Limitation of spatial distribution of ammonia-oxidizing microorganisms in the Haihe River, China, by heavy metals

Comparative analysis of water quality prediction performance based on LSTM in the Haihe River Basin, China

As the most water shortage and water polluted area in China, the water quality prediction is of utmost needed and important in Haihe River Basin for its water resource management. The long short-term memory (LSTM) has been a widely used tool for water quality forecast in recent years. The performance and adaptability of LSTM for water quality prediction of different indicators needs to be discussed before it adopted in a specific basin. However, literature contains very few studies on the comparative analysis of the various prediction accuracy of different water quality indicators and the causes, especially in Haihe River Basin. In this study, LSTM was employed to predict biochemical oxygen demand (BOD), permanganate index (COD_Mn), dissolved oxygen (DO), ammonia nitrogen (NH₃-N), total phosphorus (TP), hydrogen ion concentration (pH), and chemical oxygen demand digested by potassium dichromate (COD_Cr). According to results under 24 different input conditions, it is demonstrated that LSTMs present better predicting on BOD, COD_Mn, COD_Cr, and TP (median Nash-Sutcliffe efficiency reaching 0.766, 0.835, 0.837, and 0.711, respectively) than NH₃-N, DO, and pH (median Nash-Sutcliffe efficiency of 0.638, 0.625, and 0.229, respectively). Besides, the performance of LSTM to predict water quality is linearly related to the maximum value of temporal autocorrelation and cross-correlation coefficients of water quality indicators calculated by maximal information coefficient with the coefficients of determination of 0.79 to approximately 0.80. This study would provide new knowledge and support for the practical application and improvement of the LSTM in water quality prediction.

Ecological and health risk assessments and water quality criteria of heavy metals in the Haihe River

Heavy metal pollution is an issue of wide concern owing to the toxic and bioaccumulative properties of many heavy metals and their tendencies to persist in the environment. The Haihe River is an important river in the Beijing-Tianjin-Hebei region, and heavy metal pollution of the basin has attracted considerable attention. This study determined the concentrations of 14 heavy metals (As, Ba, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Se, Sn, and Zn) in water, sediments, and fish samples from the Haihe River basin. The results showed that the concentrations ranged from 0.08 μg L^-1 to 60.49 μg L^-1 in water, 0.11 mg kg^-1 to 229.20 mg kg^-1 in sediments, and 0.01 mg kg^-1 to 11.72 mg kg^-1 in fish. We derived the ambient water quality criteria (AWQC) value of each heavy metal with respect to human health, and then performed a comprehensive risk assessment according to the native parameters. The human health AWQC values for the assessed 14 heavy metals ranged from 0.16 μg L^-1 to 726.53 μg L^-1. The health risks posed by As, Cr, Hg, and Sb and the ecological risks associated with Ni, Cu, Cr, Zn, Cd, Co, Hg, and Sn were found to be issues of concern. The results of a sensitivity analysis revealed that the highest contributing parameter was i) the concentration in water (Cw) for Cd, Co, Mn, Sb, and Sn; ii) the intake rate of water (IR_w) for As, Ba, Cr, Hg, Ni, Pb, and Se; and iii) the concentration in fish (C_f) for Cu and Zn. The results of this research could contribute to the information required for water quality assessments and the development of water quality standards.

Contribution of heavy metal in driving microbial distribution in a eutrophic river

Urban rivers represent an important source of freshwater. Accelerated urban development has resulted in imbalances in the water ecological environment and even eutrophication. Moreover, both natural and anthropogenic sources result in frequent heavy metal pollution in urban rivers. However, the combined impact of eutrophication and heavy metal pollution on the diversity and structure of the river microbial communities has not been adequately addressed. The microbial community distribution and predicted functions were examined in six water and sediment samples from the Laojingshui (LJS) River using metagenomic sequencing. The results showed that there were distinct differences in the microbial composition along the river. Redundancy analysis (RDA) revealed that the redox potential (Eh) was the most influential factor, explaining 76.5% of the variation (p = 0.002), and the heavy metals Zn and Cu explained 4.5 and 3.9%, respectively (p < 0.05). The results revealed that high nitrogen and phosphorus concentrations may have affected the proliferation of opportunistic plant species, such as Eichhornia crassipes, but Eh and heavy metals may have had greater impacts than N and P on the microorganisms in the water and sediment. The sensitivities of Deltaproteobacteria, Acidobacteria, Gemmatimonadetes and Nitrospira were most significant under Zn and Cu contamination when accompanied by eutrophic conditions. The expression ratio of the CYS (Cystain) gene might explain why the spatial distribution of each metal differed. This study suggests that heavy metals in eutrophication water continue to be the main factors determining the composition of microbial community, so the treatment of eutrophic water still needs to attach great importance to the complex pollution of heavy metals.

Three kinds of ammonia oxidizing microorganisms play an important role in ammonia nitrogen self-purification in the Yellow River

To develop the microbial resources of the Yellow River, seven water samples were collected along the Lanzhou region of the river from upstream to downstream for testing. Analysis of various physico-chemical indexes was conducted, and key parameters influencing the water quality were selected through principal component analysis, after which the decisive factors impacting water quality were determined by correlation and regression analysis. The results indicated that (1) DO, NH₃-N, NO₂^--N, TN, TC, As, Cr⁶⁺ and Pb were the main physico-chemical factors influencing water quality in the Lanzhou region, with NH₃-N having the greatest effect. (2) Ammonia-oxidizing microorganisms [ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), and anaerobic ammonia-oxidizing bacteria (AMX)] were found to mediate the transformation of NH₃-N in the studied section. AOA was the primary microbe community among the two aerobic ammonia-oxidizing microorganisms (AOA and AOB) in the Yellow River. (3) Phylogenetic analysis showed that there were some known groups, and there were still many unknown species in the water of the studied section, especially within the AMX population. (4) Correlation analysis revealed that AOA has strong adaptability to unhealthy environments, and that some environmental factors (higher concentrations of carbon, nitrogen and some heavy metals) could increase the AOA gene abundance. Overall, these results suggested there are rich ammonia-oxidizing microbial resources, especially AOA, in the Lanzhou section of the Yellow River, which have the potential for application in nitrogen sewage treatment.

Response of ammonia-oxidizing archaea to heavy metal contamination in freshwater sediment

It has been well-documented that the distribution of ammonia-oxidizing bacteria (AOB) and archaea (AOA) in soils can be affected by heavy metal contamination, whereas information about the impact of heavy metal on these ammonia-oxidizing microorganisms in freshwater sediment is still lacking. The present study explored the change of sediment ammonia-oxidizing microorganisms in a freshwater reservoir after being accidentally contaminated by industrial discharge containing high levels of metals. Bacterial amoA gene was found to be below the quantitative PCR detection and was not successfully amplified by conventional PCR. The number of archaeal amoA gene in reservoir sediments were 9.62 × 10²-1.35 × 10⁷ copies per gram dry sediment. AOA abundance continuously decreased, and AOA richness, diversity and community structure also considerably varied with time. Therefore, heavy metal pollution could have a profound impact on freshwater sediment AOA community. This work could expand our knowledge of the effect of heavy metal contamination on nitrification in natural ecosystems.

Response of freshwater sediment archaeal community to metal spill

Archaea play an important role in the biogeochemical cycling of elements in the environment. Heavy metals are ubiquitous pollutants in the environment. Previous studies have revealed a considerable influence of metal pollution on the archaeal community, but the short-term response of the archaeal community to metal pollution remains unclear. Hence, the present study investigated the short versus long-term responses of overall archaeal communities in freshwater sediments after exposure to accidental metal pollution caused by the discharge of heavy metal-containing wastewater from an indium-producing factory. Quantitative PCR was used to determine the archaeal abundance, while Illumina MiSeq sequencing was applied to characterize the diversity and structure of the archaeal community. The abundance (2.47 × 10⁵-1.55 × 10⁸ archaeal 16S rRNA gene copies per gram dry sediment), diversity (Shannon diversity index = 2.49-4.45) and structure of overall archaeal community illustrated a drastic temporal change. The archaeal communities mainly comprised the phyla Euryarchaeota, Thaumarchaeota and Bathyarchaeota. The exposure to metal pollution induced an increase in the proportion of Euryarchaeota but lowered the proportion of Thaumarchaeota. The accidental metal pollution exerted a profound impact on the archaeal community in freshwater sediment. This study could contribute our understanding of the short versus long-term response of archaeal communities to metal pollution.

Ecological risks posed by ammonia nitrogen (AN) and un-ionized ammonia (NH3) in seven major river systems of China

Previous studies showed that continuous exposure to ammonia nitrogen (AN) contributed to regional losses of benthic invertebrate diversity in China. Yet, the overall ecological risk of AN to aquatic organisms in major riverine systems of China has not been appropriately studied. Our research then investigated temporal (seasonally/yearly) and spatial distributions of AN and un-ionized ammonia (NH₃) in major Chinese river basins using historic data generated between 2007 and 2014, and developed risk assessment criteria. Our results showed that the highest average AN concentrations occurred during winter (0.82-2.76 mg/L) and the lowest during summer (0.36-0.78 mg/L). NH₃ exhibited the opposite trend with the highest average concentrations mostly observed during spring (15.13-92.84 μg/L) and the lowest concentrations mainly during winter (10.53-45.43 μg/L). Both AN and NH₃ concentrations steadily increased and reached maximum levels in 2008 (AN: 1.22 mg/L and NH₃: 50.65 μg/L), and then decreased. Temporal trends showed that the Yellow, Hai, and Huai river basins had the highest AN and NH₃ concentrations. Subsequently, conventional (hazard quotients) and probabilistic (joint probability curves) methods were applied to assess the hazards and risks posed by AN and NH₃. The results showed that the probability of exceeding the acute toxicity threshold for 5% of species (exposed to AN or NH₃) was less than 13.3% and gradually decreased over time. To protect aquatic organisms, an acute criterion of 51.4 μg NH₃/L and a chronic criterion of 1.14 mg AN/L at pH = 7.5, 20 °C were developed and are recommended for future risk assessment studies.

The cycle of nitrogen in river systems: sources, transformation, and flux

Nitrogen is a requisite and highly demanded element for living organisms on Earth. However, increasing human activities have greatly altered the global nitrogen cycle, especially in rivers and streams, resulting in eutrophication, formation of hypoxic zones, and increased production of N2O, a powerful greenhouse gas. This review focuses on three aspects of the nitrogen cycle in streams and rivers. We firstly introduce the distributions and concentrations of nitrogen compounds in streams and rivers as well as the techniques for tracing the sources of nitrogen pollution. Secondly, the overall picture of nitrogen transformations in rivers and streams conducted by organisms is described, especially focusing on the roles of suspended particle-water surfaces in overlying water, sediment-water interfaces, and riparian zones in the nitrogen cycle of streams and rivers. The coupling of nitrogen and other element (C, S, and Fe) cycles in streams and rivers is also briefly covered. Finally, we analyze the nitrogen budget of river systems as well as nitrogen loss as N2O and N2 through the fluvial network and give a summary of the effects and consequences of human activities and climate change on the riverine nitrogen cycle. In addition, future directions for the research on the nitrogen cycle in river systems are outlined.

Effects of toxic Microcystis genotypes on natural colony formation and mechanism involved

The cyanobacterium Microcystis occurs as colonies of different sizes with varying abundance of toxic genotypes versus non-toxic genotypes under natural conditions. To investigate the effects of toxic Microcystis genotypes on natural colony formation, samples collected from the mainstream of Haihe River from July to October 2015 were sieved into four colony classes with sizes of <8 μm, 8-20 μm, 20-90 μm and >90 μm. Each colony size class was analyzed for the proportion of toxic Microcystis genotypes, and microcystins (MCs) cellular production of toxic genotypes. The results showed the smallest size class of Microcystis colonies (<8 μm) showed the lowest proportion of toxic genotypes and the highest MC-RR and MC-YR cellular production. With the increasing colony sizes, the proportion of toxic Microcystis genotypes increased but the MC-RR and MC-YR cellular production decreased. A negative correlation between the MCs cellular production and the proportion of toxic genotypes was observed in all four colony size classes, suggesting that the less there were toxic Microcystis cells able to produce MCs, the more each toxic cell needed to produce that molecule. Toxic Microcystis played an important role in the colony formation in natural waters via producing MCs.

Nitrogen mineralization and geochemical characteristics of amino acids in surface sediments of a typical polluted area in the Haihe River Basin, China

Studies of nitrogen mineralization and diagenetic status of organic matter evaluated by total hydrolysable amino acids (THAAs) were designed to test the hypothesis that nitrogen mineralization in sediments was a potential source of ammonium in strongly artificially disturbed rivers such as the Ziya River watershed. Ammonium and organic nitrogen in both water and sediment samples were the major forms of nitrogen in the watershed. NH3-N was significantly correlated with organic nitrogen in both water (R = 0.823, P < 0.01) and sediments (R = 0.787, P < 0.01). Organic nitrogen with an average content of 3,275.21 ± 1,476.10 mg · kg(-1), accounted for 82.73 % of total nitrogen (TN) in sediments. Organic nitrogen was a potential source of ammonia release into overlying water. Nitrogen mineralization experiments showed that accumulated dissolved inorganic nitrogen ranged from 326.15 to 545.72 mg · kg(-1) and accumulated NH3-N ranged from 320.95 to 533.93 mg · kg(-1). Most of the mineralized nitrogen was NH3-N ( approximately 98.17%) and mineralized nitrogen in sediments ranged from 6.20 to 22.10% of TN. Twenty amino acids were detected, accounting for 45.70 % of organic nitrogen. Protein amino acids, accounting for 89.22% of THAAs, were the dominant THAAs in sediments. The ratio of L-glutamic acid to γ-aminobutyric acid and degradation index showed that the organic matter was poorly degraded and presented a high potential risk of ammonium mineralization.