Snowmelt-driven changes in dissolved organic matter and bacterioplankton communities in the Heilongjiang watershed of China

Origin, distribution and spatial characteristics of dissolved organic matter in the Heilongjiang River

The Heilongjiang River Basin is a vast area with significant DOM sources and composition differences. The mechanism of DOM degradation under spatial variation remains unclear. This research investigated the degradation characteristics of DOM in different watersheds of the Heilongjiang River. DOM levels were higher in midstream waters, while DOM degradation rates were higher in midstream and downstream waters. The parallel factor analysis (PARAFAC) results showed that the upstream amino acid fraction was significantly depleted, the midstream was dominated by the degradation of DOM of terrestrial origin, and the downstream humic acid fraction was decreased considerably. Gene sequencing results indicated that the upstream, middle, and downstream water bodies' microbial community composition and structure differed significantly. The network analysis results revealed microorganisms in upstream water bodies mainly utilized amino acid-like substances and small molecule humic acids. Microorganisms in the middle and lower reaches of the water column were characterized by the utilization of humic acid-like fractions. In this study, we further screened the key driving microorganisms (e.g., Flavobacterium and Lacibacter) responsible for the difference in the DOM utilization function of upstream-to-midstream and midstream-to-downstream microorganisms in the Heilongjiang River. These findings will help identify the cycling process of DOM under spatial variation and predict the succession pattern of microbial communities.

Interaction between the hydrochemical environment, dissolved organic matter, and microbial communities in groundwater: A case study of a vegetable cultivation area in Huaibei Plain, China

Intensive vegetable planting has a profound impact on the surrounding aquatic environment. The self-purification ability of groundwater is poor, and it is difficult to return groundwater to its original state once polluted. Therefore, it is necessary to clarify the impact of intensive vegetable planting on groundwater. This study selected the groundwater of a typical intensive vegetable planting base in the Huaibei Plain of China as the research object. This work analyzed the content of major ions, the dissolved organic matter (DOM) composition, and the bacterial community structure in groundwater. Redundancy analysis was used to explore the interactions between the major ions, the DOM composition, and the microbial community. The results showed that under the influence of intensive vegetable planting, the F- and NO3--N contents in groundwater were significantly increased; the excitation-emission matrix combined with parallel factor analysis identified four fluorescent components (C1 and C2 were humus-like components, while C3 and C4 were protein-like components), which mainly consisted of protein-like components. Proteobacteria was the dominant phylum (mean = 69.27 %), followed by Actinobacteriota (mean = 7.25 %) and Firmicutes (mean = 4.02 %), which together explained over 80 % of the total abundance; and TDS, pH, K+, and C3 were the main influencing factors affecting the microbial community structure. This study provides a better understanding of the impact of intensive vegetable cultivation on groundwater.

Spectroscopic and compositional profiles of dissolved organic matters in urban snow from 2019 to 2021: Focusing on pollution features identification

Snow owns stronger adsorption capacity for organic pollutants compared with rain. Huge amounts of anthropogenic dissolved organic matters (DOMs) in the atmosphere may enter the water environment with urban snow and increase water pollution risk. Extracting stable pollution features of urban snow is conducive to identifying the urban snow pollution from the water environment. Herein, we systematically explored the spectroscopic and compositional profiles of urban snow in Beijing from three snow events by multiple analytical tools and extracted stable pollution features of urban snow for the first time. Results showed that conventional pollutants with high concentration were detected in urban snow. The fluorescence signals of humic-like and some protein-like materials, the molecular weight distributions of chromophoric DOM at 254 nm and humic-like materials, and 172 kinds of lignin-like molecular formulas were extracted as stable features for urban snow. These stable features of urban snow laid the foundation for the identification of urban snow pollution and the analysis of the impact mechanisms of atmospheric pollution sources on the water environment.

Benthic Biofilm Bacterial Communities and Their Linkage with Water-Soluble Organic Matter in Effluent Receivers

Benthic biofilms are pioneering microbial aggregates responding to effluent discharge from wastewater treatment plants (WWTPs). However, knowledge of the characteristics and linkage of bacterial communities and water-soluble organic matter (WSOM) of benthic biofilms in effluent-receiving rivers remains unknown. Here, we investigated the quality of WSOM and the evolution of bacterial communities in benthic biofilm to evaluate the ecological impacts of effluent discharge on a representative receiving water. Tryptophan-like proteins showed an increased proportion in biofilms collected from the discharge area and downstream from the WWTP, especially in summer. Biofilm WSOM showed weak humic character and strong autochthonous components, and species turnover was proven to be the main factor governing biofilm bacteria community diversity patterns. The bacterial community alpha diversity, interspecies interaction, biological index, and humification index were signally altered in the biofilms from the discharge area, while the values were more similar in biofilms collected upstream and downstream from the WWTP, indicating that both biofilm bacterial communities and WSOM characters have resilience capacities. Although effluent discharge simplified the network pattern of the biofilm bacterial community, its metabolic functional abundance was basically stable. The functional abundance of carbohydrate metabolism and amino acid metabolism in the discharge area increased, and the key modules in the non-random co-occurrence network also verified the important ecological role of carbon metabolism in the effluent-receiving river. The study sheds light on how benthic biofilms respond to effluent discharge from both ecological and material points of view, providing new insights on the feasibility of utilizing benthic biofilms as robust indicators reflecting river ecological health.

Divergent responses of taxonomic and predicted functional profiles of bacterioplankton to reservoir impoundment

Freshwater ecosystems are undergoing extensive human disturbance of dam construction which form large amounts of reservoirs and lead to dramatic changes in hydraulic conditions. Bacterioplankton are key component of aquatic ecosystems. Investigation on their taxonomic compositions and associated functions responded to reservoir operation is essential to understand the ecological consequence of dam construction. In this study, we use the Three Gorges Reservoir as a model system. High-throughput sequencing is used to investigate the bacterioplankton community composition, and the bioinformatic tool of Tax4Fun is applied to predict the potential metabolic functions responded to reservoir impoundment. Results show that the taxonomic communities of bacterioplankton are significantly impacted by impoundment. The dominant group of Actinobacteria which accounts for 17.0%-58.1% of the retrieved sequences significantly increases after impoundment on phylum level. The influences of impoundment appear to be more apparent on order level that the relative abundances of four groups including Frankiales, Sphingomonadales, Sphingobacteriales and SubsectionI of class Cyanobacteria significantly vary after impoundment. In contrast, the predicted functional communities of bacterioplankton remain relatively stable that most of predicted functional categories including methane and nitrogen metabolisms have no significant variation after impoundment. Besides, significant distance decay patterns appear on the taxonomic communities after impoundment rather than the predicted functional communities. The environmental variables show significant impacts on the taxonomic community rather than predicted functional community, whereas the spatial variables have no effect on both taxonomic and predicted functional communities. In general, the taxonomic and predicted functional communities of bacterioplankton exhibit divergent responses to the impoundment in reservoir.

Municipal wastewater effluent influences dissolved organic matter quality and microbial community composition in an urbanized stream

Dissolved organic matter (DOM) from wastewater treatment plant (WWTP) effluent poses serious threats to the receiving aqueous ecosystems and their microbial communities. However, the correlation between effluent-derived DOM and microbial community diversity in urbanized rivers is still poorly understood. In this study, the response relationship between the microbial community dynamics and the DOM evolution process in the effluent-dominated Xiaohe River was revealed. The results showed that macromolecular humic acids were the main components of DOM in this river with more carboxylic acid groups and humic-like acid substances found upstream and protein-like substances dominated downstream. The bacterial abundance in the upstream section of Xiaohe River was low, while its community structure was unstable but exhibited good uniformity, and the bacterial diversity in the downstream was rich. The response of bacterial and eukaryotic communities to WWTP effluent was weak, while that of Actinobacteria to WWTP effluent was more prominent. Furthermore, different microbial communities were affected by different compositions and structure of DOM in the effluent of WWTP. The protein-like components in DOM had the most profound impact on the microbial community, followed by polysaccharides and components rich in hydroxyl and amino functional groups. The study grasped the migration and evolution of DOM in rivers with unconventional water recharge, and revealed their diverse effects on microbial community in urbanized rivers.

Laboratory investigation of phosphorus loss with snowmelt and rainfall runoff from a Steppe wetland catchment

Phosphorus (P) losses from terrestrial soils contribute to eutrophication of surface waters. As priority non-point source pollution ways, rainfall runoff (RS₁) and snowmelt runoff (RS₂) are the main carrier of P loss from terrestrial ecosystem. The aim of this study was to investigate the similarities and differences between P loss with RS₁ and RS₂ of the same soil type. Six types of soil were used in this experiment. Results have shown that 1), Different types of soil have different P loss with RS₁ and RS₂ under different slope, and the changes ranged from 0.003 to 0.370 mg L^-1. 2), The effects of soil type, slope and runoff type on P loss with surface runoff was not independent, both individual effects of all factors and their interaction with the other two factors effected the P loss with runoff. 3), In our experiment, some soils showed no significant difference between P content in RS₁ and RS₂. In some soils, P loss with RS₁ was higher than that with RS₂ while the opposite conclusion was showed in Bog soil (BS) which with higher soil water content. 4), The P loss with RS₁ and RS₂ of different soils were both mainly affected by soil water content (SW), Olsen-P content (OP) and soil organic matter content (OM). These results can help us understand the P loss with different patterns of surface runoff better and are expected to provide pertinent opinions on the analysis of P loss with runoff and its influencing factors of grassland soils.

Impact of phosphate additive on organic carbon component degradation during pig manure composting

Phosphate, as an additive to composting, could significantly reduce ammonia emission and nitrogen loss but may also cause adverse effects on the degradation of organic matter. However, there is little information about the influence of pH change, salt content, and phosphate on different organic fraction degradation during composting with the addition of phosphate at a higher level. In this study, the equimolar phosphoric acid (H₃PO₄), sulfuric acid (H₂SO₄), and dipotassium phosphate (K₂HPO₄) were added into pig manure composting with 0.25 mol mass per kilogram of dry matter basis addition amount to evaluate the effect of H⁺, PO₄^3-, and salinity on carbon component transformation and organic matter degradation. The results showed that both H₃PO₄ and K₂HPO₄ additives could lead to shorter duration in the thermophilic phase, lower degradation of lignocellulose, and lesser carbon loss compared to CK, even though had different pH, i.e., acidic and alkaline conditions, respectively. Besides, the addition of H₃PO₄, H₂SO₄, and K₂HPO₄ could increase the degradation of soluble protein and lipid during composting. Redundancy analysis demonstrated that the variation in different organic carbon fractions was significantly correlated with the changes of pH and the presence of PO₄^3-, but not with SO₄^2- and electrical conductivity, suggesting that pH and phosphate were the more predominant factors than salinity for the inhibition of organic matter degradation. Taken together, as acidic phosphate addition produces a true advantage of controlling nitrogen loss and lower inhibition of organics transformation during composting, the expected effects may result in more efficient composting products.

How do fungal communities and their interaction with bacterial communities influence dissolved organic matter on the stability and safety of sludge compost?

This study was conducted to assess the effect of fungal communities and their interaction with bacterial communities on the dissolved organic matter (DOM) transformation for the stability and safety of sludge composting. The results showed that fungal community had strong shifts in diverse stages of sludge composting along with the changes of temperature. Correlation analysis demonstrated that fungal communities had significant connections with bacterial communities during composting but were not directly related to the indicators of phytotoxicity and maturity. Variance partitioning analysis suggested that the interactions of fungal and bacterial communities had the biggest contribution (49.75%) to composting stability and safety. Based on structural equation modeling, the possible way of fungal community participated in the transformation of DOM components and the formation of humic-like substances of DOM by interacting with bacterial community was proposed, which will provide important information for understanding the biotic interaction in composting and improving composting fermentation process.

Improved lignocellulose-degrading performance during straw composting from diverse sources with actinomycetes inoculation by regulating the key enzyme activities

This study was conducted to assess the effect of thermophilic actinomycetes inoculation on the lignocellulose degradation, enzyme activities and microbial community during different types of straw composting from wheat, rice, corn and soybean. The results showed that actinomycetes inoculation not only changed the structure of actinomycetic and bacterial community but also accelerated the degradation of cellulose, hemicellulose and lignin and increased the key enzymes activities including CMCase, Xylanase, manganese peroxidase, lignin peroxidase and laccase during composting particularly from wheat straw and rice straw. The key enzyme and physiochemical parameters which affected organic fractions degradation have been identified by redundancy analysis. The combined application of actinomycete inoculation and urea addition as a source of nitrogen was suggested to regulate the key enzyme activities and lignocellulose degradation, which lays a foundation for effectively managing organic wastes from different types of crop straws by composting.

Organophosphorus-degrading bacterial community during composting from different sources and their roles in phosphorus transformation

The goals of this study were to identify the key culturable organophosphorus-degrading bacteria (OPDB) that contributed to regulating different phosphorus (P) fractions and evaluate the roles of OPDB and inorganic phosphate-solubilizing bacteria (IPSB) in P transformation during different composting. The results showed that the amounts, incidence and community composition of OPDB for composts from diverse sources were distinctly different but significantly related to temperature and organic matter content. Fifteen key OPDB correlated closely with different P fractions have been selected by redundancy analysis. Two structural equation models were established to compare the roles of OPDB and IPSB on P availability during composting. Variance partitioning further showed that the interactions between IPSB and OPDB communities had a greater impact on P transformation than each independent factor. Therefore, the combined regulation of IPSB and OPDB were suggested to control the transformation of P fractions during composting.

Effects of floodgates operation on nitrogen transformation in a lake based on structural equation modeling analysis

Floodgates operation is one of the primary means of flood control in lake development. However, knowledge on the linkages between floodgates operation and nitrogen transformation during the flood season is limited. In this study, water samples from six sampling sites along Lake Xingkai watershed were collected before and after floodgates operation. The causal relationships between environmental factors, bacterioplankton community composition and nitrogen fractions were determined during flood season. We found that concentrations of nitrogen fractions decreased significantly when the floodgates were opened, while the concentrations of total nitrogen (TN) and NO₃^- increased when the floodgates had been shut for a period. Further, we proposed a possible mechanism that the influence of floodgates operation on nitrogen transformation was largely mediated through changes in dissolved organic matter, dissolved oxygen and bacterioplankton community composition as revealed by structural equation modeling (SEM). We conclude that floodgates operation has a high risk for future eutrophication of downstream watershed, although it can reduce nitrogen content temporarily. Therefore, the environmental impacts of floodgates operation should be carefully evaluated before the floodwaters were discharged into downstream watershed.

Impact of phosphate-solubilizing bacteria inoculation methods on phosphorus transformation and long-term utilization in composting

This study aimed to assess the effect of phosphate-solubilizing bacteria (PSB) application and inoculation methods on rock phosphate (RP) solubilization and bacterial community during composting. The results showed that PSB inoculation in different stages of composting, especially both in the beginning and cooling stages, not only improved the diversity and abundance of PSB and bacterial community, but also distinctly increased the content of potential available phosphorus. Redundancy analysis indicated that the combined inoculation of PSB in the initial stage with higher inoculation amount and in the cooling stage with lower inoculation amount was the best way to improve the inoculation effect and increase the solubilization and utilization of RP during composting. Besides, we suggested three methods to improve phosphorus transformation and long-term utilization efficiency in composts based on biological fixation of phosphates by humic substance and phosphate-accumulating organisms.

Effect of actinobacteria agent inoculation methods on cellulose degradation during composting based on redundancy analysis

In this study, actinobacteria agent including Streptomyces sp. and Micromonospora sp. were inoculated during chicken manure composting by different inoculation methods. The effect of different treatments on cellulose degradation and the relationship between inoculants and indigenous actinobacteria were investigated during composting. The results showed that inoculation in different stages of composting all improved the actinobacteria community diversity particularly in the cooling stage of composting (M3). Moreover, inoculation could distinctly accelerate the degradation of organic matters (OM) especially celluloses. Redundancy analysis indicated that the correlation between indigenous actinobacteria and degradation of OM and cellulose were regulated by inoculants and there were significant differences between different inoculation methods. Furthermore, synergy between indigenous actinobacteria and inoculants for degradation of OM and cellulose in M3 was better than other treatments. Conclusively, we suggested an inoculation method to regulate the indigenous actinobacteria based on the relationship between inoculants and indigenous actinobacteria and degradation content.