Research trends and hotspots related to ammonia oxidation based on bibliometric analysis

Bibliometric analysis of residual cardiovascular risk: trends and frontiers

BACKGROUND

The presence of residual cardiovascular risk is an important cause of cardiovascular events. Despite the significant advances in our understanding of residual cardiovascular risk, a comprehensive analysis through bibliometrics has not been performed to date. Our objective is to conduct bibliometric studies to analyze and visualize the current research hotspots and trends related to residual cardiovascular risk. This will aid in understanding the future directions of both basic and clinical research in this area.

METHODS

The literature was obtained from the Web of Science Core Collection database. The literature search date was September 28, 2022. Bibliometric indicators were analyzed using CiteSpace, VOSviewer, Bibliometrix (an R package), and Microsoft Excel.

RESULT

A total of 1167 papers were included, and the number of publications is increasing rapidly in recent years. The United States and Harvard Medical School are the leading country and institution, respectively, in the study of residual cardiovascular risk. Ridker PM and Boden WE are outstanding investigators in this field. According to our research results, the New England Journal of Medicine is the most influential journal in the field of residual cardiovascular risk, whereas Atherosclerosis boasts the highest number of publications on this topic. Analysis of keywords and landmark literature identified current research hotspots including complications of residual cardiovascular risk, risk factors, and pharmacological prevention strategies.

CONCLUSION

In recent times, global attention toward residual cardiovascular risk has significantly increased. Current research is focused on comprehensive lipid-lowering, residual inflammation risk, and dual-pathway inhibition strategies. Future efforts should emphasize strengthening international communication and cooperation to promote the comprehensive evaluation and management of residual cardiovascular risk.

Prognosis of coronary heart disease after percutaneous coronary intervention: a bibliometric analysis over the period 2004-2022

BACKGROUND

As the complexity and diversity of the percutaneous coronary intervention (PCI) are being explored and reported, burgeoning research has progressed in this field. However, there is no comprehensive analysis available on PCI-related studies published in the literature. This study aimed to analyze and visualize the changes of scientific output regarding prognosis of coronary heart disease (CHD) after PCI over the past 20 years and to reveal the knowledge domain and development trends in this field by using CiteSpace software.

METHODS

Relevant articles published over the period 2004-2022 were retrieved from the Web of Science Core Collection database. After manual selection, qualified documents were included and recorded with the information of their title, abstract, keyword, author, descriptor, citation, identifier, publishing year and publishing organization. We transferred the data to CiteSpace V5.8.R2 (Version 5.8.R2) to draw knowledge maps and to conduct co-occurrence analysis, cluster analysis, timeline analysis, burst term detection and citation analysis.

RESULTS

A total of 14,699 literature records were found relating prognosis of CHD after PCI in the past 20 years (2004-2022), including 14,212 original articles and reviews, and they were published in 153 different journals. Publication production has increased annually and a total of 1182 authors, 796 institutes and 147 countries have contributed to these publications. Moreover, the most representative author was Gregg W Stone from the CardioVascular Research Foundation (CVRF) with 368 publications, whose team mainly focused on exploring the efficacy and safety of revascularization and the characteristics of susceptible population. The global productivity ranking was led by the USA with 3326 published papers, followed by Italy (n = 1355), Japan (n = 1080), China (n = 1075) and Germany (n = 937). And the keywords of these publications were "percutaneous coronary intervention" (n = 2271), "outcome" (n = 1756), "mortality" (n = 1730) and "impact" (n = 1334). Other commonly-used words were "predictor" (n = 1324), "intervention" (n = 1310), "angioplasty" (n = 1299), "risk" (n = 1144), "acute myocardial infarction" (n = 1136) and "artery disease" (n = 1098). Cluster analysis showed that 15 high connected clusters were generated with a modularity Q of 0.831 and a weighted mean silhouette of 0.9388 by applying the log-likelihood ratio algorithm, and the top 5 clusters were #0 optical coherence tomography, #1 dual antiplatelet therapy, #2 bleeding, #3 clopidogrel and #4 thrombus aspiration. Furthermore, the frontiers in the field of prognosis of CHD after PCI mainly involved "decision making", "reperfusion", "angioplasty", "balloon", "unstable angina", "dual antiplatelet therapy", "cardiac surgical score", "restenosis", "reperfusion", "thrombolytic therapy", etc. CONCLUSIONS: To sum up, efficacy and safety of different types of stents, the risk factors of restenosis and thrombotic events after PCI, early risk assessment, and secondary prevention and complications of patients with CHD after PCI were research hotspots and frontier topics in the area by bibliometric analysis. The results could provide a comprehensive overview of the research hotspots and frontier topics relating prognosis of CHD after PCI, promoting a better understanding of the knowledge domain and development trends in this field during the past 20 years.

Visual analysis of greenhouse gas emissions from sewage treatment plants based on CiteSpace: from the perspective of bibliometrics

With the global reduction actions of greenhouse gas (GHG) emissions, environmental facilities, including sewage treatment plants (STPs), need to reduce pollutants while minimizing GHG emissions. Therefore, more and more publications revealed the formation mechanism of GHGs in STPs and committed to finding better reduction schemes. From the perspective of bibliometrics, this study used CiteSpace to conduct quantitative and visual analysis based on 1,543 publications retrieved from Web of Science between 2000 and 2021 around the world. We have systematically evaluated the structure, development trend, hot spots, and research frontier in the field of GHG emissions from STPs and compared with the contents of top journals to verify the scientificity of the analysis. The results show that the number of publications has increased year by year, and the networks of authors and institutions show a strong correlation. Among them, the clusters of nitrous oxide, anaerobic digestion, and life cycle assessment (LCA) started earlier and received extensive attention, which derived other clusters in the research process. With the development of the field, researchers have gradually changed from single water treatment facilities to multi-carriers that can realize energy regeneration and utilization simultaneously. Accordingly, the GHG reduction of STPs through energy regeneration and resource recovery has become a hot point and frontier direction, which also challenges the breakthroughs in relevant technologies. Furthermore, it provides scientific support for the formulation of relevant incentive policies and economic subsidy systems, so as to alleviate the pressure of global warming and realize the sustainable development of STPs concurrently.

Hotspots and trends of biological water treatment based on bibliometric review and patents analysis

In order to reveal the hotspots and trends of biological water treatment from the perspectives of scientific and technological innovation, both of the bibliometric review and patents analysis were performed in this study. The Web of Science Core Collection database and Derwent Innovation Index database recorded 30023 SCI papers and 50326 patents, respectively were analyzed via information visualization technology. The results showed that China ranked the first in both papers and patents, while the United States and Japan had advantages in papers and patents, respectively. It was concluded through literature metrology analysis that microbial population characteristics, biodegradation mechanism, toxicity analysis, nitrogen and phosphorus removal and biological treatment of micro-polluted wastewater were the research hotspots of SCI papers. Activated sludge process and anaerobic-aerobic combined process were the two mainstream technologies on the basis of patent technology classification analysis. Technology evolution path of biological water treatment was also elucidated in three stages based on the citation network analysis. Furthermore, the future directions including research on the law of interaction and regulation of biological phases and pollutants and the technology innovations towards the targeted biotransformation or selective biodegradation of pollutants and resource reuse of wastewater were prospected.

Study on Intestinal Flora and Asthma: Knowledge Graph Analysis Based on CiteSpace (2001-2021)

Asthma is a common chronic inflammatory disease of the airway. Intestinal flora, a significant risk factor for asthma, has become a widespread concern in the pathogenesis of asthma. To review the literature related to intestinal flora in asthma, summarize research direction, and report trends, this study used CiteSpace to perform bibliometric statistics and analysis on the research papers of intestinal flora and asthma collected in the Web of science core collection from 2001 to 2021. Eventually, a total of 613 articles were included. The results demonstrated that research on gut flora and asthma continued to heat up, with article numbers increasing, especially in the last decade. Moreover, analysis of the keywords showed that the research topics of intestinal flora and asthma range from confirming the link between intestinal flora and asthma to investigating mechanisms and then to asthma treatment. According to the summary of research hotspots, we expand on three emerging issues that require attention in the intestinal flora and asthma research, including (regulatory T)Treg cells, probiotics, and chain fatty acid. Evidence illustrated that Treg cells play a crucial role in the pathogenesis of asthma caused by dysbiosis of the gut flora. Furthermore, in contrast to probiotic supplements, which do not reduce the risk of developing asthma, short-chain fatty acids supplements do. Overall, the research direction in the field of intestinal flora and asthma has recently evolved from macro to micro with depth broadened. As a robust scientific evaluation, our study provided a comprehensive overview of the area, particularly for research focus, which could more precisely direct scholars on future research and clinical diagnosis, therapy, and individualized prevention.

Research hotspot and trend analysis in the diagnosis of inflammatory bowel disease: A machine learning bibliometric analysis from 2012 to 2021

Aims

This study aimed to conduct a bibliometric analysis of the relevant literature on the diagnosis of inflammatory bowel disease (IBD), and show its current status, hot spots, and development trends.

Methods

The literature on IBD diagnosis was acquired from the Science Citation Index Expanded of the Web of Science Core Collection. Co-occurrence and cooperation relationship analysis of authors, institutions, countries, journals, references, and keywords in the literature were carried out through CiteSpace software and the Online Analysis platform of Literature Metrology. At the same time, the relevant knowledge maps were drawn, and the keywords cluster analysis and emergence analysis were performed.

Results

14,742 related articles were included, showing that the number of articles in this field has increased in recent years. The results showed that PEYRIN-BIROULET L from the University Hospital of Nancy-Brabois was the author with the most cumulative number of articles. The institution with the most articles was Mayo Clin, and the United States was far ahead in the article output and had a dominant role. Keywords analysis showed that there was a total of 818 keywords, which were mainly focused on the research of related diseases caused or coexisted by IBD, such as colorectal cancer and autoimmune diseases, and the diagnosis and treatment methods of IBD. Emerging analysis showed that future research hotspots and trends might be the treatment of IBD and precision medicine.

Conclusion

This research was the first bibliometric analysis of publications in the field of IBD diagnosis using visualization software and data information mining, and obtained the current status, hotspots, and development of this field. The future research hotspot might be the precision medicine of IBD, and the mechanism needed to be explored in depth to provide a theoretical basis for its clinical application.

Knowledge mapping of planetary boundaries based on bibliometrics analysis

The planetary boundaries concept has triggered a vast amount of pure and applied scientific research, as well as policy and governance activities globally. Indeed, it has rapidly become a centerpiece of sustainability study. It is crucial to review the scientific state of the planetary boundaries (PB) concept systematically. However, there is a lack of research on drawing a scientific investigation map of planetary boundaries. Therefore, to clarify the spatial and temporal distribution characteristics, research hotspots, and frontiers of planetary boundaries, a scientometric analysis was performed based on 530 academic publications on planetary boundaries from 2009 to 2021. This paper conducted the analysis by visualizing the social network, dual-map overlay, co-cited references, structure variation article, and co-occurrence keywords with CiteSpace. The results show that as a new achievement and paradigm in sustainable development research, the planetary boundaries framework is gradually getting global attention and promotion, which has increasingly become an interdisciplinary hot research topic. The most productive authors and institutions are concentrated in England, the USA, Germany, and Sweden. Relevant articles were mainly published in journals focusing on ecology, earth, marine, veterinary, animal, economics, and politics. In addition, we summarized four predominant research themes by clustering keywords: the calculation of single boundary threshold and present value, the integration with assessment methods such as life cycle assessment and footprint families, the downscaling of planetary boundaries, and the expansion to economic and social domains. For scholars who are interested in this topic, this paper would be a useful reference and guideline.

Knowledge domain and emerging trends in diabetic cardiomyopathy: A scientometric review based on CiteSpace analysis

Objective

To review the literature related to diabetic cardiomyopathy (DCM), and investigate research hotspots and development trends of this field in the relevant studies based on CiteSpace software of text mining and visualization in scientific literature.

Methods

The relevant literature from the last 20 years was retrieved from the Web of Science (WoS) Core Collection database. After manual selection, each document record includes title, authors, year, organization, abstract, keywords, citation, descriptors, and identifiers. We imported the downloaded data into CiteSpace V (version 5.8.R2) to draw the knowledge map and conduct cooperative network analysis, cluster analysis, burst keyword analysis, and co-citation analysis.

Results

After manual screening, there were 3,547 relevant pieces of literature published in the last 18 years (from 2004 to 2021), including 2,935 articles and reviews, which contained 15,533 references, and the number was increasing year by year. The publications of DCM were dedicated by 778 authors of 512 institutions in 116 countries. The People's Republic of China dominated this field (1,117), followed by the USA (768) and Canada (176). In general, most articles were published with a focus on “oxidative stress,” “heart failure,” “diabetic cardiomyopathy,” “dysfunction,” “cardiomyopathy,” “expression,” “heart,” “mechanism,” and “insulin resistance.” Then, 10 main clusters were generated with a modularity Q of 0.6442 and a weighted mean silhouette of 0.8325 by the log-likelihood ratio (LLR) algorithm, including #0 heart failure, #1 perfused heart, #2 metabolic disease, #3 protective effect, #4 diabetic patient, #5 cardiac fibrosis, #6 vascular complication, #7 mitochondrial dynamics, #8 sarcoplasmic reticulum, and #9 zinc supplementation. The top five references with the strongest citation bursts include “Boudina and Abel”, “Jia et al.”, “Fang et al.”, “Poornima et al.”, and “Aneja et al.”.

Conclusion

The global field of DCM has expanded in the last 20 years. The People's Republic of China contributes the most. However, there is little cooperation among authors and institutions. Overall, this bibliometric study identified the hotspots in DCM research, including “stress state,” “energy metabolism,” “autophagy,” “apoptosis,” “inflammation,” “fibrosis,” “PPAR,” etc. Thus, further research focuses on these topics that may be more helpful to identify, prevent DCM and improve prophylaxis strategies to bring benefit to patients in the near future.

Land quality evaluation for sustainable development goals: a structured review using bibliometric and social network analysis

This literature review focuses on land quality evaluation (LQE) and its effects on sustainable development through quantitative analysis and value-added information. In contrast to the traditional perspective, a structured review based on bibliometric indicators and social network analysis allows identifying hidden evidence for answering the following research questions: (i) What: What is the application of LQE? (ii) Which: Which sustainable development goals does the application contribute to? (iii) Why and how: What are the main applications and methods of each topic? (iv) Where: Where is the hotspot of the problem? What is the future research orientation of the topic? (v) How and when: How has the topic grown since 2000? Data investigation explores 4029 articles in 2000-2019 from four publishers. With the support of VOSviewer software, six clusters corresponding to six main applications of LQE are classified. Overlapping keywords in several clusters are resolved by the binary term frequency counter for the cluster preference determination. After conducting the data verification and editing process, a structured review is performed again with systematic research questions. This research offers a synthesis of traditional and novel quantitative analysis for literature review, which is comprehensive, accurate, and reliable.

Trends in research on corneal cross linking from 2001 to 2020: a bibliometric analysis

CLINICAL RELEVANCE

The research status and hotspots in the field of corneal cross linking (CXL) can benefit clinicians, researchers and the general public.

BACKGROUND

The purpose of this study was to map the publishing trend on CXL research and explore the research hotspots.

METHODS

A bibliometric analysis was performed using the Web of Science Core Collection to investigate the publishing trend on CXL research. VOSviewer was used to build the knowledge map to visualise the number of annual publications, distribution of countries and institutions, international cooperation, author productivity, source journals and research hotspots in the field of CXL.

RESULTS

A total of 2061 peer-reviewed articles on CXL research were collected from 2001 to 2020, and the annual research production increased over time. The United States was the country with the largest number of published articles, and the University of Zurich was the most active institution. Hafezi F published the largest number of articles on CXL, while Cornea was the journal with the largest number of studies on CXL. The most frequently cited references mainly focus on CXL in the treatment of keratoconus. The keywords were divided in 5 categories: 1) CXL mechanism, 2) ectasia diseases and refractive surgery, 3) corneal biomechanics, 4) efficacy evaluation, 5) treatment of infectious keratitis.

CONCLUSION

The quantity and quality of articles on CXL were evaluated using bibliometric techniques by extracting the data from the Web of Science Core Collection. The research hotspots could provide insights on CXL research, providing valuable information for clinicians to perform research in this field and find potential partners.

Mapping Research on Microbial Fuel Cells in Wastewater Treatment: A Co-Citation Analysis

Microbial fuel cells (MFCs) are promising technologies, aiming at treating different types of industrial and domestic wastewater. In recent years, more and more publications focusing on wastewater treatment have been published. Based on the retrieval of publications from Web of Science Core Collection database, the new emerging trends of microbial fuel cells in wastewater treatment was evaluated with a scientometric analysis method from 1995 to 2020. All publications downloaded from (WOS) were screened by inclusion criteria, and 2233 publications were obtained for further analysis. Document co-citation and burst detection of MFCs in wastewater treatment were analyzed and visualized by software of CiteSpace. Our study indicated that “Environmental Science” is the most popular discipline, while the journal of Bioresource Technology published the greatest quantity of articles in the field of MFCs applied wastewater treatment. China and the Chinese Academy of Science are the most productive country and institution, respectively. “Azo dye” has become the new research topic, which indicates the application area and the development of MFCs. The performance of MFCs for wastewater treatment has been widely discussed. The findings of this study may ameliorate the researcher in seizing the frontier of MFCs in wastewater treatment.

Blockchain technology in the smart city: a bibliometric review

Blockchain can function as a foundational technology with numerous applications in smart cities. The objective of this paper is twofold. First, it provides a detailed overview of the extant literature on blockchain applications in smart cities; second, it reveals the trends and suggests future research directions for scholars who wish to contribute to this rapidly growing field. We conducted a bibliometric review using a keyword co-occurrence network and article co-citation analysis. The analysis includes the assessment of 148 articles published between 2016 and 2020 in 76 academic journals. The review results demonstrate that the number of articles devoted to the study of blockchain applications and smart cities has increased exponentially in recent years. More importantly, the research identifies some of the most influential studies in this area. The paper discusses trends and highlights the challenges related to the deployment of blockchain in smart cities. To the authors’ best knowledge, this represents the first study to review the literature from leading journals on blockchain applications in smart cities using bibliometric techniques.

Bibliometric Analysis of Bronchopulmonary Dysplasia in Extremely Premature Infants in the Web of Science Database Using CiteSpace Software

Objectives: To review the literature related to bronchopulmonary dysplasia in extremely pre-mature infants, summarize research direction, and report trends. Methods: CiteSpace is a Java application which supports visual exploration with knowledge discovery in bibliographic databases. Relevant articles from 2008 to 2020 were retrieved from the Web of Science Core Collection database, and we extracted the following data: title, abstract, year, keywords, author, organization, journal and cited literature. We downloaded the data into CiteSpace (version 5.7.R3) to summarize countries, institutions, journals, and authors. We visualized the data with a knowledge map, collaborative network analysis, cluster analysis, and burst keyword analysis. Results: We identified 610 articles on bronchopulmonary dysplasia in extremely pre-mature infants. The United States had the most articles on this topic (302 articles), followed by Canada (49 articles) and Germany (44 articles). The top three institutions, high-yield journals, and authors were all from the United States. The most common keywords were neurodevelopmental disorders, active perinatal care, mechanical ventilation, inflammation, pulmonary hypertension, low-dose hydrocortisone, development, and patent ductus arteriosus. Conclusions: This study illustrates the trends and frontiers in the study of bronchopulmonary dysplasia in extremely pre-mature infants. The current research direction is to identify the risk factors in developing bronchopulmonary dysplasia in extremely pre-mature infants.

Whether glycine betaine improves the thermotolerance of mesophilic anammox consortia

While the application of mesophilic anammox process is currently the state of the art, the feasibility of a thermophilic anammox bioprocess is still unclear. In this study, we investigate whether glycine betaine (GB) addition can enhance the thermotolerance of mesophilic anammox biomass in the upflow anaerobic sludge blanket (UASB) reactors fed with synthetic wastewater at a nitrogen loading of approximately 4 kg N m^-3 d^-1. The results showed that during a long-term operation at 45°C with GB (0, 0.1, 1, 2 mM) addition, anammox performance became worse with the final effluent concentrations of NO₂ ^-N of 145 ± 11.6 mg L^-1 and nitrogen removal efficiency decreased from 92.3-6.9%. Specific anammox activity decreased from 392.1 ± 12.1-6.0 ± 0.8 mg N g^-1 VSS d^-1, which were not significantly higher than those in the control reactor. The content of heme c showed a stronger downward trend in T₁ (with GB addition) than in the control reactor T₀. The qPCR results showed that the relative abundance of Candidatus Kuenenia decreased in both the experimental (from 53.5-28.8%) and control reactors (from 54.1-35.1%). Overall, continuous addition of exogenous GB did not improve the thermotolerance of mesophilic anammox consortia at 45°C.

Research Trends and Hotspot Analysis of Conjunctival Bacteria Based on CiteSpace Software

OBJECTIVE

To sort out the literature related to conjunctival bacteria and summarize research hotspots and trends of this field.

MATERIALS AND METHODS

The relevant literature data from 1900 to 2019 was retrieved from the Web of Science Core Collection database. After manual selection, each document record includes title, author, keywords, abstract, year, organization, and citation. We imported the downloaded data into CiteSpace V (version 5.5R2) to draw the knowledge map and conduct cooperative network analysis, discipline and journal analysis, cluster analysis, and burst keyword analysis.

RESULTS

After manual screening, there were 285 relevant papers published in the last 28 years (from 1991 to 2019), and the number is increasing year by year. The publications of conjunctival bacteria were dedicated by 1381 authors of 451 institutions in 56 countries/regions. The United States dominates this field (82 literatures), followed by Germany (23 literatures) and Japan (23 literatures). Overall, most cited papers were published with a focus on molecular biology, genetics, nursing, and toxicology. Most papers fall into the category of ophthalmology, veterinary sciences, and pharmacology and pharmacy. The only organized cluster is the "postantibiotic effect," and the top 5 keywords with the strongest citation bursts include "postoperative endophthalmiti(s)," "infectious keratoconjunctiviti(s)," "conjunctiviti(s)," "resistance," and "diversity".

CONCLUSION

The global field of conjunctival bacteria has expanded in the last 28 years. The United States contributes most. However, there are little cooperation among authors and institutions. Overall, this bibliometric study organized one cluster, "postantibiotic effect", and identified the top 5 hotspots in conjunctival bacteria research: "postoperative endophthalmiti(s)," "infectious keratoconjunctiviti(s)," "conjunctiviti(s)," "resistance," and "diversity". Thus, further research focuses on these topics that may be more helpful to prevent ocular infection and improve prophylaxis strategies to bring a benefit to patients in the near future.

Identification, interactions, nitrogen removal pathways and performances of culturable heterotrophic nitrification-aerobic denitrification bacteria from mariculture water by using cell culture and metagenomics

The rapid expansion of aquaculture industry brings about significant environmental concerns, especially nitrogen pollution. Compared to nitrogen bioconversion implemented by the conventional autotrophic nitrifiers and anaerobic denitrifiers, bacteria capable of heterotrophic nitrification-aerobic denitrification (HNAD) in mariculture environments have yet to be well understood. In this study, twenty-five species of new halophilic HNAD bacteria were isolated and identified from mariculture water. By these strains co-cultured in the synthetic mariculture water (ammonia: 5 mg/L, C/N: 5, salinity: 30‰), microbial dynamic analysis showed that ammonia were mainly removed by dominant genera of Marinomonas, Marinobacterium, Halomonas, and Cobetia which simultaneously had positive correlations to total nitrogen removal. Metagenomic annotations revealed that inorganic-N was converted into gaseous-N and organic-N by these HNAD bacteria through nitrogen metabolism pathways of assimilation, partial nitrification, nitroalkane oxidation, nitrate/nitrite dissimilation reduction, and denitrification. Among them, due to the interspecific coexistence and cooperation, Marinomonas communis &Halomonas titanicae, Marinomonas communis &Cobetia marina, Marinomonas aquimarina &Halomonas titanicae, and Marinomonas aquimarina &Cobetia marina exhibited significantly better inorganic-N removal efficiency and stability. The four novel bacterial consortia could transform approximately 60% of initial ammonia into intracellular organic-N (18-20%) and gaseous-N (36-38%), which were significantly higher than those of their single strains. These findings will contribute to understanding and developing the culturable HNAD bacteria as promising candidates for nitrogen pollution control and water bioremediation in mariculture or other saline environments.

Visualization and analysis of mapping knowledge domain of oxidation studies of sulfide ores

The oxidation of sulfide ores is a common phenomenon. To better understand the current development and status of oxidation studies of sulfide ores (OSSO), a bibliometric analysis of OSSO was conducted by mapping the spatiotemporal distribution of the knowledge domain and the research focus using VOSviewer and Citespace tools. The data were derived from the Web of Science (WOS) core collection database from 2000 to 2018. Study emphases covered publication outputs, countries/regions, organizations, top journals, research focus and keyword co-occurrence network, and theme development. The results include the following findings: (1) The line of the 3-year moving average of publications (3-year MAP), h-index (3-year MAH), and authors (3-year MAA) increased from 2001 to 2018. Conversely, the h-index continuously declined. (2) Asia had the most publications, with 1052, followed by Europe, with 923, and America, with 767. China, the USA, and Australia are the most active countries. (3) The top 10 organizations with the most publications are five Chinese organizations and one organization from each of the following countries: Russia, Australia, France, and India. (4) Hydrometallurgy, Minerals Engineering, Ore Geology Reviews, Geochimica Et Cosmochimica Acta, and Economic Geology are among the top 10 journals that researchers are most concerned about. (5) Cooperation among different organization or different countries is the most effective way to produce the most influential papers. (6) The OSSO is still focused on the process of oxidation by using different methods and techniques. In future work, it is necessary to progress new methods to understand the process of self-heating and prevent spontaneous combustion disaster of sulfide ore which result from OSSO.

Genome-resolved metagenomics analysis provides insights into the ecological role of Thaumarchaeota in the Amazon River and its plume

Background

Thaumarchaeota are abundant in the Amazon River, where they are the only ammonia-oxidizing archaea. Despite the importance of Thaumarchaeota, little is known about their physiology, mainly because few isolates are available for study. Therefore, information about Thaumarchaeota was obtained primarily from genomic studies. The aim of this study was to investigate the ecological roles of Thaumarchaeota in the Amazon River and the Amazon River plume.

Results

The archaeal community of the shallow in Amazon River and its plume is dominated by Thaumarchaeota lineages from group 1.1a, which are mainly affiliated to Candidatus Nitrosotenuis uzonensis, members of order Nitrosopumilales, Candidatus Nitrosoarchaeum, and Candidatus Nitrosopelagicus sp. While Thaumarchaeota sequences have decreased their relative abundance in the plume, Candidatus Nitrosopelagicus has increased. One genome was recovered from metagenomic data of the Amazon River (ThauR71 [1.05 Mpb]), and two from metagenomic data of the Amazon River plume (ThauP25 [0.94 Mpb] and ThauP41 [1.26 Mpb]). Phylogenetic analysis placed all three Amazon genome bins in Thaumarchaeota Group 1.1a. The annotation revealed that most genes are assigned to the COG subcategory coenzyme transport and metabolism. All three genomes contain genes involved in the hydroxypropionate/hydroxybutyrate cycle, glycolysis, tricarboxylic acid cycle, oxidative phosphorylation. However, ammonia-monooxygenase genes were detected only in ThauP41 and ThauR71. Glycoside hydrolases and auxiliary activities genes were detected only in ThauP25.

Conclusions

Our data indicate that Amazon River is a source of Thaumarchaeota, where these organisms are important for primary production, vitamin production, and nitrification.

Nanomaterials in the Environment: Research Hotspots and Trends

Research on the field of nanomaterials in environment has continued to be a major area of interest in recent years. To present the up-to-date progress in this field, a bibliometric study is conducted to analyze 7087 related publications in the Science Citation Index (SCI) core collection of Web of Science based on the expanded SCI. These publications are identified through using representative keywords in the research directions environment of the Web of Science. This study finds that China and the United States dominate the field; one difference between them is that China issued more independent publications and the United States issued more cooperative publications. In addition, the number of the related publications in Asian countries has exceeded that of European and American ones. A Chinese institution, the Chinese Academy of Sciences, has an absolute dominance in this field. Traditional high-impact environmental journals have ruled this field. The number of publications in the Energy and Environmental Science field has gradually decreased. In addition, a co-citation analysis shows that previous studies in this field can be divided into four major branches, and that graphene oxide and nano-inorganic particles are increasingly becoming research hotspots.

Abundance and community composition of comammox bacteria in different ecosystems by a universal primer set

Complete ammonia oxidizing bacteria (CAOB) have been recognized as a new member of ammonia-oxidizing microorganisms (AOMs) due to its single-step nitrification capability. However, the abundance and diversity of CAOB in environmental ecosystems were still far from known owing to the lack of specific molecular marker. Herein, a universal primer set specifically targeting both clades of CAOB amoA gene with high specificity and coverage was successfully designed. Intriguingly, real-time quantitative PCR tests revealed that CAOB were ubiquitous and unexpectedly abundant in agricultural soils, river sediments, intertidal zones, drinking water and wastewater treatment systems. Phylogenetic analysis indicated that clade A existed in all the five types of ecosystems, while clade B were only detected in soil and sediment samples. Four sub-clusters were further classified within clade A, in which N. nitrosa cluster dominated CAOB amoA in activated sludge samples while the new recognized soil cluster was the primary constitute in soils. Moreover, the niche specialization between different CAOB species and the environmental conditions were supposed to be the primary driven force to shape the diversity and community of CAOB. This study provided a strong evidence in support of the ubiquities and high abundances of CAOB in various environmental ecosystems and highlighted the significance of including CAOB as the new member of AOMs to re-evaluate the biogeochemical nitrogen cycle.

The effects of graphene oxide on nitrification and N2O emission: Dose and exposure time dependent

With the extensive application of graphene oxide (GO), its leakage and release into wastewater treatment plants become inevitable. However, the toxicity of graphene oxide (GO) on nitrification process and the underlying mechanisms still remain unclear. In this study, the toxic effects of GO at concentration of 10 and 100 mg/L in 4 h and 10 days were evaluated with sealed reactors operated in sequencing batch mode. In the initial 4 h, both GO concentrations showed no negative effect on nitrogen conversion. However, the exposure to 100 mg/L GO significantly weakened the NH+ 4-N and NO- 2-N conversion capabilities and intensified the nitrous oxide (N₂O) generation after 10 days. Extracellular polymeric substance (EPS) analysis suggested that 100 mg/L GO decreased the protein content of the nitrifying activated sludge. Moreover, reactive oxygen species (ROS) level was promoted by 100 mg/L GO owing to the impaired endogenous antioxidant enzymes including superoxide dismutase (SOD) and catalase (CAT), which caused oxidative stress to bacteria. Finally, quantitative PCR results confirmed that nitrite-oxidizing bacteria (NOB) and complete ammonia oxidizing bacteria (CAOB) were more sensitive to GO, which was the primary cause for the significant promotion of N₂O generation in the high GO concentration. This study offered new insights in the toxicity of GO on nitrification and N₂O generation in the terms of dose and exposure time.

Global research trends in microbiome-gut-brain axis during 2009-2018: a bibliometric and visualized study

BACKGROUND

The pathways and mechanism by which associations between the gut microbiome and the brain, termed the microbiome-gut-brain axis (MGBA), are manifest but remain to be fully elucidated. This study aims to use bibliometric analysis to estimate the global activity within this rapidly developing field and to identify particular areas of focus that are of current relevance to the MGBA during the last decade (2009-2018).

METHODS

The current study uses the Scopus for data collection. We used the key terms "microbiome-gut-brain axis" and its synonyms because we are concerned with MGBA per se as a new concept in research rather than related topics. A VOSviewer version 1.6.11 was used to visualize collaboration pattern between countries and authors, and evolving research topics by analysis of the term co-occurrence in the title and abstract of publications.

RESULTS

Between 2009 and 2018, there were 51,504 published documents related to the microbiome, including 1713 articles related to the MGBA: 829 (48.4%) original articles, 658(38.4%) reviews, and 226 (13.2%) other articles such as notes, editorials or letters. The USA took the first place with 385 appearances, followed by Ireland (n = 161), China (n = 155), and Canada (n = 144).The overall citation h-index was 106, and the countries with the highest h-index values were the USA (69), Ireland (58), and Canada (43). The cluster analysis demonstrated that the dominant fields of the MGBA include four clusters with four research directions: "modeling MGBA in animal systems", "interplay between the gut microbiota and the immune system", "irritable bowel syndrome related to gut microbiota", and "neurodegenerative diseases related to gut microbiota".

CONCLUSIONS

This study demonstrates that the research on the MGBA has been becoming progressively more extensive at global level over the past 10 years. Overall, our study found that a large amount of work on MGBA focused on immunomodulation, irritable bowel syndrome, and neurodevelopmental disorders. Despite considerable progress illustrating the communication between the gut microbiome and the brain over the past 10 years, many issues remain about their relevance for therapeutic intervention of many diseases.

A Bibliometric Analysis of Research on Acid Rain

With the continuation of industrialization and urbanization, acid rain (AR) has aroused extensive concern because of its potential negative effects on ecosystems. However, analysis of the current status and development trends in AR research area has seldom been systematically studied. Therefore, we motivated to conduct a bibliometric analysis of AR publications (1900–2018) using HistCite and CiteSpace software programs. Compared to traditional reviews by experts, this study offers an alternative method to quantitatively analyze and visualize the development of AR field at a large time scale. The results indicated that the overall concern of AR research studies had increased from 1900 to 2018. The most productive country was the United States, while the institution with the most publications was Chinese Academy of Sciences. “Environmental Sciences” was the most popular subject category, Water, Air, and Soil Pollution was the dominant journal, and C.T. Driscoll was the most prominent author in AR field. There were three hotspots in the field of AR, including analyzing AR status and its control policies in Europe, the United States, and China in the past few decades, investigating the ecological consequences of AR on plant histological, physiological, and biochemical traits, as well as surface water and soil properties, and the model application for quantitatively assessing AR and its effects on terrestrial and aquatic ecosystems at regional scale. Further, “behavior”, “phosphorus”, “fractionation”, “soil acidification”, “corrosion”, “performance”, “recovery”, “rainwater”, “trace element”, and “surface water” have been emerging active topics in recent years. This study can help new researchers to find out the most relevant subject categories, countries, institutions, journals, authors, and articles, and identify research trends and frontiers in the field of AR.

Transcriptional activity and diversity of comammox bacteria as a previously overlooked ammonia oxidizing prokaryote in full-scale wastewater treatment plants

The discovery of complete ammonia oxidizing bacteria (CAOB) has fundamentally overturned the traditional recognition of nitrification. However, little was known about the transcriptional activity and diversity of the newly recognized ammonia oxidizing prokaryote in engineered ecosystems. To fill this gap, transcriptional investigations of CAOB amoA genes were carried out comparatively with the canonical ammonia oxidizing bacteria (AOB) and archaea (AOA) in eight full-scale wastewater treatment plants (WWTPs). Remarkably, qPCR results revealed the transcriptional levels of CAOB amoA gene were unexpectedly high in most of samples with the highest 24-fold that of AOB amoA, suggesting CAOB were actively participating in ammonia oxidation while they were previously overlooked. This result also well explained the confusing high abundances of genus Nitrospira which were frequently detected in WWTPs. Furthermore, phylogenetic analysis based on high throughput sequencing indicated the CAOB amoA gene sequences formed three well-supported clusters and Nitrospira nitrosa cluster accounted for 97% of all the retrieved sequences, which was supposed to be the dominant taxon of CAOB in the ammonia-intensive environment due to niche partitioning. This study highlighted the significance of including the newly discovered ammonia oxidizing bacterial member when assessing the nitrification process and ecological function in the future.

Newly designed primer pair revealed dominant and diverse comammox amoA gene in full-scale wastewater treatment plants

The discovery of complete ammonia oxidizing bacteria (CAOB) capable of performing the two-step nitrification process on their own has fundamentally upended our traditional perception. However, their environmental distribution and ecological significance in driving ammonia oxidation are still urgently awaited to be assessed. In this study, the diversity and abundance of CAOB amoA gene in wastewater treatment plants (WWTPs) were presented taking advantage of a newly designed primer pair specifically targeting CAOB amoA gene. Phylogenetic results demonstrated the novel amoA gene formed a clearly distinct cluster from the canonical amoA and pmoA genes. Among the five well-supported sub-clusters, Nitrospira nitrosa cluster accounted for 94.34% of all the currently retrieved sequences from WWTPs. More importantly, qPCR results demonstrated a remarkably high abundance of CAOB amoA gene, which were up to 182.7-fold more abundant than AOB amoA gene. This study provided new dimension and fundamental basis for future researches towards biogeochemical nitrogen cycle.

Research Trends and Hotspots Analysis Related to the Effects of Xenobiotics on Glucose Metabolism in Male Testes

This study aimed to integrate and analyze the existing studies and to explore research trends and hotspots related to the effects of xenobiotics on glucose metabolism in male testes. All articles were retrieved from the PubMed database, from an inception date up to 10 June 2017. CiteSpace software (version 5.1.R8 SE) was used for the co-word cluster analysis. A total of 165 eligible publications were included in this study. In 1949⁻1959, only two articles were published. After 1960, the number of articles increased steadily. These articles were published in 97 journals, in particular, in the Indian Journal of Experimental Biology (11 articles, 6.7%). Most of the authors (87.0%) only published one article. Only a few established research teams, mostly from the USA, worked consistently in this field. The main xenobiotics that had been studied were medicine and common environmental pollutants, e.g., gossypol, cadmium, di-n-butyl phthalate, and alpha-chlorohydrin. The hotspot keywords were Sertoli cell, lactate dehydrogenase, 6-phosphate dehydrogenase, oxidative stress, and glucose metabolism. The focus of research had been changed overtime. This is the first bibliometric study between xenobiotics and glucose metabolism in the male testes. The findings suggest that environmental pollutants have become a huge concern, and related research should be strengthened.

A scientometric review of emerging trends and new developments in agricultural ecological compensation

Agricultural ecological compensation has drawn an increasingly broad range of interest since early 1990s. In recent years, the volume of the literature grows rapidly. As a result, a systematic review of the diverse research field and its current trends becomes essential. This paper surveys the literature of agricultural ecological compensation between 1990 and 2016. Specifically, by employing CiteSpace information visualization software, we firstly identified the research hotspots and evolution path and then illustrated the frontier and developing trend of the domain in core and broader perspectives. It is found that the focus of the academic community has always been researches on the theoretical policy and application of the payment for agro-ecosystem services, agricultural ecological compensation based on contingent valuation method, and ecological compensation of farmland landscape and organic food production as well as willingness to accept/pay for land use and ecological protection. Meanwhile, we also found that, in recent years, qualitative research has received more and more attention in the field of agricultural ecological compensation, since global warming, agricultural carbon emissions, and other emerging environmental issues have aroused widespread concern of the people around the world. Moreover, we believed that more and more scholars will employ case study methodology to analyze agricultural ecological compensation in specific systems, regions, or circumstances in the future.

Unifying the global phylogeny and environmental distribution of ammonia-oxidising archaea based on amoA genes

Ammonia-oxidising archaea (AOA) are ubiquitous and abundant in nature and play a major role in nitrogen cycling. AOA have been studied intensively based on the amoA gene (encoding ammonia monooxygenase subunit A), making it the most sequenced functional marker gene. Here, based on extensive phylogenetic and meta-data analyses of 33,378 curated archaeal amoA sequences, we define a highly resolved taxonomy and uncover global environmental patterns that challenge many earlier generalisations. Particularly, we show: (i) the global frequency of AOA is extremely uneven, with few clades dominating AOA diversity in most ecosystems; (ii) characterised AOA do not represent most predominant clades in nature, including soils and oceans; (iii) the functional role of the most prevalent environmental AOA clade remains unclear; and (iv) AOA harbour molecular signatures that possibly reflect phenotypic traits. Our work synthesises information from a decade of research and provides the first integrative framework to study AOA in a global context.

Past, current, and future research on microalga-derived biodiesel: a critical review and bibliometric analysis

Microalga-derived biodiesel plays a crucial role in the sustainable development of biodiesel in recent years. Literature related to microalga-derived biodiesel had an increasing trend with the expanding research outputs. Based on the Science Citation Index Expanded (SCI-Expanded) of the Web of Science, a bibliometric analysis was conducted to characterize the body of knowledge on microalga-derived biodiesel between 1993 and 2016. From the 30 most frequently used author keywords, the following research hotspots are extracted: lipid preparation from different microalga species, microalga-derived lipid and environmental applications, lipid-producing microalgae cultivation, microalgae growth reactor, and microalga harvest and lipid extraction. Other keywords, i.e., microalga mixotrophic cultivation, symbiotic system between microalga and other oleaginous yeast, microalga genetic engineering, and other applications of lipid-producing microalga are future focal points of research. Graphical abstract.

Dominant role of ammonia-oxidizing bacteria in nitrification due to ammonia accumulation in sediments of Danjiangkou reservoir, China

Surface sediments are the inner source of contaminations in aquatic systems and usually maintain aerobic conditions. As the key participators of nitrification process, little is known about the activities and contributions of ammonia-oxidizing archaea (AOA) and bacteria (AOB) in the surface sediments. In this study, we determined the net and potential nitrification rates and used 1-octyne as an AOB specific inhibitor to detect the contributions of AOA and AOB to nitrification in surface sediments of Danjiangkou reservoir, which is the water source area of the middle route of South-to-North Water Diversion Project in China. Quantitative PCR and Illumina high-throughput sequencing were used to evaluate the abundance and diversity of the amoA gene. The net and potential nitrification rates ranged from 0.42 to 1.93 and 2.06 to 8.79 mg N kg^-1 dry sediments d^-1, respectively. AOB dominated in both net and potential nitrification, whose contribution accounted for 52.7-78.6% and 59.9-88.1%, respectively. The cell-specific ammonia oxidation rate calculation also revealed the cell-specific rates of AOB were higher than that of AOA. The Spearman's rank correlation analysis suggested that ammonia accumulation led to the AOB predominant role in net nitrification activity, and AOB abundance played the key role in potential nitrification activity. Furthermore, phylogenetic analysis suggested AOB were predominantly characterized by the Nitrosospira cluster, while AOA by the Nitrososphaera and Nitrososphaera sister clusters. This study will help us to better understand the contributions and characteristics of AOA and AOB in aquatic sediments and provide improved strategies for nitrogen control in large reservoirs.

Benchmarking the scientific research on wastewater-energy nexus by using bibliometric analysis

With an exponential increase in urbanization and industrialization, water pollution is an inevitable consequence of relatively lagging wastewater treatment facilities. The conventional activated sludge process for wastewater treatment primarily emphasizes the removal of harmful substances to maintain increasingly stringent effluent discharged standards, which is considered an energy-intensive technique. Therefore, innovative and sustainable wastewater treatment should pay more attention to energy and resource recovery in dealing with fossil fuel depletion, global-scale energy security, and climate change. A bibliometric analysis was applied to trace wastewater-energy nexus-related research during the period 1991 to 2015, with respect to the Science Citation Index EXPANDED (SCI-EXPANDED) database. Journal of Hazardous Materials, ranking 1st in h-index (79), was the most productive journal (431, 4.5%) during the same time, followed by International Journal of Hydrogen Energy (422, 4.4%) and Water Research (393, 4.1%) journal, the latter owning a topmost journal impact factor. Though, China (2154, 22.5%) was the most productive country, while the USA with highest h-index (88) was the favorest collaborative country. The Chinese Academy of Sciences, China (241, 2.5%) produced the maximum publications. A novel method called "word cluster analysis" showed that the emerging sustainable processes and novel renewable energy application are applied in response to the desire for a net wastewater-energy nexus system. Based on different wastewater types, the emerging energy and sources recovery treatment processes of Anammox, anaerobic digestion, and microbial fuel cells gained extensive innovation. Evaluation indicators including sustainability, life cycle assessment, and environmental impact were appropriately used to dissert feasibility of the novel treatment methods in regard of renewable energy utilization, energy savings, and energy recovery. The transformation of the new concept of "broaden income source, economize on expenditures and exploit inner potential" should be generalized in order to achieve an environmentally sustainable development of wastewater-energy nexus system.