ambient temperature led to the conversion of POC to labile C, whereas MAOC increased with soil moisture. CHO, CHNO, and CHOS increased with soil moisture and temperature. The highest temperature (35 °C) promoted conversion of lignin/cram (carboxyl-rich alicyclic molecules)-like molecules into aromatic components with the highest intragroup transformation of DOM (74%). The highest soil moisture (60%) favoured the conversion of refractory substances into aliphatic/peptide and lipid components with the highest intergroup transformation (28%). Compared to fungi, bacteria were the main driving force for DOM transformation. Therefore, changes in ambient temperature and soil moisture control the microbial formation and transformation of different C fractions (i.e. POC and MAOC) via DOM differently at the molecular level.

Photochemical evolution of the molecular composition of dissolved organic carbon and dissolved brown carbon from wood smoldering

Recently, extreme wildfires occur frequently around the world and emit substantial brown carbon (BrC) into the atmosphere, whereas the molecular compositions and photochemical evolution of BrC remain poorly understood. In this work, primary smoke aerosols were generated from wood smoldering, and secondary smoke aerosols were formed by the OH radical photooxidation in an oxidation flow reactor, where both primary and secondary smoke samples were collected on filters. After solvent extraction of filter samples, the molecular composition of dissolved organic carbon (DOC) was determined by Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). The molecular composition of dissolved BrC was obtained based on the constraints of DOC formulae. The proportion of dissolved BrC fractions accounted for approximately 1/3–1/2 molecular formulae of DOC. The molecular characteristics of dissolved BrC showed higher levels of carbon oxidation state, double bond equivalents, and modified aromaticity index than those of DOC, indicating that dissolved BrC fractions were a class of organic structures with relatively higher oxidation state, unsaturated and aromatic degree in DOC fractions. The comparative analysis suggested that aliphatic and olefinic structures dominated DOC fractions (contributing to 70.1%-76.9%), while olefinic, aromatic, and condensed aromatic structures dominated dissolved BrC fractions (contributing to 97.5%-99.9%). It is worth noting that dissolved BrC fractions only contained carboxylic-rich alicyclic molecules (CRAMs)-like structures, unsaturated hydrocarbons, aromatic structures, and highly oxygenated compounds. CRAMs-like structures were the most abundant species in both DOC and dissolved BrC fractions. Nevertheless, the specific molecular characteristics for DOC and dissolved BrC fractions varied with subgroups after aging. The results highlight the similarities and differences in the molecular compositions and characteristics of DOC and dissolved BrC fractions with aging. This work will provide insights into understanding the molecular composition of DOC and dissolved BrC in smoke.

Continuous planting of Chinese fir monocultures significantly influences dissolved organic matter content and microbial assembly processes

Monoculture plantations in China, characterized by the continuous cultivation of a single species, pose challenges to timber accumulation and understory biodiversity, raising concerns about sustainability. This study investigated the impact of continuous monoculture plantings of Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) on soil properties, dissolved organic matter (DOM), and microorganisms over multiple generations. Soil samples from first to fourth-generation plantations were analyzed for basic chemical properties, DOM composition using Fourier transform ion cyclotron resonance mass spectrometry, and microorganisms via high-throughput sequencing. Results revealed a significant decline in nitrate nitrogen content with successive rotations, accompanied by an increase in easily degradable compounds like carbohydrates, aliphatic/proteins, tannins, Carbon, Hydrogen, Oxygen and Nitrogen- (CHON) and Carbon, Hydrogen, Oxygen and Sulfur- (CHOS) containing compounds. However, the recalcitrant compounds, such as lignin and carboxyl-rich alicyclic molecules (CRAMs), condensed aromatics and Carbon, Hydrogen and Oxygen- (CHO) containing compounds decreased. Microorganism diversity, abundance, and structure decreased with successive plantations, affecting the ecological niche breadth of fungal communities. Bacterial communities were strongly influenced by DOM composition, particularly lignin/CRAMs and tannins. Continuous monoculture led to reduced soil nitrate, lignin/CRAMs, and compromised soil quality, altering chemical properties and DOM composition, influencing microbial community assembly. This shift increased easily degraded DOM, accelerating soil carbon and nitrogen cycling, ultimately reducing soil carbon sequestration. From environmental point of view, the study emphasizes the importance of sustainable soil management practices in continuous monoculture systems. Particularly the findings offer valuable insights for addressing challenges associated with monoculture plantations and promoting long-term ecological sustainability.

Characteristics of dissolved organic matter during the cyanobacterial degradation in Chaohu Lake, China.

The actual DOM in Chaohu Lake was used to feed cyanobacterial to explore the changes of microbial communities, fluorescence spectral characteristics and molecular composition of DOM during the degradation of cyanobacteria. It is found that cyanobacterial grow periodically depending on the concentration of nutrients with the decreasing concentration of nutrient salts. Both Bacteroidetes and Actinobacteria have strong correlation with algae growth. Bacteroidetes has a positive correlation with algae growth, relationship on the contrary, Actinobacteria has a negative relationship. The humus-like components in the four groups are similar, but the protein-like component (C3) shows periodic changes with the life process of cyanobacteria. The average molecular weight of each sample detected by Orbitrap high-resolution mass spectrometer increases slightly and the DOM increase aromaticity in the end. In this study, the molecule of Carboxyl-Rich Alicyclic Molecules (CRAM) is difficult to be done by photodegradation and biodegradation in the early periods, but some molecules of CRAM are selectively degraded by microorganisms in the final period. The growth of cyanobacterial lead to increasing the concentration of protein-like and carbohydrate-like molecule of DOM in the water. In the final stage, the molecule group of CHO disappear significantly and the molecule group of heteroatomic group increase.

Effects of burning and photochemical degradation of Macondo surrogate oil on its composition and toxicity.

Petroleum products in the environment can produce significant toxicity through photochemically driven processes. Burning surface oil and photochemical degradation were two mechanisms for oil removal after the Deepwater Horizon (DWH) oil spill in the Gulf of Mexico. After burning, residual oil remains in the environment and may undergo further weathering, a poorly understood fate. Although photochemistry was a major degradation pathway of the DWH oil, its effect on burned oil residue in the environment is under studied. Here, we ignited Macondo surrogate crude oil and allowed it to burn to exhaustion. Water-accommodated fractions (WAFs) of the burn residue were created in full sunlight to determine the effects of photochemical weathering on the burned oil residue. Our findings show that increased dissolved organic carbon concentrations (DOC) for the light unburned and light burned after sunlight exposure positively correlated to decreased microbial growth and production inhibition (i.e. more toxic) when compared to the dark controls. Optical and molecular analytical techniques were used to identify the classes of compounds contributing to the toxicity in the dark and light burned and dark and light unburned WAFs. After light exposure, the optical composition between the light unburned and light burned differed significantly (p < 0.05), revealing key fluorescence signatures commonly identified as crude oil degradation products. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis showed more condensed aromatic, reduced oxygenated compounds present in the light burned than in the light unburned. FT-ICR MS also showed an increase in the percent relative abundance of carboxyl-rich alicyclic molecules (CRAM) like compounds in the light burned compared to light unburned. The increase in CRAM suggests that the composition of the light burned is more photorefractory, i.e., reduced, explaining the residual toxicity observed in microbial activity. Overall, these data indicate burning removes some but not all toxic compounds, leaving behind compounds which retain considerable toxicity. This study shows that burn oil residues are photolabile breaking down further into complex reduced compounds.

“Gray Carbon” in Sewage Treatment Plants: A Neglected Carbon Sink

This study introduces the concept of “gray carbon,” emphasizing its critical role in carbon sequestration in sewage treatment. By focusing on recalcitrant dissolved organic carbon (RDOC) in sewage effluents and its subsequent transformation in marine environments, we underscore the significant impact of sewage-derived organic carbon on the efficiency of carbon sequestration. Through analysis of carboxylic-rich alicyclic molecules, this study illuminates the convergence in the molecular compositions of RDOC across various aquatic systems. Dark-culture experiments reveal marked variations in the microbial community structures of the aforementioned molecules, indicating that these changes may play an important role in the degradation and subsequent transformation of organic matter in marine environments. These insights lay the groundwork for advancing technologies designed to enhance wastewater alkalinity, which will improve the sustainability of wastewater treatment and preserve marine ecosystems. Enhancing sewage alkalinity can influence microbial processes and chemical equilibria, potentially affecting the formation and accumulation of gray carbon. Further investigation is necessary to understand the potential effect of alkalinity enhancement on the microbial communities and biochemical pathways involved in gray carbon formation. Our findings support the integration of gray carbon strategies into broader carbon neutrality initiatives, providing a scientific and technological blueprint for enhancing global carbon management and mitigating climate change.

Selectivity of β-Cyclodextrin Polymer toward Aquatic Contaminants: Insights from Ultrahigh-Resolution Mass Spectrometry of Dissolved Organic Matter.

Selectivity in solid-phase extraction (SPE) materials has become increasingly important for analyte enrichment in sensitive analytical workflows to alleviate detrimental matrix effects. Molecular-level investigation of matrix constituents, which are preferentially extracted or excluded, can provide the analytical chemist with valuable information to learn about their control on sorbent selectivity. In this work, we employ nontargeted Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) to elucidate the molecular chemodiversity of freshwater-derived dissolved organic matter (DOM) extracted by the selective model sorbent β-cyclodextrin polymer (β-CDP) in comparison to conventional, universal SPE sorbents (i.e., Oasis HLB, Supel-Select HLB, and LiChrolut EN). Statistical analysis of MS data corroborated the highly selective nature of β-CDP by revealing the extracted DOM spectra that are most dissimilar to original compositions. We found that its selectivity was characterized by pronounced discrimination against highly oxygenated and unsaturated DOM compounds, which were associated with the classes of lignin-like, tannin-like, and carboxylic-rich alicyclic molecules. In contrast, conventional sorbents excluded less highly oxygenated compounds and showed a more universal extraction behavior for a wide range of DOM compositional space. We lay these findings in a larger context that aids the analyst in obtaining an a priori estimate of sorbent selectivity toward any target analyte of interest serving thereby an optimization of sample preparation. This study highlights the great value of nontargeted ultrahigh-resolution MS for better understanding of targeted analytics and provides new insights into the selective sorption behavior of novel sorbents.

Hypoxia diversifies molecular composition of dissolved organic matter and enhances preservation of terrestrial organic carbon in the Yangtze River Estuary

Dissolved organic matter (DOM) is an essential component of the global carbon cycle, and estuaries link the rivers and the oceans, thus playing important roles in land-ocean DOM transformation and transport. However, the effects of hypoxia on DOM transport and fate in estuaries and coastal oceans remains poorly understood. To address this gap, we characterized the molecular composition of DOM in bottom water (BW) and sediment porewater (PW) at hypoxic and non-hypoxic sites in the Yangtze River Estuary (YRE) using ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry. Our results showed significant differences in DOM molecular composition between hypoxic and non-hypoxic areas for both BW and PW. Specifically, DOM in hypoxic sites was more recalcitrant than that in non-hypoxic areas for both BW and PW, with lower H/C, and higher O/C, double bond equivalent, and modified aromaticity index. The presence of higher polyphenols, and black carbon in hypoxic areas suggested that hypoxic conditions could facilitate the preservation of terrestrial organic matter. Furthermore, we identified a much higher number of hypoxia-unique formulas than ocean-non-hypoxia-unique formulas, indicating that hypoxia could diversify the DOM pool. Within hypoxia-unique formulas for PW, both biologically labile (unsaturated aliphatic compounds and peptides) and recalcitrant formulas (carboxyl-rich alicyclic molecules) were found, suggesting that hypoxia could facilitate the preservation of labile formulas and the production of recalcitrant formulas. In addition, we formulated that the sulfurization is more important in PW than BW in hypoxic areas based on the higher dissolved organic sulfur (DOS) abundance and larger number of hypoxia-only formulas in hypoxic PW, and also the precursor analysis results. Overall, our study provides insights into the effect of hypoxia on the molecular characteristics and preservation of DOM in estuaries and coastal oceans, highlighting the importance of considering hypoxia in understanding the biogeochemical processes of these ecosystems.