Refractory Dissolved Organic Matter Research Articles

Mechanism and molecular level insight of refractory dissolved organic matter in landfill leachate treated by electroflocculation coupled with ozone

Landfill leachate usually contains a substantial content of refractory dissolved organic matter (RDOM), which mainly consists of humic acid and fulvic acid. RDOM easily blocks the membrane pores resulting in membrane fouling when treated using membrane technologies, and it costs abundant chemical regents using advanced oxidation processes. To achieve the economically efficient treatment of RDOM in landfill leachate, this study established the electroflocculation coupled with ozone (EFCO) system. EFCO achieved a higher removal for organic matter than single electroflocculation and ozonation, where RDOM was the main component of organic matter in landfill leachate. The remaining RDOM was decomposed into lots of micromolecule weight and biodegradable proteins and lipids. Ozone improved the removal of dissolved organic matter (DOM) mainly by changing the functional groups on its surface and improving the mechanical strength of flocs to enhance the complexation between flocs and DOM. Cl2− and ClO produced in the EFCO system were the main active chlorine species, which enhanced the mineralization of RDOM. The study provides an efficient method for the pretreatment of RDOM in landfill leachate.

Improved solid-state 13C and 15N NMR reveals fundamental compositional divide between refractory dissolved organic carbon and nitrogen in the sea

Marine dissolved organic matter (DOM) is one of the largest reservoirs of organic carbon and nitrogen reservoirs in the world. Yet, despite its global importance, most DOM remains molecularly uncharacterized. Solid-state nuclear magnetic resonance (NMR) spectroscopy of isolated DOM fractions represents one of the most powerful techniques to understand overall structural composition. However, it is well known that standard cross polarization magic angle spinning (CP/MAS) NMR, the technique used for almost all past solid-state NMR studies of DOM, is at best “semi-quantitative,” and underestimates fully substituted NMR active nuclei. Additionally, almost all past solid-state NMR work analyzed high molecular weight (HMW) material isolated by ultrafiltration, which is now understood to represent mostly 14C-young, “semi-labile” compounds. In contrast, there is far less information regarding the composition of older, low molecular weight (LMW) DOM, which represents the vast majority of the ocean’s accumulated refractory DOM pool.Here, we applied 13C and 15N solid-state multiCP/MAS NMR, improved NMR methods optimized to more quantitatively resolve fully substituted NMR nuclei, to both HMW and LMW DOM isolated from the surface and deep North Pacific Subtropical Gyre. These methods confirm past work indicating most nitrogen containing HMW DOM as amide compounds, but also demonstrate a modest heterocyclic N component not previously identified. In contrast, we found that LMW DON is almost entirely aromatic heterocyclic N, consistent with the hypothesis that heterocyclic N structures may be largely responsible for the accumulation of the ocean’s refractory DON pool. Surprisingly, however, we find DOC aromatic functionalities still represent only a very minor portion of either the HMW or the LMW refractory carbon pools, in marked contrast to refractory DON composition. Together, these more quantitative solid-state NMR techniques likely represent the most accurate picture of DON and DOC functional and compound-class makeup to date, and so have broad implications for our understanding of marine DOM structure and cycling. Specifically, our new data suggests that while chemical composition likely acts as a key control on DOM lability, the most refractory components of DOC and DON have very different compositions, sources, and cycling, supporting the idea that DOC and DON cycling in the ocean may be largely decoupled.

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.

Long-term straw return promotes accumulation of stable soil dissolved organic matter by driving molecular-level activity and diversity

Agricultural management practices play pivotal roles in affecting soil organic matter formation pathways, particularly through changes in soil dissolved organic matter (DOM). The potential effects of long-term straw return on the molecular-level activity and transformations of DOM were investigated in this study. Over 9 years, the soil DOM from straw return had high stability, unsaturation degree, oxidability, and strong aromaticity. The relative abundance of biologically refractory DOM components (i.e., lignin-, condensed aromatic-, and tannin-like formulas) increased by 4 %-33 %, whereas the relative abundance of biodegradable DOM components (i.e., lipids and protein/amino sugar-like formulas) decreased by 15 %-23 % under straw return. Furthermore, the positive effects of straw return on the intergroup transformations of biologically refractory compounds favored the persistence of recalcitrant organic matter in the soil. Importantly, intragroup transformations of recalcitrant-active molecules and biologically refractory compounds were critical pathways for stable DOM accumulation under straw return, and these were mainly driven by thermodynamically limited processes. Thus, our results indicated that long-term straw return promoted the persistence of stabilized soil DOM by driving molecular-level activity and diversity.

Microbial stratification and DOM removal in drinking water biofilters: Implications for enhanced performance

Biofiltration is a low-cost, low-energy technology that employs a biologically activated bed of porous medium to reduce the biodegradable fraction of the dissolved organic matter (DOM) pool in source water, resulting in the production of drinking water. Microbial communities at different bed depths within the biofilter play crucial roles in the degradation and removal of dissolved organic carbon (DOC), ultimately impacting its performance. However, the relationships between the composition of microbial communities inhabiting different biofilter depths and their utilisation of various DOC fractions remain poorly understood. To address this knowledge gap, we conducted an experimental study where microbial communities from the upper (i.e., top 10 cm) and lower (i.e., bottom 10 cm) sections of a 30-cm long laboratory-scale biofilter were recovered. These communities were then individually incubated for 10 days using the same source water as the biofilter influent. Our study revealed that the bottom microbial community exhibited lower diversity yet had a co-occurrence network with a higher degree of interconnections among its members compared to the top microbial community. Moreover, we established a direct correlation between the composition and network structure of the microbial communities and their ability to utilise various DOM compounds within a DOM pool. Interestingly, although the bottom microbial community had only 20 % of the total cell abundance compared to the top community at the beginning of the incubation, it utilised and hence removed approximately 60 % more total DOC from the DOM pool than the top community. While both communities rapidly utilised labile carbon fractions, such as low-molecular-weight neutrals, the utilisation of more refractory carbon fractions, like high-molecular-weight humic substances with an average molecular weight of more than ca. 1451 g/mol, was exclusive to the bottom microbial community. By employing techniques that capture microbial diversity (i.e., flow cytometry and 16S rRNA amplicon sequencing) and considering the complexities of DOM (i.e., LCOCD), our study provides novel insights into how microbial community structure could influence the microbial-mediated processes of engineering significance in drinking water production. Finally, our findings could offer the opportunity to improve biofilter performances via engineering interventions that shape the compositions of biofilter microbial communities and enhance their utilisation and removal of DOM, most notably the more classically humified and refractory DOM compound groups.

Polystyrene microplastics facilitate formation of refractory dissolved organic matter and reduce CO2 emissions

Microplastics, as a type of anthropogenic pollution in aquatic ecosystems, affect the carbon cycle of organic matter. Although some studies have investigated the effects of microplastics on dissolved organic matter (DOM), the impact of alterations in the chemical properties of microplastics on refractory DOM and carbon release remains unclear. Here, we observed that microplastic treatments (e.g., polystyrene, PS) altered the composition and function of microbial community, notably increasing the abundance of microbial families involved in consuming easily degradable organic matter. During the process in which microbial community decomposed organic matter into DOM, PS underwent surface oxidation. The oxidized PS aggregated with DOM and microorganisms through electrostatic interactions and chemical bonds. Moreover, these interactions between oxidized PS and microbial community affect the utilization of organic matter, resulting in a significant decrease in CO2 emissions. Specifically, total CO2 emissions decreased by approximately 23.76 % with 0.1 mg/L PS treatment and by 44.97 % with 10 mg/L PS treatment compared to those in PS-free treatments over the entire reaction. These findings underscored the significance of the chemical properties of PS in the interactions among DOM and microorganisms, emphasizing the potential impact of PS microplastics on the carbon cycle in ecosystems.

Spatiotemporal dynamics of dissolved organic matter in subtropical karst cave waters identified by optical properties

Natural dissolved organic matter (DOM) is ubiquitous in aquatic environments and is an essential component in the carbon cycle in karst areas. To improve understanding of the carbon cycle in karst caves with heterogeneous hydrological processes, we examined the spatiotemporal variability of DOM composition and further uncovered its source and fate. Results may also provide insights into the feedbacks of organic carbon to carbon sinks in karst regions. In this study, concentrations and compositions of DOM, partial pressure of aqueous carbon dioxide (pCO2), dissolved inorganic carbon, and other physicochemical parameters were investigated in a karst cave at Mahuang, Southwest China. Ultraviolet-visible absorption spectroscopy was coupled with multiple statistical analyses to identify the compositional variations and potential fates of DOM in cave waters. The results showed that DOM dynamics were regulated by both terrigenous and biogenic drivers under the control of meteorological conditions. With higher air temperature, precipitation, and microbial activity, fulvic fractions were consumed to generate CO2, leading to the accumulation of refractory DOM in cave waters and changing the hydrochemical features. When temperature and precipitation decreased, DOM was dominated by lignin fractions, which served as an indicator of terrestrial inputs and vascular plants, suggesting variation in the preferential fraction of biological consumption. In addition, different hydrological path patterns influenced DOM properties in cave waters due to differences in recharging, the leaching process, and subsurface reworking. Thus, hydrology could serve as an important constraint on the coupling between dissolved organic and inorganic carbon.

Optical properties and molecular differences in dissolved organic matter at the Bermuda Atlantic and Hawai'i ALOHA time-series stations

Fundamental differences in DOM composition at the Bermuda Atlantic time series (BATS) station versus the Hawai’i time series station ALOHA.

The ratio but not individual of fragile to refractory DOM affects greenhouse gases release in different trophic level lakes

Inland shallow lakes are recognized as an important source of greenhouse gases (GHGs), and their contribution is expected to increase due to global eutrophication. The generation and release of GHGs involved multiple variables, leading to many uncertain potential factors. This study examined the emission characteristics of GHGs at the water-air interface in 12 shallow lakes categorized into four eutrophic levels in the Yangtze River basin. The average emission rates of CH4, CO2 and N2O were 1.55, 3.43, 18.13 and 30.47 mg m−2 h−1, 4.12, 14.64, 25.11 and 69.84 mg m−2 h−1, and 0.2, 0.25, 0.43 and 0.79 mg m−2 day−1 in the oligotrophic, mesotrophic, eutrophic and hypereutrophic lakes, respectively. There were significant correlations between eutrophic levels and the emission rates of CH4 and CO2 (p < 0.05). Redundancy analysis and Mantel test were conducted to further examine the key factors influencing carbon emissions from eutrophic water. It was found that the presence of algae and nutrients in the overlying water played a crucial role in the release of GHGs, indicating the importance of ecosystem productivity in the carbon budget of the lake. In order to assess the bioavailability of organic matter, a new indicator called R(P/H) was proposed. This indicator represents the ratio of protein and humus-like components, which were obtained through EEMs-PARAFAC modeling. The relationship between R(P/H) and CH4 was found to be exponential (R2 = 0.90). Additionally, R(P/H) showed a linear relationship with CO2 and N2O (R2 = 0.68, R2 = 0.75). Therefore, it is crucial to consider R(P/H) as an important factor in accurately estimating global GHG emission fluxes in the future, especially with advancements in the database.

Evidence for a Temporary Positive Priming Effect in Aquatic Systems With Certain Substrates and Isotopic Discrimination of DOM Sources

AbstractThe “priming effect” (PE) is well documented in soils, but contradictory results have been reported in aquatic ecosystems. To investigate whether the presence of easily degradable labile dissolved organic matter (LDOM) changes the biodegradation rate of nonlabile DOM (NLDOM), incubations were performed with various sources of LDOM and NLDOM in microcosms amended with nutrients and a microbial inoculum. Stable carbon isotope analysis of DOM was used for the first time to estimate the PE. A significant positive PE (15%–34% of initial NLDOM) was measured, but only with mixed (glucose + amino acids) or complex (disaccharide) LDOM. Regardless of the LDOM, no PE was measured with more recalcitrant NLDOM (half‐life of 128 days here), when the LDOM/NLDOM ratio was low (0.3 versus 1), and when the NLDOM and microbial inoculum were from the same water (no mixing). The presence of sediments likely enhances microbial diversity and NLDOM degradation rate, but it did not increase the PE. Microbial use of LDOM produced new microbial NLDOM that should be discriminated (here by isotopes) from initial NLDOM for accurate PE measurements. The PE was temporary and lasted about 2 weeks after one LDOM addition. In areas with frequent additions of LDOM, a long‐lasting PE is expected and may significantly increase CO2 production. Exposing refractory DOM to new conditions and microbial community, such as during natural water mixing, seems to exert a strong control on its dynamics. Here, the addition of a new microbial inoculum had a stronger effect than adding LDOM or the PE.

Impact of wild fish faeces to deep sea dissolved organic carbon as revealed by laboratory incubations

Wild fish faeces play an important role in mediating marine organic carbon cycle, but the faeces released dissolved organic matter (DOM) and its potentials in contribution to refractory DOM is unclear. The mixed wild fish (including mesopelagic fish) faecal samples from the northern South China Sea were collected and incubated up to 75 days using the waters collected at 800 m depth of the same site, to reveal the stability of the faeces released DOM. Our result showed that the faeces had a good DOM-release potential and dissolved organic carbon (DOC) was up to 150 μM in the first day and reached its maximum (172 μM) on day 3, which was in comparison with the control group (DOC <70 μM). In our incubations, the faecal samples contributed net DOC of 110 μM, in which 42 μM persisted till the end of the 75 days dark incubation. This meant the faeces-released DOC had a refractory fraction of 38% over the incubation. Total hydrolysable amino acids concentration significantly dropped, and amino acids molecular indicator (DI, AA C yield and D/L ratio) changes over the incubation indicated the fast transformation of DOM from labile to degraded stage. Network analysis was applied to the amino acids and our result showed that D-form amino acids were almost positively coupled with L-form amino acids. Glycine (in mol%) showed consistent accumulation along with DOM degradation. Also, glycine was at the key node position of the amino acids network, showing negative relation with all other amino acids enantiomers in both faeces group and control group. Our results proved that glycine had the potential in tracing the refractory fraction over the DOM degradation.

Molecular characterization of DOM: A full-scale case study of an advanced DWTP from Jintan, China

Since the variation of dissolved organic matter (DOM) is complicated in the aquatic environment and water treatment processes, adverse effects on human and water quality may be triggered. Nowadays, more drinking water treatment plants (DWTPs) begin using pretreatment and advanced treatment processes for removing refractory DOM to pursue high-quality drinking water, including preozonation, and combined ozonation and biological activated carbon (BAC) treatment. The combination of Fourier transform ion cyclotron resonance mass spectrometer and fluorescence spectroscopy could comprehensively unveil the DOM composition and characteristics in water treatments. This study aims to provide information on the molecular and spectroscopic characteristics of DOM in a DWTP with pretreatments and advanced treatment processes and summarize the removal efficiency of different treatment processes on DOM. Results showed that the removal rate of low-molecular-weight acids (LMW-acids) by BAC was as high as 98 %, reflecting the synergistic effect of ozonation/BAC unit. Pre-ozonation enhanced coagulation process had a good removal effect on CHONS. The diversity of DOM increased and the relative abundance of DOM intensity was reduced in preozonation/coagulation/sedimentation and O3/BAC units. The preozonation-coagulation/sedimentation unit reduced the aromaticity or unsaturation of DOM while the advanced treatment process had a higher removal rate of fluorescent components.

FexO/FeNC modified activated carbon packing media for biological slow filtration to enhance the removal of dissolved organic matter in reused water

The biological slow filtration reactor (BSFR) process has been found to be moderately effective for the removal of refractory dissolved organic matter (DOM) in the treatment of reused water. In this study, bench scale experiments were conducted using a mixture of landscape water and concentrated landfill leachate as feed water, to compare a novel iron oxide (FexO)/FeNC modified activated carbon (FexO@AC) packed BSFR, with a conventional activated carbon packed BSFR (AC-BSFR), operated in parallel. The results showed that the FexO@AC packed BSFR had a refractory DOM removal rate of 90%, operated at a hydraulic retention time (HRT) of 10 h at room temperature for 30 weeks, while under the same conditions the removal by the AC-BSFR was only 70%. As a consequence, the treatment by the FexO@AC packed BSFR substantially reduced the formation potential of trihalomethanes, and to a less extent, haloacetic acids. The modification of FexO/FeNC media raised the conductivity and the oxygen reduction reaction (ORR) efficiency of the AC media to accelerate the anaerobic digestion by consuming the electrons that are generated by anaerobic digestion itself, which lead to the marked improvement in refractory DOM removal.

Molecular insights into the dissolved organic matter of leather wastewater in leather industrial park wastewater treatment plant

Leather wastewater (LW) effluent is characterized by complex organic matter, high salinity, and poor biodegradability. To meet the discharge standards, LW effluent is often mixed with municipal wastewater (MW) before being treated at a leather industrial park wastewater treatment plant (LIPWWTP). However, whether this method efficiently removes the dissolved organic matter (DOM) from LW effluent (LWDOM) remains debatable. In this study, the transformation of DOM during full-scale treatment was revealed using spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry. LWDOM exhibited higher aromaticity and lower molecular weight than DOM in MW (MWDOM). The DOM properties in mixed wastewater (MixW) were similar to those in LWDOM and MWDOM. The MixW was treated using a flocculation/primary sedimentation tank (FL1/PST), anoxic/oxic (A/O) process, secondary sedimentation tank (SST), flocculation/sedimentation tank, denitrification filter (FL2/ST-DNF), and an ozonation contact reactor (O3). The FL1/PST unit preferentially removed the peptide-like compounds. The A/O-SST units had the highest removal efficiencies for dissolved organic carbon (DOC) (61.34 %) and soluble chemical oxygen demand (SCOD) (52.2 %). The FL2/ST-DNF treatment removed the lignin-like compounds. The final treatment showed poor DOM mineralization efficiency. The correlation between water quality indices, spectral indices, and molecular-level parameters indicated that lignin-like compounds were strongly correlated with spectral indices and CHOS compounds considerably contributed to the SCOD and DOC. Although the effluent SCOD met the discharge standard, some refractory DOM from LW remained in the effluent. This study illustrates the composition and transformation of DOM and provides theoretical guidance for improving the current treatment processes.

Treatment of high chlorine-containing composting leachate biochemical effluent by Ti/RuO2-IrO2 anodic electrochemical oxidation: Optimization and evolution of pollutants

In this paper, the treatment of composting leachate biochemical effluent (CLBE) by electrochemical oxidation (EO) with Ti/RuO2-IrO2 as the anode could achieve concomitantly organic carbon removal (COD) and nitrogen removal (NH3-N), taking advantage of CLBE inherent high chlorine content. Under the optimal conditions of the current density of 90 mA·cm−2, A/V= 14 m−1, d= 1 cm, pH= 8.34 and Cl- concentration of 6000 mg·L−1, 92.19% of NH3-N and 85.59% of TN could be removed in 40 min, and 100% COD removal could be achieved after 100 min, with energy consumption (EC) of 111.09 kW·h·m−3. And the increase in Cl- concentration could reduce the EC of the EO process. In parallel, constructed simple quantitative accounting models for chlorine-containing and nitrogen-containing species in the EO system and clarified the evolution of them. Cl species eventually exist mainly in the form of free chlorine (by-product AOCL concentration always below 2 mg·L−1), while N species were mainly removed in the form of N2 (≈95.10% of TN). Additionally, the gas chromatography-mass spectrometry (GC-MS) showed that the dominant component of dissolved organic matter (DOM) in CLBE was degraded from the initial unsaturated C21-C30 to the saturated C11-C20. And the Excitation emission matrix (EEM) showed that the EO achieved the non-selective degradation of DOM, the refractory humic acid DOM reached 90.74% removal and the readily biodegradable non-humic DOM removal 92.02%, respectively. These results indicated that the EO technology has broad application potential in the treatment of chlorinated wastewater, and coupling it after biological treatment would be a promising method.

Sea Ice Melting Drives Substantial Change in Dissolved Organic Matter in Surface Water off Prydz Bay, East Antarctic

AbstractPolar ecosystems are vulnerable to climate warming and are characterized by a rapid decline in sea ice. In this study, we collected surface water samples from the Southern Ocean off Prydz Bay during and after sea ice melting season (SIM). After removing the particles using 0.7‐μm pore size filters, we measured the concentration of dissolved organic carbon (DOC) and the composition of dissolved organic matter (DOM) by using the total organic carbon analyzer, optical spectrometry, and high‐resolution mass spectrometry. The mean DOC concentration during the SIM is 1.3‐fold higher than that during the post‐SIM, whereas the a350 value, an indicator of chromophoric DOM, showed the opposite trend by a factor of 2.8. Five components, including three humic‐like and two protein‐like substances were identified by fluorescence excitation emission matrices‐parallel factor analysis, in which protein‐like biolabile components were enriched during the SIM and humic‐like refractory components were enriched during the post‐SIM. The DOM in the water samples from the SIM had higher proportions of polycyclic aromatics, polyphenols, and compounds within molecular lability boundary, whereas the DOM from the post‐SIM had a higher proportion of highly unsaturated compounds. Statistical analysis revealed significant correlations between DOM and sea‐ice melting/associated environmental factors (e.g., water temperature, salinity, and microbial activity). These collective results suggest that sea ice melting is an important factor driving a transition from more labile to more refractory DOM in the surface water off Prydz Bay, which may have a profound impact on the marine ecosystem and carbon cycle in the polar ocean.

Direct Production of Bio-Recalcitrant Carboxyl-Rich Alicyclic Molecules Evidenced in a Bacterium-Induced Steroid Degradation Experiment.

Carboxyl-rich alicyclic molecules (CRAM) are highly unsaturated compounds extensively distributed throughout aquatic environments and sediments. This molecular group is widely referred to as a major proxy of recalcitrant organic materials, but its direct biosynthesis remains unclear. Steroids are a typical anthropogenic contaminant and have been previously suggested to be precursors of CRAM; however, experimental evidence to support this hypothesis is lacking. Here, a steroid-degrading bacterium, Comamonas testosteroni ATCC 11996, was incubated in a liquid medium supplemented with testosterone (a typical steroid) as the sole carbon source for 90 days. Testosterone-induced metabolites (TIM) were extracted for molecular characterization and to examine the bioavailability during an additional 90-day incubation after inoculation with a natural coastal microbial assemblage. The results showed that 1,775 molecular formulas (MFs) were assigned to TIM using ultrahigh-resolution mass spectrometry, with 66.99% categorized as CRAM-like constituents. A large fraction of TIM was respired or transformed during the additional 90-day seawater incubation; nevertheless, 638 MFs of the TIM persisted or increased during incubation. Among the 638 MFs, 394 were commonly assigned in natural deep seawater samples (depths of 500 to 2,000 m) from the South China Sea. Compared to the catabolites of the well-established testosterone degradation pathway, we compiled a list of bio-refractory MFs and potential chemical structures, some of which shared structural homology with CRAM. These results demonstrated direct microbial production of bio-refractory CRAM from steroid hormones and indicated that some of the biogenic CRAM resisted microbial decomposition, potentially contributing to the aquatic refractory dissolved organic matter (DOM) pool. IMPORTANCE CRAM are an operationally defined DOM group comprising a complex mixture of carboxylated and fused alicyclic structures. This DOM group is majorly characterized as refractory DOM in the marine environment. However, the origins of the complex CRAM remain unclear. In this study, we demonstrated that testosterone (a typical steroid) could be transformed into bio-refractory CRAM by a single bacterial strain and observed that some of the CRAM highly resisted microbial degradation. Through molecular comparison and screening, potential chemical structures of steroid-induced CRAM were suggested. This study established the biological connection between steroids and bio-refractory CRAM, and it provides a novel perspective explaining the fate of terrestrial contaminants in aquatic environments.

Regulation of the Nutrient Cycle Pathway and the Microbial Loop Structure by Different Types of Dissolved Organic Matter Decomposition in Lakes.

To explore the effect of different types of dissolved organic matter (DOM) decomposition on nutrient cycling pathways and the microbial loop, four lakes with different DOM sources were investigated monthly. In Lake Tangxun, Dolichospermum decay released highly labile dissolved organic nitrogen into the water column. This induced bacterial organic nitrogen decomposition, as indicated by the increased abundance of gltB, gltD, gdh, and glnA as well as aminopeptidase activity. Genes associated with dissimilatory nitrate reduction to ammonium further fueled ammonium accumulation, driving Microcystis blooms in the summer. In Lake Zhiyin, fish bait deposits (high nitrogen, similar to Dolichospermum detritus) also caused Microcystis blooms. Detritus from Microcystis decomposition then produced high levels of labile dissolved organic phosphorus, inducing phosphatase activity and increasing soluble reactive phosphorus concentrations from September to April in Lakes Tangxun and Zhiyin. The high refractory DOM from macrophytes in Lake Houguan led to insufficient nutrient availability, leading to nutrient mutualism between algae and bacteria. The high levels of labile dissolved organic carbon from terrestrial detritus in Lake Yandong increased bacterial biomass and production, resulting in low chlorophyll content due to the competitive relationship between algal and bacterial nutrient requirements. Therefore, different DOM compositions induce unique connections among available nutrients, algae, and bacteria in the microbial loop.

The efficiency of the microbial carbon pump as seen from the relationship between apparent oxygen utilization and fluorescent dissolved organic matter

The mechanism behind the source/production of refractory dissolved organic matter (RDOM) in the ocean remains an enigma, but the biological origin of RDOM has now been recognized and conceptualized as the microbial carbon pump (MCP). Consequently, the development of a proxy for the efficiency of the MCP production of RDOM is desirable. It has widely been observed that marine humic-like fluorescent dissolved organic matter (FDOMH) is linearly correlated with apparent oxygen utilization (AOU), and hence it is promising for exploring this relationship. Here, we discuss whether the slope of the AOU–FDOMH correlation can be an indicator for evaluating the MCP efficiency in the dark ocean. We gathered all the peer-reviewed literature on AOU–FDOMH relationships and summarized the general results and factors that may affect this relationship. Finally, we propose to make the AOU–FDOMH relationship comparable and applicable at a global scale in future studies to better constrain sources and sinks of FDOMH and its link to heterotrophic microbial respiration, namely, the FDOM-based MCP efficiency.

Co-substrate promoting the biodegradation of refractory DOM in semi-coking wastewater: DOM evolution and microbial community

Semi-coking wastewater treatment has become a difficult issue because of its complexity, heterogeneity and toxicity. In this study, co-metabolism with ethanol or sodium acetate as co-substrates was applied to improve the biological degradation of refractory dissolved organic matter (DOM) in semi-coking wastewater. Kinetic model simulation results indicated that both co-substrates addition improved COD removal efficiency and biodegradation rate. Excitation-emission matrix (EEM) spectroscopy and size exclusion chromatography with organic carbon detector (SEC-OCD) analysis were applied to evaluate the DOM biodegradation in terms of fluorescence and molecular weight (MW). Although co-substrates resulted in a comparable enhanced removal efficiency of protein-like and humic-like fluorophores, there are distinct differences in their specific protein-like fluorescent peaks (Peak C1&C2). Ethanol addition was more conducive to the removal of humic substance with apparent molecular weight (AMW) ranging from 1.1 kDa to 20 kDa. Acetate addition resulted in higher removal of building blocks of humic substances (BB, AMW 0.45–1.1 kDa) and proteinaceous biopolymers (BP, AMW >20 kDa). Co-metabolism triggered the shift of microbial community structure and metabolic functions. Using co-occurrence network analysis, nine keystone genera were identified, which maintained the functional stability of bacterial community responding to organic toxicants stress. LefSe analysis elucidated that the potential major genera involved in refractory organics biodegradation with acetate addition were Stenotrophomonas (40.3%) and Pseudomonas (21.2%), while Pseudoxanthomonas (10.4%) with ethanol addition. Phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt2) analysis showed that benzoate, styrene, xylene and naphthalene degradation pathway were enhanced by acetate addition as compared to ethanol.